{"slug":"fiber","name":"Dietary fiber","aliases":["Fiber intake","Dietary fibre","Soluble and insoluble fiber"],"kind":"lever","system":"gut-health","unit":"grams per day","unitNote":"The Institute of Medicine sets its Adequate Intake at 14 g of total fiber per 1,000 kcal, which is where the 25 g for women and 38 g for men come from; the cohorts count total fiber from food questionnaires; the lipid and glucose trials dose one soluble fiber (psyllium, oat beta-glucan, guar) in grams on top of the diet. Each figure is quoted as its source counts it.","answer":"In the WHO-commissioned series of 185 prospective studies and 58 trials, the highest fiber consumers had 15 to 30 percent lower all-cause and cardiovascular mortality and lower incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer than the lowest, with the greatest risk reduction at 25 to 29 g a day and dose-response curves that kept falling above it; the Institute of Medicine's Adequate Intake is 25 g a day for women and 38 g for men, the UK's is 30 g, and the average American eats about 16 to 17 g. In randomized trials, about 10 g a day of psyllium lowers LDL cholesterol by 0.33 mmol/L and ApoB by 0.05 g/L, and raising fiber in people with diabetes lowers HbA1c, fasting glucose, insulin and HOMA-IR.","summary":"Dietary fiber is the part of plant food that human enzymes cannot digest, so it reaches the colon, where it is either fermented by bacteria into short-chain fatty acids or holds water and bulks the stool. The cohort evidence for lower mortality, heart disease, diabetes and colorectal cancer with higher intake is among the most consistent in nutrition, the trials show measurable effects on LDL, ApoB, glucose, insulin and C-reactive protein, and almost no one in the United States eats the recommended amount.","whatItIs":[{"text":"The Institute of Medicine defines dietary fiber as nondigestible carbohydrates and lignin that are intrinsic and intact in plants, and sets the Adequate Intake at 14 g of total fiber per 1,000 kcal, which works out to 25 g a day for adult women and 38 g for adult men, on the basis of research showing protection against coronary heart disease. The Academy of Nutrition and Dietetics, restating that standard in 2015, adds that the mean intake in the United States is 17 g a day and that only 5 percent of the population meets the Adequate Intake. The UK's Scientific Advisory Committee on Nutrition reached a similar figure by a different route in 2015: a population average of 30 g a day for adults, measured by the AOAC methods, chosen because the dose-response plots for cardiovascular disease, type 2 diabetes and colorectal cancer showed the greatest benefit at 30 g and above and the confidence intervals widened beyond it.","cites":["dahl2015","iom2005","sacn2015"],"grade":"A"},{"text":"Fiber is not one substance. SACN's definition covers all carbohydrates that are neither digested nor absorbed in the small intestine and have a degree of polymerization of three or more, plus lignin; a food's label lumps them together, but they behave differently in the gut. The viscous, soluble fibers (beta-glucan from oats, pectin, guar gum, psyllium) form gels that slow glucose absorption and bind bile acids; the fermentable ones feed colonic bacteria; the insoluble, poorly fermented ones such as wheat bran hold water and add bulk. A 1980 experiment in Nature made the distinction physical: cabbage fiber was extensively broken down in the human colon and stimulated microbial growth, while wheat fiber stayed largely undigested and retained water in the gut lumen, and the authors showed that the main component of human feces is bacteria. The same group listed the effects of fiber on the colon as more stool, dilution of its contents, faster passage and altered handling of minerals, nitrogen and bile acids.","cites":["sacn2015","stephen1980","cummings1980","wanders2011","jovanovski2018","gunness2010"],"grade":"C"},{"text":"The reason a physician asks about fiber is that it sits on the pathway to several numbers on this shelf at once. The Dietary Guidelines for Americans 2020-2025 call dietary fiber a dietary component of public health concern, note that more than 90 percent of women and 97 percent of men do not meet recommended intakes, and state that the recommendations are based on levels observed to reduce risk of coronary heart disease. The 2018 umbrella review of 18 meta-analyses covering 298 prospective studies found convincing evidence for lower cardiovascular and all-cause mortality with higher fiber intake, and highly suggestive evidence for lower cardiovascular disease and coronary artery disease; convincing is that review's highest tier, and only 3 of its 21 outcomes reached it.","cites":["dga2020","veronese2018"],"grade":"A"}],"origins":[{"text":"The human gut is built for far more fiber than it now receives. In 1985 Eaton and Konner proposed in the New England Journal of Medicine that departures from the diet and activity of our hunter-gatherer ancestors contribute to the chronic diseases of modern life, the evolutionary discordance hypothesis; reviewing it twenty-five years later, they wrote that the anthropological evidence continued to show ancestral diets with much lower refined carbohydrate and sodium, much higher fiber and protein, and comparable fat and cholesterol, and that official recommendations had moved toward hunter-gatherer values in the interval. The modern Hadza of Tanzania make the point in bacteria rather than grams. Their gut microbiota holds more microbial richness and diversity than that of urban Italian controls, is enriched in Prevotella, Treponema and unclassified Bacteroidetes that may help them extract nutrition from fibrous plant foods, and differs between the sexes in a way that probably reflects the division of foraging labor; many of the tubers they eat are so fibrous that the quid is chewed and spat out. Sampled across more than a year, the Hadza microbiome also cycles with the seasons, some taxa vanishing and returning, and the taxa that are most seasonally volatile in the Hadza are the same ones that separate industrialized from traditional populations across 18 populations in 16 countries: lineages that have decreased in prevalence and abundance as societies modernized.","cites":["eaton1985","konner2010","schnorr2014","smits2017"],"grade":"C"},{"text":"What a low-fiber diet does to that ecosystem has been tested in mice carrying a human microbiota. On a diet low in microbiota-accessible carbohydrates, the fiber fraction bacteria can use, the loss of diversity within one generation was largely reversible when fiber returned; over several generations it compounded into a progressive loss that reintroducing fiber could not recover, because taxa driven to low abundance were inefficiently passed to the next generation and went extinct within the isolated population. Restoring the original community required both the missing bacteria and the fiber. A second experiment showed where the bacteria turn when fiber is scarce: in gnotobiotic mice with a defined human gut community, chronic or intermittent fiber deprivation drove the microbiota to feed on the host's own mucus glycoproteins, eroding the colonic mucus barrier, bringing the epithelium closer to a mucosal pathogen and producing lethal colitis. Both are mouse experiments and are cited for mechanism, not as human outcomes.","cites":["sonnenburg2016","desai2016"],"grade":"C"},{"text":"On the shelf, fiber sits upstream of the lipid, glucose, inflammatory and bowel markers through four routes. First, bile acids: viscous soluble fibers bind bile salt micelles in the small intestine so that less is reabsorbed and more is excreted, the liver draws on cholesterol to make more, and LDL and ApoB fall; that is the best-supported of three proposed mechanisms, the others being a lower glycemic response with less insulin-driven hepatic cholesterol synthesis and an effect of the fermentation product propionate, for which the evidence is inconclusive. Second, glucose: in 1978 volunteers took a 50 g glucose load with guar, pectin, tragacanth, methylcellulose, wheat bran or cholestyramine, each flattened the glucose curve and generally lowered insulin, the flattening correlated with viscosity (r = 0.926), and hydrolyzing guar into a non-viscous form abolished its effect. Third, fermentation: anaerobic bacteria turn fermentable fiber into short-chain fatty acids that activate G-protein-coupled receptors, inhibit histone deacetylases and serve as fuel; butyrate, made only by colonic bacteria, accounted for 73 to 75 percent of the oxygen consumption of isolated human colonocytes when offered alone, the basis of the hypothesis that bacterial fatty acids are the main energy source of the colonic lining. Fourth, appetite: propionate delivered to the colon of overweight adults raised peptide YY and GLP-1, reduced energy intake acutely, and over 24 weeks reduced weight gain, intra-abdominal fat and liver fat. ApoB, LDL, non-HDL, fasting glucose, fasting insulin, HOMA-IR, HbA1c, triglycerides and hs-CRP each have an entry here, and fiber is a lever on every one of them.","cites":["gunness2010","jenkins1978","koh2016","roediger1980","chambers2015","makki2018"],"grade":"C"}],"labRange":{"typical":"Adequate Intake 14 g per 1,000 kcal: 25 g a day for adult women and 38 g for adult men (Institute of Medicine 2005, restated by the Academy of Nutrition and Dietetics 2015); 30 g a day for adults by the AOAC methods (SACN 2015); the Dietary Guidelines for Americans 2020-2025 set goals from the same Dietary Reference Intakes","claims":[{"text":"The Adequate Intake of 14 g of total fiber per 1,000 kcal, 25 g a day for adult women and 38 g for adult men, was set on research demonstrating protection against coronary heart disease, and the Academy's 2015 position paper carries it forward with the finding that mean US intake is 17 g a day and 5 percent of the population meets it. The Institute of Medicine's report, which the Academy quotes, is the primary source; its chapter text could not be read on the publisher's site for this entry, so its figures are quoted through the position paper.","cites":["dahl2015","iom2005"],"grade":"A"},{"text":"SACN set the UK dietary reference value for adults at 30 g a day in 2015 after reviewing the non-linear dose-response plots from the Aune and Threapleton meta-analyses of cardiovascular disease, coronary heart disease, stroke, type 2 diabetes and colorectal cancer: intakes of 30 g a day and above were associated with the greatest reduction in those diseases, the confidence intervals widened beyond 30 g, and the previous reference value of 18 g of non-starch polysaccharide equated to about 23 to 24 g by the newer methods, so the new figure was a real increase. SACN also wrote that most of the evidence comes from fiber eaten as a natural component of a variety of foods, that isolated fibers have shown effects on blood lipids and bowel function, and that it is not known whether those isolated fibers confer the full range of benefits.","cites":["sacn2015"],"grade":"A"},{"text":"Against those figures, actual intake is low. In the National Health and Nutrition Examination Survey from 2001 to 2010, mean fiber intake was 16.1 g a day in adults aged 19 to 50 and 16.1 g in adults 51 and over, with vegetables (22.6 percent), grain mixtures (12.0 percent) and fruits (11.1 percent) the main sources; the Dietary Guidelines put more than 90 percent of women and 97 percent of men below the recommendation, in line with the more than 85 percent of adults who underconsume fruits, vegetables and whole grains. The survey analysis declares industry affiliations among its authors.","cites":["mcgill2015","dga2020"],"grade":"B"}]},"evidence":{"headline":"Highest versus lowest fiber intake carried 15 to 30 percent lower all-cause and cardiovascular mortality and lower coronary heart disease, stroke, type 2 diabetes and colorectal cancer across 185 prospective studies, with the greatest reduction at 25 to 29 g a day and more benefit above it; trials show lower LDL, ApoB, glucose, insulin and C-reactive protein; no trial has measured mortality or events.","scale":{"min":0,"max":40,"ticks":[0,5,10,15,20,25,30,35,40]},"bands":[{"label":"Adequate Intake: 25 g a day for women, 38 g for men (14 g per 1,000 kcal); 30 g in the UK","low":25,"high":38,"tone":"lab","source":"Institute of Medicine 2005 via Academy 2015; SACN 2015"},{"label":"Below about 15 g a day: no measurable protection against type 2 diabetes in the dose-response curve (RR 0.98 at 15 g)","low":0,"high":15,"tone":"caution","source":"Yao 2014"},{"label":"Typical US intake: about 16 to 17 g a day; more than 90 percent of women and 97 percent of men below the goal","low":16,"high":17,"tone":"caution","source":"McGill 2015; Dahl 2015; Dietary Guidelines 2020-2025"},{"label":"Greatest risk reduction across mortality, heart disease, stroke, diabetes and colorectal cancer: 25 to 29 g a day","low":25,"high":29,"tone":"evidence","source":"Reynolds 2019"},{"label":"Above 30 g: dose-response curves still falling (diabetes RR 0.76 at 30 g, 0.66 at 35 g); confidence intervals widen","low":30,"high":40,"tone":"neutral","source":"Reynolds 2019; Yao 2014; SACN 2015"}],"claims":[{"text":"The WHO-commissioned series pooled just under 135 million person-years from 185 prospective studies and 58 clinical trials with 4,635 adults, excluding people with chronic disease, weight-loss trials and supplement trials. Comparing the highest with the lowest fiber consumers, the observational data showed a 15 to 30 percent decrease in all-cause and cardiovascular mortality and in the incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer; the trials showed lower body weight, systolic blood pressure and total cholesterol with higher intake. Risk reduction across the critical outcomes was greatest at a daily intake of 25 to 29 g, and the dose-response curves suggested that higher intakes could protect further against cardiovascular disease, type 2 diabetes and colorectal and breast cancer. The certainty of the evidence for fiber was graded moderate, against low to moderate for whole grains and low to very low for glycemic index and load, and the authors wrote that the dose-response evidence was striking enough that the relationships could be causal.","cites":["reynolds2019"],"grade":"A"},{"text":"Mortality has two independent meta-analyses that agree. Across seven cohorts with 62,314 deaths among 908,135 participants, the highest fiber intake carried a relative risk of 0.77 for total mortality against the lowest, and each 10 g a day was associated with a relative risk of 0.89, with cereal and, to a lesser extent, vegetable fiber carrying the association and fruit fiber showing none. Across 17 cohorts with 67,260 deaths among 982,411 people, the top tertile carried a relative risk of 0.84 against the bottom, each 10 g a day a 10 percent lower risk (0.90), robust across subgroups and with no publication bias detected.","cites":["kim2014","yang2015"],"grade":"B"},{"text":"For the heart, 22 cohort publications found each 7 g a day of total fiber associated with a 9 percent lower risk of cardiovascular disease (risk ratio 0.91, 0.88 to 0.94) and of coronary heart disease (0.91, 0.87 to 0.94); insoluble fiber and fiber from cereals and vegetables were associated with lower risk of both, and fruit fiber with lower cardiovascular disease. Trials have not followed events: the Cochrane review of 23 randomized trials with 1,513 participants found none that reported mortality or cardiovascular events, most shorter than 12 weeks, with pooled reductions in total cholesterol of 0.23 mmol/L and LDL of 0.14 mmol/L, no change in triglycerides, a very small fall in HDL (0.03 mmol/L), a 1.77 mm Hg fall in diastolic pressure and a non-significant 1.92 mm Hg fall in systolic, and no clear difference between supplements and food or between soluble and insoluble fiber in the small subgroups.","cites":["threapleton2013","hartley2016"],"grade":"B"},{"text":"For type 2 diabetes, 17 prospective cohorts with 19,033 cases among 488,293 participants gave a relative risk of 0.81 for total fiber, 0.77 for cereal fiber, 0.94 for fruit fiber and 0.75 for insoluble fiber, highest against lowest. The relationship was nonlinear: relative risks of 0.98 at 15 g a day, 0.97 at 20, 0.89 at 25, 0.76 at 30 and 0.66 at 35, a curve that is flat until the mid-twenties and then falls. In the EPIC-InterAct case-cohort of 11,559 cases across eight European countries, the top quarter of intake carried a hazard ratio of 0.82 after adjustment for lifestyle and diet, an association that attenuated and lost significance after adjustment for body mass index, and the accompanying meta-analysis of 19 cohorts put the relative risk per 10 g a day at 0.91 for total fiber and 0.75 for cereal fiber; the authors read the EPIC result as evidence that part of the association runs through body weight.","cites":["yao2014","interact2015"],"grade":"B"},{"text":"For colorectal cancer, 25 prospective studies gave a summary relative risk of 0.90 per 10 g a day of total fiber (0.86 to 0.94, with no heterogeneity across the 16 studies), 0.90 for cereal fiber, no significant association for fruit or vegetable fiber, and 0.83 per three daily servings of whole grains. The mechanism has been tested in people: when African Americans, whose colon cancer rate is 65 per 100,000, swapped diets for two weeks with rural South Africans, whose rate is under 5 per 100,000, the high-fiber, low-fat African-style diet produced reciprocal changes in mucosal biomarkers of cancer risk, more saccharolytic fermentation and butyrate production, and suppressed secondary bile acid synthesis in the Americans, while the Africans on a Western-style diet moved the other way.","cites":["aune2011","okeefe2015"],"grade":"B"},{"text":"In people who already have diabetes, raising fiber moves the markers. Across 42 trials with 1,789 adults with prediabetes, type 1 or type 2 diabetes, increasing fiber lowered HbA1c by 2.00 mmol/mol (33 trials), fasting glucose by 0.56 mmol/L, about 10 mg/dL (34 trials), insulin by a standardized mean difference of 2.03 (19 trials), HOMA-IR by 1.24 (9 trials), total cholesterol by 0.34 mmol/L, LDL by 0.17 mmol/L, triglycerides by 0.16 mmol/L, body weight by 0.56 kg and C-reactive protein by a standardized mean difference of 2.80 (7 trials), with high heterogeneity throughout; two cohorts of 8,300 adults with diabetes followed 8.8 years showed 14 fewer deaths per 1,000 when a 35 g a day intake was compared with the average 19 g, with a clear dose response. An earlier pooling of 11 trials in 605 patients with type 2 diabetes, using high-fiber foods up to 42.5 g a day or soluble fiber supplements up to 15 g a day for 8 to 24 weeks, lowered HbA1c by 0.55 percentage points and fasting glucose by 9.97 mg/dL.","cites":["reynolds2020","silva2013"],"grade":"A"},{"text":"The lipid effect is the most precisely measured. Across 28 randomized trials with 1,924 participants, a median dose of about 10.2 g a day of psyllium for three weeks or more lowered LDL cholesterol by 0.33 mmol/L, about 13 mg/dL, non-HDL cholesterol by 0.39 mmol/L and ApoB by 0.05 g/L, about 5 mg/dL, in people with and without high cholesterol; the ApoB estimate was graded high quality and the LDL and non-HDL estimates moderate. Across 28 trials of 3 g a day or more of oat beta-glucan, LDL fell 0.25 mmol/L and total cholesterol 0.30 mmol/L with no change in HDL or triglycerides, no evidence that dose (3.0 to 12.4 g a day) or duration (2 to 12 weeks) changed the result, and a larger fall in people with higher baseline LDL. The 1999 regression across 67 controlled trials put the effect at 0.057 mmol/L of LDL per gram of soluble fiber between 2 and 10 g a day, with oats, psyllium and pectin indistinguishable, and concluded that soluble fiber makes a small contribution within the practical range of intake.","cites":["jovanovski2018","whitehead2014","brown1999"],"grade":"A"},{"text":"Weight and appetite answer more modestly. In the POUNDS Lost trial, where 345 adults with overweight or obesity ate calorie-restricted diets for six months and lost a mean of 7.27 kg, fiber intake was the most influential predictor of weight loss in a model that already included energy density, fat, age and adherence, and it was strongly associated with adherence to the diet itself; this is a secondary analysis within a randomized trial, not a randomized comparison of fiber. Across 58 fiber-versus-control comparisons of appetite, the more viscous fibers (pectins, beta-glucans, guar gum) reduced appetite in 59 percent of comparisons against 14 percent for less viscous ones, and acute energy intake in 69 percent against 30 percent; effects on long-term energy intake and body weight were small and showed no clear dose response.","cites":["miketinas2019","wanders2011"],"grade":"B"},{"text":"Inflammation follows, with limits. Across 14 randomized trials in overweight and obese adults, fiber or fiber-rich food lowered circulating C-reactive protein by 0.37 mg/L, a reduction seen only in the trials where the intervention group ate at least 8 g a day more fiber than control. In a crossover trial of 35 adults eating 30 g a day from either a DASH-style diet or a supplement after a baseline of 11.9 g, C-reactive protein fell from 4.4 to 3.8 mg/L on the high-fiber diet and to 3.6 mg/L on the supplement, but the fall was confined to the 18 lean normotensive participants (2.0 to 1.4 and 1.2 mg/L) and did not reach significance in the 17 obese hypertensive ones. In a 17-week randomized study of 18 healthy adults per arm, a high-fiber diet increased the microbiome's glycan-degrading enzymes but left microbial diversity and the primary cytokine score unchanged, while a high-fermented-food diet steadily raised diversity and lowered inflammatory markers; the response to fiber split into three trajectories by baseline microbiota diversity, which is a reason the same intake does different things in different people.","cites":["jiao2015","king2007","wastyk2021"],"grade":"B"},{"text":"The microbiome evidence is beginning to move from association to trial. In a randomized study of isoenergetic diets in patients with type 2 diabetes, with fecal shotgun metagenomics, a select group of short-chain fatty acid producers was promoted by dietary fibers while most other potential producers were diminished or unchanged; participants in whom those fiber-promoted producers were more diverse and abundant had a greater improvement in HbA1c, partly through increased GLP-1, and the responders crowded out producers of indole and hydrogen sulfide.","cites":["zhao2018"],"grade":"B"},{"text":"For the bowel itself, the evidence is older and plainer. In the EPIC-Oxford cohort of 47,033 adults followed 11.6 years with 812 cases of diverticular disease, the highest fifth of fiber intake (25.5 g a day or more for women, 26.1 g for men) carried a 41 percent lower risk of admission or death from diverticular disease than the lowest fifth (under 14 g), and vegetarians a 31 percent lower risk, each independent of the other. Across five randomized trials in constipation, fiber increased stool frequency (odds ratio 1.19) without clearly changing stool consistency, laxative use or painful defecation. Across 14 trials with 906 patients with irritable bowel syndrome, fiber reduced the chance of remaining symptomatic (relative risk 0.86, number needed to treat 10), a benefit seen only with soluble fiber (0.83, number needed to treat 7) and not with bran (0.90), though bran showed no evidence of harm.","cites":["crowe2011","yang2012","moayyedi2014"],"grade":"B"}]},"levers":[{"name":"Whole plant foods before anything isolated","effect":"the form of fiber the outcome evidence was built on","claim":{"text":"The Reynolds series excluded supplement trials by design and found its 15 to 30 percent risk reductions in people eating fiber as food; SACN wrote that most of the evidence for the wide range of benefits comes from fiber present as a naturally integrated component of a variety of foods, and that it is not known whether isolated fibers confer the full range; the Academy's position is that adults reach adequate intake by eating more plant foods while cutting energy from foods high in added sugar and fat and low in fiber, with whole grains, legumes, vegetables, fruits and nuts as the message. In the US survey, vegetables, grain mixtures and fruits already supply nearly half of the fiber people eat, so the gap closes by eating more of what is already on the plate.","cites":["reynolds2019","sacn2015","dahl2015","mcgill2015"],"grade":"A"}},{"name":"Cereal fiber and whole grains","effect":"the fiber source most consistently tied to lower mortality, diabetes and colorectal cancer in the cohorts","claim":{"text":"By source, cereal fiber carried the mortality association where fruit fiber did not; for type 2 diabetes, cereal fiber gave a relative risk of 0.77 highest against lowest and 0.94 per 2 g a day in one meta-analysis and 0.75 per 10 g a day in another, against 0.93 for vegetable fiber and no association for fruit fiber in EPIC-InterAct; for colorectal cancer, cereal fiber (0.90 per 10 g) and whole grains (0.83 per three servings a day) carried the association. The Dietary Guidelines ask for refined grains to be replaced with whole grains for this reason. Whether the cereal signal is the fiber or the whole-grain package it arrives in cannot be separated by cohorts.","cites":["kim2014","yao2014","interact2015","aune2011","dga2020"],"grade":"B"}},{"name":"Oats and other viscous soluble fibers","effect":"lowers LDL and total cholesterol; no effect on HDL or triglycerides","claim":{"text":"Three grams a day or more of oat beta-glucan, the amount behind the approved health claims, lowered LDL by 0.25 mmol/L and total cholesterol by 0.30 mmol/L across 28 randomized trials, with the effect larger in people with higher baseline LDL and in those with diabetes and no dependence on dose above 3 g or on duration from 2 to 12 weeks. The 1999 regression put oats, psyllium and pectin at the same effect per gram and illustrated it with three 28 g servings of oatmeal, about 3 g of soluble fiber, lowering LDL by roughly 0.13 mmol/L. The mechanism is bile acid binding in the small intestine, which is why the fiber has to be eaten with meals to work.","cites":["whitehead2014","brown1999","gunness2010"],"grade":"A"}},{"name":"Psyllium, about 10 g a day with meals","effect":"lowers LDL by about 0.33 mmol/L, non-HDL by 0.39 and ApoB by 0.05 g/L","claim":{"text":"Psyllium is the one isolated fiber with a high-quality ApoB estimate: a median of about 10.2 g a day for three weeks or more lowered LDL by 0.33 mmol/L, non-HDL cholesterol by 0.39 mmol/L and ApoB by 0.05 g/L across 28 trials of 1,924 people with or without hypercholesterolemia. It is a viscous, gel-forming fiber, which is the class that bound bile acids and flattened glucose curves in the mechanism studies, and the effect appears within the three-week trial window. It adds to a food-first diet rather than replacing one, since the outcome cohorts were built on food.","cites":["jovanovski2018","gunness2010","jenkins1978","sacn2015"],"grade":"A"}},{"name":"Legumes, vegetables and fruit","effect":"fermentable fiber for the colon; vegetable fiber tracks lower heart disease, fruit fiber less consistently","claim":{"text":"Vegetable fiber was associated with lower cardiovascular and coronary disease and fruit fiber with lower cardiovascular disease across 22 cohorts; for diabetes, vegetable fiber tracked lower risk in EPIC-InterAct and fruit fiber gave a relative risk of 0.94 in the pooled cohorts; for colorectal cancer, legume fiber gave a point estimate of 0.62 per 10 g that did not reach significance on four studies, and fruit and vegetable fiber showed no association. Legumes and vegetables carry the fermentable fraction that the two-week diet swap used to raise butyrate production and suppress secondary bile acids in the colon. The sources differ in what they do, which is the argument for eating several.","cites":["threapleton2013","interact2015","yao2014","aune2011","okeefe2015"],"grade":"B"}},{"name":"Pairing with meals, and the ramp-up","effect":"viscous fiber works at the meal it is eaten with; fermentable fiber needs a slow start","claim":{"text":"The glucose-flattening and bile-acid-binding effects belong to the meal: in the 1978 experiment the fiber was taken with the glucose load, and the bile salt mechanism acts in the small intestine as food passes. Fermentable fibers announce themselves: in 26 healthy adults, 5 and 10 g doses of inulin and oligofructose mildly raised gas and bloating, the 10 g oligofructose dose substantially so, and the authors set tolerance at up to 10 g a day of native inulin and 5 g of oligofructose. In the Cochrane trials, adverse events were mostly mild to moderate gastrointestinal symptoms and more common in the fiber groups. The Reynolds diabetes review noted that the largest glycemic gains came in people moving from low to moderate or high intakes, and proposed 15 g a day more, or 35 g, as a reasonable target in diabetes; moving there over weeks rather than days is a tolerance question, not an efficacy one.","cites":["jenkins1978","gunness2010","bonnema2010","hartley2016","reynolds2020"],"grade":"C"}},{"name":"Time to effect","effect":"lipids and glucose move within weeks; the cohort outcomes are decades","claim":{"text":"Psyllium trials of three weeks or more and oat beta-glucan trials of 2 to 12 weeks showed their full lipid effect within the trial, with no evidence that longer treatment lowered LDL further; the diabetes trials ran 8 to 24 weeks for a 0.55-point HbA1c change; the C-reactive protein crossover showed its fall within each diet period after a three-week baseline. The mortality, heart disease, diabetes and cancer associations come from cohorts followed for years to decades, and no randomized trial of fiber has run long enough to count an event, so the near-term numbers are the only direct test a person gets.","cites":["jovanovski2018","whitehead2014","silva2013","king2007","hartley2016"],"grade":"B"}}],"caveats":[{"text":"The outcome evidence is observational, and fiber travels with other habits. The 15 to 30 percent reductions in the WHO series, the mortality meta-analyses and the diabetes and colorectal cancer dose-response curves are all from cohorts that measured diet by questionnaire; the Reynolds authors graded the certainty moderate and called the dose-response striking enough that the relationships could be causal, which is a carefully chosen verb. The trials that exist measured LDL, glucose, weight, blood pressure and C-reactive protein over weeks, and the Cochrane review found none that reported a cardiovascular event or a death. In EPIC-InterAct the diabetes association weakened to non-significance once body mass index entered the model, so part of the signal may run through weight rather than fiber itself.","cites":["reynolds2019","hartley2016","interact2015"],"grade":"B"},{"text":"Gas, bloating and a change in bowel habit are the usual price of a fast increase, and fermentable fibers charge more of it than bulking ones. Inulin and oligofructose raised flatulence and bloating in healthy adults at 10 g doses; the Cochrane trials reported mostly mild to moderate gastrointestinal effects, more often in the fiber groups; in irritable bowel syndrome, bran did not help where soluble fiber did, and only the soluble fibers earned a number needed to treat. The practical reading is that intake rises over weeks, with water, and that a person with a sensitive bowel starts with soluble fiber rather than bran.","cites":["bonnema2010","hartley2016","moayyedi2014"],"grade":"C"},{"text":"High-fiber foods, especially whole grains and legumes, carry phytate, which inhibits iron absorption in a dose-dependent way in single-meal studies; iron bioavailability is estimated at 14 to 18 percent from mixed diets and 5 to 12 percent from vegetarian diets in people with no iron stores, and the influence of nondigestible carbohydrates themselves on iron absorption remains unresolved. In whole diets with many inhibitors and enhancers the effect of any one component is more modest, and a person's iron status matters more than their diet's composition. Someone with low ferritin who moves to a plant-heavy, high-fiber diet deserves a repeat ferritin and transferrin saturation rather than an assumption either way.","cites":["hurrell2010"],"grade":"C"},{"text":"Fiber is not for every gut. The American College of Gastroenterology's 2022 gastroparesis guideline recommends a small particle diet to improve symptom relief and gastric emptying, which is the opposite direction from a bulking fiber program; a stomach that empties slowly, a bowel narrowed by stricture or recent surgery, or a flare of inflammatory bowel disease changes the advice, and a physician who knows the anatomy should set it.","cites":["camilleri2022"],"grade":"C"},{"text":"Isolated fibers are not interchangeable with food, and the strongest trial evidence belongs to a short list of them. SACN's reference value was set on food-based intake and it wrote that isolated fibers have shown effects on lipids and bowel function but that the full range of benefits is unproven for them; the psyllium and oat beta-glucan estimates are precise because those two fibers were tested repeatedly, not because every product labeled fiber does the same. A fortified bar is not a lentil.","cites":["sacn2015","jovanovski2018","whitehead2014"],"grade":"B"}],"seeAPhysician":["Blood in the stool, black or tarry stools, or a change in bowel habit that persists for more than a few weeks, especially with a family history of colorectal cancer; fiber is not the first step, an evaluation is.","Unexplained weight loss, waking at night with abdominal pain, fever with abdominal pain, or vomiting, with or without a change in bowel habit.","Severe or worsening abdominal pain, distension or constipation after increasing fiber, particularly with a history of bowel surgery, Crohn's disease, a known stricture or radiation to the abdomen or pelvis.","Early fullness, nausea or vomiting after meals, or a diagnosis of gastroparesis, where a bulking fiber program can make things worse and the diet needs to be set by someone who knows the gastric emptying.","Difficulty swallowing or any sense of a supplement sticking in the throat or chest; gel-forming fibers must be taken with enough water and are not for people with swallowing problems.","Low ferritin or anemia in someone eating a plant-heavy diet, or a planned change to one, so that iron can be followed rather than guessed at.","A lipid plan under review: a physician should know what the person eats before adding or removing a cholesterol-lowering medication, because a change in fiber shifts LDL and ApoB within weeks."],"openQuestions":["No randomized trial has tested fiber against mortality, heart attack, stroke, diabetes incidence or colorectal cancer. The outcome curve is observational, graded moderate certainty by the WHO series; the trials stop at risk markers over weeks to months.","The ideal intake is not fixed. The WHO series found the greatest reduction at 25 to 29 g a day with curves still falling above it, SACN chose 30 g because confidence intervals widened beyond it, the Institute of Medicine's Adequate Intake is 25 and 38 g by sex, and the diabetes dose-response was still improving at 35 g; whether more than about 35 to 40 g a day adds benefit in a Western population, or at what point tolerance limits it, has not been mapped.","Which fiber matters most is unsettled. Cereal fiber carries the cohort signal for mortality, diabetes and colorectal cancer, viscous soluble fibers carry the lipid and glucose trials, fermentable fibers carry the microbiome mechanism, and no study has randomized people to one class against another on a hard outcome.","Whether food fiber and isolated fiber are equivalent is open. SACN says the full range of benefit is unproven for isolated fibers; psyllium has a high-quality ApoB estimate; nothing has compared a supplement with the same grams from food on an outcome.","Why the same intake does different things in different people is only beginning to be measured. In the Stanford study the immune response to a high-fiber diet split into three trajectories by baseline microbiota diversity, and in the Shanghai trial HbA1c improved most in those whose fiber-promoted short-chain fatty acid producers flourished; whether a person's microbiome can be read in advance to predict their response is a research question, not a clinical test.","How much of the diabetes association runs through body weight is unknown. The EPIC-InterAct association attenuated after adjustment for body mass index; adjusting away a mediator understates a true effect and adjusting away a confounder corrects a false one, and the cohort cannot tell which it did.","The multi-generation microbiota extinction seen in mice has not been shown in people. The Hadza and modernization data are consistent with it; a human test would take generations."],"questions":[{"q":"How much fiber should I eat a day?","a":"The Institute of Medicine's Adequate Intake is 14 g per 1,000 kcal, which comes to 25 g a day for adult women and 38 g for adult men, set on evidence of protection against coronary heart disease; the UK's SACN set 30 g a day for adults in 2015 on the dose-response curves for heart disease, diabetes and colorectal cancer. The WHO-commissioned Lancet series found the greatest risk reduction across those outcomes at 25 to 29 g a day, with curves still falling above it. The average American eats about 16 to 17 g, and more than 90 percent of women and 97 percent of men fall short, so for most people the honest answer is more than now, from food, raised over weeks."},{"q":"Does fiber lower cholesterol and ApoB?","a":"Yes, by a measurable and modest amount, and the viscous soluble fibers do it. Across 28 randomized trials, about 10 g a day of psyllium lowered LDL by 0.33 mmol/L (about 13 mg/dL), non-HDL by 0.39 mmol/L and ApoB by 0.05 g/L (about 5 mg/dL); across 28 trials, 3 g a day or more of oat beta-glucan lowered LDL by 0.25 mmol/L. The mechanism is bile acid binding in the small intestine, so the fiber has to be eaten with meals, and the effect is complete within a few weeks. It is a small fraction of what a statin does, and it is an honest first move or an addition, not a substitute, which is a decision to make with the physician who is watching the numbers."},{"q":"Does fiber help blood sugar and insulin?","a":"In people with prediabetes or diabetes, raising fiber lowered HbA1c by 2 mmol/mol, fasting glucose by about 10 mg/dL, insulin and HOMA-IR across 42 randomized trials, with the largest glycemic gains in people moving from low to moderate or high intakes; an earlier pooling of 11 trials put the HbA1c change at 0.55 percentage points. In people without diabetes, the cohorts put the risk of developing type 2 diabetes at 0.76 at 30 g a day and 0.66 at 35 g relative to the lowest intakes, with little difference below about 20 g. Viscous fiber works at the meal by slowing glucose absorption; fermentable fiber works more slowly through the colon and its bacteria."},{"q":"Is a fiber supplement as good as fiber from food?","a":"Not proven, and the outcome evidence says food. The WHO series excluded supplement trials and found its 15 to 30 percent risk reductions in people eating fiber as food; SACN wrote that isolated fibers have shown effects on blood lipids and bowel function but that it is not known whether they confer the full range of benefits. Psyllium has a high-quality trial estimate for ApoB and LDL, and oat beta-glucan for LDL, so those two have earned a specific use. For the rest, legumes, whole grains, vegetables, fruit and nuts are what the long cohorts measured."},{"q":"Why does fiber make me bloated, and will it stop?","a":"Fermentable fibers, including inulin, oligofructose and the fiber in beans and some vegetables, are food for colonic bacteria, and gas is what fermentation makes; in healthy adults a 10 g dose of oligofructose raised gas and bloating noticeably. The usual course is that symptoms are worst in the first days of an increase and settle as intake becomes steady, which is why the increase is spread over weeks with enough water. If a sensitive bowel is the issue, the irritable bowel trials found soluble fiber helped where bran did not, and if bloating comes with pain, vomiting, weight loss or blood, that is a physician's question before it is a fiber question."}],"citations":[{"id":"reynolds2019","authors":"Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L","title":"Carbohydrate quality and human health: a series of systematic reviews and meta-analyses","journal":"Lancet","year":2019,"pmid":"30638909","doi":"10.1016/S0140-6736(18)31809-9","type":"meta-analysis","finding":"Across 185 prospective studies (just under 135 million person-years) and 58 trials (4,635 adults), the highest fiber consumers had a 15 to 30 percent decrease in all-cause and cardiovascular mortality and in coronary heart disease, stroke, type 2 diabetes and colorectal cancer; trials showed lower body weight, systolic blood pressure and total cholesterol; risk reduction was greatest at 25 to 29 g a day with dose-response curves suggesting further benefit above; certainty graded moderate for fiber; WHO among the funders.","url":"https://pubmed.ncbi.nlm.nih.gov/30638909/"},{"id":"sacn2015","authors":"Scientific Advisory Committee on Nutrition","title":"Carbohydrates and Health","journal":"The Stationery Office, London (Public Health England)","year":2015,"url":"https://assets.publishing.service.gov.uk/media/5a7f7cc3ed915d74e622ac2a/SACN_Carbohydrates_and_Health.pdf","type":"guideline","finding":"Recommends a population average dietary fiber intake of 30 g a day for adults by the AOAC methods (previous 18 g of non-starch polysaccharide equated to about 23 to 24 g), chosen from the non-linear dose-response plots for cardiovascular disease, coronary heart disease, stroke, type 2 diabetes and colorectal cancer, on which 30 g and above carried the greatest benefit and confidence intervals widened beyond it; notes that most evidence comes from fiber eaten as food and that isolated fibers have not been shown to confer the full range of benefits."},{"id":"dahl2015","authors":"Dahl WJ, Stewart ML","title":"Position of the Academy of Nutrition and Dietetics: Health Implications of Dietary Fiber","journal":"Journal of the Academy of Nutrition and Dietetics","year":2015,"pmid":"26514720","doi":"10.1016/j.jand.2015.09.003","type":"consensus","finding":"Restates the Institute of Medicine definition (nondigestible carbohydrates and lignin intrinsic and intact in plants) and Adequate Intake of 14 g per 1,000 kcal, 25 g for adult women and 38 g for adult men, based on protection against coronary heart disease; mean US intake 17 g a day with 5 percent meeting the Adequate Intake; adequate intake is reached through whole grains, legumes, vegetables, fruits and nuts.","url":"https://pubmed.ncbi.nlm.nih.gov/26514720/"},{"id":"iom2005","authors":"Institute of Medicine, Food and Nutrition Board","title":"Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids","journal":"National Academies Press, Washington, DC","year":2005,"doi":"10.17226/10490","url":"https://doi.org/10.17226/10490","type":"guideline","finding":"The report that defines dietary, functional and total fiber and sets the Adequate Intake of 14 g per 1,000 kcal; its chapter text was not readable on the publisher's site for this entry, so its definition and figures are quoted through the Academy of Nutrition and Dietetics 2015 position paper, cited alongside it."},{"id":"dga2020","authors":"US Department of Agriculture and US Department of Health and Human Services","title":"Dietary Guidelines for Americans, 2020-2025","journal":"Ninth edition, December 2020","year":2020,"url":"https://www.dietaryguidelines.gov/sites/default/files/2021-03/Dietary_Guidelines_for_Americans-2020-2025.pdf","type":"guideline","finding":"Names dietary fiber a dietary component of public health concern; states that more than 90 percent of women and 97 percent of men do not meet recommended intakes, in line with more than 85 percent of adults underconsuming fruits, vegetables and whole grains; sets fiber goals from the Dietary Reference Intakes, based on levels observed to reduce risk of coronary heart disease; asks for refined grains to be replaced with whole grains."},{"id":"mcgill2015","authors":"McGill CR, Fulgoni VL 3rd, Devareddy L","title":"Ten-year trends in fiber and whole grain intakes and food sources for the United States population: National Health and Nutrition Examination Survey 2001-2010","journal":"Nutrients","year":2015,"pmid":"25671414","doi":"10.3390/nu7021119","type":"cross-sectional","finding":"In NHANES 2001 to 2010 (24,809 adults), mean fiber intake was 16.1 g a day for adults 19 to 50 and 16.1 g for adults 51 and over; main sources for adults were vegetables (22.6 percent), other foods (14.3 percent), grain mixtures (12.0 percent) and fruits (11.1 percent); authors declare food-industry affiliations.","url":"https://pubmed.ncbi.nlm.nih.gov/25671414/"},{"id":"veronese2018","authors":"Veronese N, Solmi M, Caruso MG, et al.","title":"Dietary fiber and health outcomes: an umbrella review of systematic reviews and meta-analyses","journal":"American Journal of Clinical Nutrition","year":2018,"pmid":"29566200","doi":"10.1093/ajcn/nqx082","type":"review","finding":"Across 18 meta-analyses of 298 prospective observational studies and 21 outcomes, convincing evidence for lower cardiovascular mortality and all-cause mortality with higher fiber intake (and pancreatic cancer, not from prospective studies), and highly suggestive evidence for lower cardiovascular disease and coronary artery disease.","url":"https://pubmed.ncbi.nlm.nih.gov/29566200/"},{"id":"kim2014","authors":"Kim Y, Je Y","title":"Dietary fiber intake and total mortality: a meta-analysis of prospective cohort studies","journal":"American Journal of Epidemiology","year":2014,"pmid":"25143474","doi":"10.1093/aje/kwu174","type":"meta-analysis","finding":"Across seven cohorts with 62,314 deaths among 908,135 participants, the highest fiber intake carried a pooled relative risk of 0.77 (0.74 to 0.80) for total mortality against the lowest, and 0.89 (0.85 to 0.92) per 10 g a day; cereal and, less so, vegetable fiber carried the association, fruit fiber did not.","url":"https://pubmed.ncbi.nlm.nih.gov/25143474/"},{"id":"yang2015","authors":"Yang Y, Zhao LG, Wu QJ, Ma X, Xiang YB","title":"Association between dietary fiber and lower risk of all-cause mortality: a meta-analysis of cohort studies","journal":"American Journal of Epidemiology","year":2015,"pmid":"25552267","doi":"10.1093/aje/kwu257","type":"meta-analysis","finding":"Across 17 prospective studies with 67,260 deaths among 982,411 people, the top tertile of fiber intake carried a relative risk of 0.84 (0.80 to 0.87) for all-cause mortality against the bottom, and each 10 g a day a relative risk of 0.90 (0.86 to 0.94); robust across subgroups, no publication bias.","url":"https://pubmed.ncbi.nlm.nih.gov/25552267/"},{"id":"threapleton2013","authors":"Threapleton DE, Greenwood DC, Evans CE, et al.","title":"Dietary fibre intake and risk of cardiovascular disease: systematic review and meta-analysis","journal":"BMJ","year":2013,"pmid":"24355537","doi":"10.1136/bmj.f6879","type":"meta-analysis","finding":"Across 22 cohort publications, each 7 g a day of total fiber carried a risk ratio of 0.91 (0.88 to 0.94) for cardiovascular disease and 0.91 (0.87 to 0.94) for coronary heart disease; insoluble fiber and cereal and vegetable fiber were inversely associated with both, fruit fiber with cardiovascular disease; the first author declares industry support.","url":"https://pubmed.ncbi.nlm.nih.gov/24355537/"},{"id":"hartley2016","authors":"Hartley L, May MD, Loveman E, Colquitt JL, Rees K","title":"Dietary fibre for the primary prevention of cardiovascular disease","journal":"Cochrane Database of Systematic Reviews","year":2016,"pmid":"26758499","doi":"10.1002/14651858.CD011472.pub2","type":"meta-analysis","finding":"Across 23 randomized trials (1,513 participants), none reported mortality or cardiovascular events; increased fiber lowered total cholesterol by 0.23 mmol/L and LDL by 0.14 mmol/L, did not change triglycerides, lowered HDL by 0.03 mmol/L and diastolic pressure by 1.77 mm Hg (systolic 1.92, not significant); no clear subgroup effect by supplement versus food or soluble versus insoluble; adverse events mostly mild to moderate gastrointestinal, more in fiber groups.","url":"https://pubmed.ncbi.nlm.nih.gov/26758499/"},{"id":"yao2014","authors":"Yao B, Fang H, Xu W, et al.","title":"Dietary fiber intake and risk of type 2 diabetes: a dose-response analysis of prospective studies","journal":"European Journal of Epidemiology","year":2014,"pmid":"24389767","doi":"10.1007/s10654-013-9876-x","type":"meta-analysis","finding":"Across 17 cohorts with 19,033 cases among 488,293 participants, relative risks of type 2 diabetes were 0.81 for total fiber, 0.77 for cereal, 0.94 for fruit and 0.75 for insoluble fiber; nonlinear dose-response with relative risks of 0.98, 0.97, 0.89, 0.76 and 0.66 at 15, 20, 25, 30 and 35 g a day; 0.94 per 2 g a day of cereal fiber.","url":"https://pubmed.ncbi.nlm.nih.gov/24389767/"},{"id":"interact2015","authors":"InterAct Consortium","title":"Dietary fibre and incidence of type 2 diabetes in eight European countries: the EPIC-InterAct Study and a meta-analysis of prospective studies","journal":"Diabetologia","year":2015,"pmid":"26021487","doi":"10.1007/s00125-015-3585-9","type":"cohort","finding":"In a case-cohort of 11,559 incident cases over 10.8 years, the top quarter of fiber intake carried a hazard ratio of 0.82 (0.69 to 0.97) for type 2 diabetes after lifestyle and dietary adjustment, attenuated and non-significant after adjustment for body mass index; in a meta-analysis of 19 cohorts, relative risks per 10 g a day were 0.91 for total fiber, 0.75 for cereal fiber and 0.93 for vegetable fiber.","url":"https://pubmed.ncbi.nlm.nih.gov/26021487/"},{"id":"aune2011","authors":"Aune D, Chan DS, Lau R, et al.","title":"Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies","journal":"BMJ","year":2011,"pmid":"22074852","doi":"10.1136/bmj.d6617","type":"meta-analysis","finding":"Across 25 prospective studies, the summary relative risk of colorectal cancer per 10 g a day was 0.90 (0.86 to 0.94) for total fiber, 0.93 for fruit fiber, 0.98 for vegetable fiber, 0.62 (0.27 to 1.42) for legume fiber and 0.90 (0.83 to 0.97) for cereal fiber; 0.83 per three daily servings of whole grains.","url":"https://pubmed.ncbi.nlm.nih.gov/22074852/"},{"id":"okeefe2015","authors":"O'Keefe SJ, Li JV, Lahti L, et al.","title":"Fat, fibre and cancer risk in African Americans and rural Africans","journal":"Nature Communications","year":2015,"pmid":"25919227","doi":"10.1038/ncomms7342","type":"laboratory","finding":"A two-week supervised diet exchange between African Americans (colon cancer 65 per 100,000) and rural South Africans (under 5 per 100,000) produced reciprocal changes in mucosal biomarkers of cancer risk and in the microbiota and metabolome, with increased saccharolytic fermentation and butyrogenesis and suppressed secondary bile acid synthesis in the Americans on the high-fiber, low-fat diet.","url":"https://pubmed.ncbi.nlm.nih.gov/25919227/"},{"id":"reynolds2020","authors":"Reynolds AN, Akerman AP, Mann J","title":"Dietary fibre and whole grains in diabetes management: Systematic review and meta-analyses","journal":"PLoS Medicine","year":2020,"pmid":"32142510","doi":"10.1371/journal.pmed.1003053","type":"meta-analysis","finding":"Two cohorts of 8,300 adults with diabetes followed 8.8 years: 14 fewer deaths per 1,000 comparing 35 g a day with the average 19 g; across 42 trials (1,789 adults with prediabetes or diabetes) increasing fiber lowered HbA1c by 2.00 mmol/mol, fasting glucose 0.56 mmol/L, insulin (SMD 2.03), HOMA-IR 1.24, total cholesterol 0.34 mmol/L, LDL 0.17 mmol/L, triglycerides 0.16 mmol/L, weight 0.56 kg and C-reactive protein (SMD 2.80), high heterogeneity; proposes 15 g a day more, or 35 g, as a reasonable target in diabetes.","url":"https://pubmed.ncbi.nlm.nih.gov/32142510/"},{"id":"silva2013","authors":"Silva FM, Kramer CK, de Almeida JC, Steemburgo T, Gross JL, Azevedo MJ","title":"Fiber intake and glycemic control in patients with type 2 diabetes mellitus: a systematic review with meta-analysis of randomized controlled trials","journal":"Nutrition Reviews","year":2013,"pmid":"24180564","doi":"10.1111/nure.12076","type":"meta-analysis","finding":"Across 11 randomized trials (13 comparisons, 605 patients, 8 to 24 weeks) of high-fiber diets (foods up to 42.5 g a day or soluble fiber supplements up to 15 g a day), HbA1c fell 0.55 percentage points (0.13 to 0.96) and fasting glucose 9.97 mg/dL (1.78 to 18.16).","url":"https://pubmed.ncbi.nlm.nih.gov/24180564/"},{"id":"jovanovski2018","authors":"Jovanovski E, Yashpal S, Komishon A, et al.","title":"Effect of psyllium (Plantago ovata) fiber on LDL cholesterol and alternative lipid targets, non-HDL cholesterol and apolipoprotein B: a systematic review and meta-analysis of randomized controlled trials","journal":"American Journal of Clinical Nutrition","year":2018,"pmid":"30239559","doi":"10.1093/ajcn/nqy115","type":"meta-analysis","finding":"Across 28 randomized trials (1,924 participants) of three weeks or more, a median of about 10.2 g a day of psyllium lowered LDL by 0.33 mmol/L (0.27 to 0.38), non-HDL cholesterol by 0.39 mmol/L and ApoB by 0.05 g/L (0.03 to 0.08) in people with or without hypercholesterolemia; GRADE moderate for LDL and non-HDL, high for ApoB.","url":"https://pubmed.ncbi.nlm.nih.gov/30239559/"},{"id":"whitehead2014","authors":"Whitehead A, Beck EJ, Tosh S, Wolever TM","title":"Cholesterol-lowering effects of oat β-glucan: a meta-analysis of randomized controlled trials","journal":"American Journal of Clinical Nutrition","year":2014,"pmid":"25411276","doi":"10.3945/ajcn.114.086108","type":"meta-analysis","finding":"Across 28 randomized trials, 3 g a day or more of oat beta-glucan lowered LDL by 0.25 mmol/L (0.20 to 0.30) and total cholesterol by 0.30 mmol/L with no effect on HDL or triglycerides, no influence of dose (3.0 to 12.4 g a day) or duration (2 to 12 weeks), and a larger LDL effect with higher baseline LDL and in diabetes.","url":"https://pubmed.ncbi.nlm.nih.gov/25411276/"},{"id":"brown1999","authors":"Brown L, Rosner B, Willett WW, Sacks FM","title":"Cholesterol-lowering effects of dietary fiber: a meta-analysis","journal":"American Journal of Clinical Nutrition","year":1999,"pmid":"9925120","doi":"10.1093/ajcn/69.1.30","type":"meta-analysis","finding":"Across 67 controlled trials, 2 to 10 g a day of soluble fiber lowered total cholesterol by 0.045 mmol/L and LDL by 0.057 mmol/L per gram, with oats, psyllium and pectin not significantly different and no effect on triglycerides or HDL; 3 g of soluble fiber from oats (three 28 g servings of oatmeal) lowers total and LDL cholesterol by about 0.13 mmol/L; the effect is small within the practical range of intake.","url":"https://pubmed.ncbi.nlm.nih.gov/9925120/"},{"id":"gunness2010","authors":"Gunness P, Gidley MJ","title":"Mechanisms underlying the cholesterol-lowering properties of soluble dietary fibre polysaccharides","journal":"Food & Function","year":2010,"pmid":"21776465","doi":"10.1039/c0fo00080a","type":"review","finding":"Three proposed mechanisms for the cholesterol lowering of soluble fibers (beta-glucan, pectin, guar gum, psyllium): prevention of bile salt reabsorption in the small intestine with greater fecal excretion, supported by in vivo, ex vivo and in vitro evidence that bile salt micelles bind to the fiber; a reduced glycemic response with less insulin-stimulated hepatic cholesterol synthesis; and effects of the fermentation product propionate, for which evidence is inconclusive.","url":"https://pubmed.ncbi.nlm.nih.gov/21776465/"},{"id":"jenkins1978","authors":"Jenkins DJ, Wolever TM, Leeds AR, et al.","title":"Dietary fibres, fibre analogues, and glucose tolerance: importance of viscosity","journal":"British Medical Journal","year":1978,"pmid":"647304","doi":"10.1136/bmj.1.6124.1392","type":"rct","finding":"In groups of four to six volunteers taking 50 g glucose tolerance tests with and without 12 g of guar, pectin, gum tragacanth, methylcellulose, wheat bran or cholestyramine, each reduced blood glucose and generally insulin; guar flattened the curve most and lost its effect when hydrolyzed to a non-viscous form; the reduction in peak glucose correlated with viscosity (r = 0.926).","url":"https://pubmed.ncbi.nlm.nih.gov/647304/"},{"id":"koh2016","authors":"Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F","title":"From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites","journal":"Cell","year":2016,"pmid":"27259147","doi":"10.1016/j.cell.2016.05.041","type":"review","finding":"Short-chain fatty acids, the major class of bacterial metabolites made from dietary fiber, directly activate G-protein-coupled receptors, inhibit histone deacetylases and serve as energy substrates, and through these routes affect host physiology in health and disease.","url":"https://pubmed.ncbi.nlm.nih.gov/27259147/"},{"id":"makki2018","authors":"Makki K, Deehan EC, Walter J, Bäckhed F","title":"The Impact of Dietary Fiber on Gut Microbiota in Host Health and Disease","journal":"Cell Host & Microbe","year":2018,"pmid":"29902436","doi":"10.1016/j.chom.2018.05.012","type":"review","finding":"Dietary fibers interact directly with gut microbes and lead to production of key metabolites such as short-chain fatty acids; the type, quality and origin of food shape gut microbial ecology, host physiology and health.","url":"https://pubmed.ncbi.nlm.nih.gov/29902436/"},{"id":"roediger1980","authors":"Roediger WE","title":"Role of anaerobic bacteria in the metabolic welfare of the colonic mucosa in man","journal":"Gut","year":1980,"pmid":"7429343","doi":"10.1136/gut.21.9.793","type":"laboratory","finding":"In isolated human colonocytes from ascending and descending colon, n-butyrate, produced only by anaerobic bacteria in the lumen, accounted for 73 and 75 percent of oxygen consumption when offered alone and suppressed glucose oxidation; the basis of the hypothesis that bacterial fatty acids are a major energy source for the colonic mucosa, especially the distal colon.","url":"https://pubmed.ncbi.nlm.nih.gov/7429343/"},{"id":"stephen1980","authors":"Stephen AM, Cummings JH","title":"Mechanism of action of dietary fibre in the human colon","journal":"Nature","year":1980,"pmid":"7360261","doi":"10.1038/284283a0","type":"laboratory","finding":"Fiber increases stool output, dilutes colonic contents, speeds passage and alters colonic handling of minerals, nitrogen and bile acids; the main component of human feces is bacteria; cabbage fiber is extensively degraded and stimulates microbial growth while wheat fiber remains largely undigested and retains water in the lumen.","url":"https://pubmed.ncbi.nlm.nih.gov/7360261/"},{"id":"cummings1980","authors":"Cummings JH, Stephen AM","title":"The role of dietary fibre in the human colon","journal":"Canadian Medical Association Journal","year":1980,"pmid":"6257366","type":"review","finding":"The magnitude of fiber's effects on colonic function depends on the source and the physical and chemical composition of the fiber and on the individual; three modes of action are discussed: water-holding capacity, production of short-chain fatty acids in the colon, and alteration of the colonic microflora.","url":"https://pubmed.ncbi.nlm.nih.gov/6257366/"},{"id":"chambers2015","authors":"Chambers ES, Viardot A, Psichas A, et al.","title":"Effects of targeted delivery of propionate to the human colon on appetite regulation, body weight maintenance and adiposity in overweight adults","journal":"Gut","year":2015,"pmid":"25500202","doi":"10.1136/gutjnl-2014-307913","type":"rct","finding":"Propionate stimulated PYY and GLP-1 release from human colonic cells; 10 g of an inulin-propionate ester acutely raised postprandial PYY and GLP-1 and reduced energy intake; over 24 weeks in 60 overweight adults, 10 g a day reduced weight gain, intra-abdominal fat and liver fat and prevented the fall in insulin sensitivity seen in the inulin-control group.","url":"https://pubmed.ncbi.nlm.nih.gov/25500202/"},{"id":"wanders2011","authors":"Wanders AJ, van den Borne JJ, de Graaf C, et al.","title":"Effects of dietary fibre on subjective appetite, energy intake and body weight: a systematic review of randomized controlled trials","journal":"Obesity Reviews","year":2011,"pmid":"21676152","doi":"10.1111/j.1467-789X.2011.00895.x","type":"review","finding":"Across 58 appetite, 26 acute intake, 38 long-term intake and 66 body-weight comparisons, more viscous fibers (pectins, beta-glucans, guar gum) reduced appetite in 59 percent of comparisons against 14 percent for less viscous ones, and acute energy intake in 69 against 30 percent; effects on energy intake and body weight were relatively small with no clear dose-response.","url":"https://pubmed.ncbi.nlm.nih.gov/21676152/"},{"id":"miketinas2019","authors":"Miketinas DC, Bray GA, Beyl RA, Ryan DH, Sacks FM, Champagne CM","title":"Fiber Intake Predicts Weight Loss and Dietary Adherence in Adults Consuming Calorie-Restricted Diets: The POUNDS Lost (Preventing Overweight Using Novel Dietary Strategies) Study","journal":"Journal of Nutrition","year":2019,"pmid":"31174214","doi":"10.1093/jn/nxz117","type":"rct","finding":"Secondary analysis of 345 participants (mean age 52.5, BMI 32.6) with dietary records at baseline and six months on calorie-restricted diets; mean weight change 7.27 kg; fiber intake was the most influential predictor of weight loss in the best-fit model (R squared 0.41) independent of macronutrient and energy intake, and was strongly associated with adherence.","url":"https://pubmed.ncbi.nlm.nih.gov/31174214/"},{"id":"jiao2015","authors":"Jiao J, Xu JY, Zhang W, Han S, Qin LQ","title":"Effect of dietary fiber on circulating C-reactive protein in overweight and obese adults: a meta-analysis of randomized controlled trials","journal":"International Journal of Food Sciences and Nutrition","year":2015,"pmid":"25578759","doi":"10.3109/09637486.2014.959898","type":"meta-analysis","finding":"Across 14 randomized trials in overweight and obese adults, dietary fiber or fiber-rich food lowered circulating C-reactive protein by 0.37 mg/L (0 to 0.74), a reduction seen only when the intervention group ate at least 8 g a day more fiber than control; no heterogeneity or publication bias.","url":"https://pubmed.ncbi.nlm.nih.gov/25578759/"},{"id":"king2007","authors":"King DE, Egan BM, Woolson RF, Mainous AG 3rd, Al-Solaiman Y, Jesri A","title":"Effect of a high-fiber diet vs a fiber-supplemented diet on C-reactive protein level","journal":"Archives of Internal Medicine","year":2007,"pmid":"17353499","doi":"10.1001/archinte.167.5.502","type":"rct","finding":"In a randomized crossover of 35 adults (18 lean normotensive, 17 obese hypertensive) moving from 11.9 g a day to about 30 g from a DASH diet (27.7 g) or a supplement (26.3 g), C-reactive protein fell from 4.4 to 3.8 mg/L on the diet and 3.6 mg/L on the supplement, significant only in the lean normotensive participants (2.0 to 1.4 and 1.2 mg/L).","url":"https://pubmed.ncbi.nlm.nih.gov/17353499/"},{"id":"wastyk2021","authors":"Wastyk HC, Fragiadakis GK, Perelman D, et al.","title":"Gut-microbiota-targeted diets modulate human immune status","journal":"Cell","year":2021,"pmid":"34256014","doi":"10.1016/j.cell.2021.06.019","type":"rct","finding":"In a 17-week randomized study of healthy adults (18 per arm), a high-fiber diet increased microbiome-encoded glycan-degrading enzymes with stable diversity and an unchanged primary cytokine score, with three immunological trajectories corresponding to baseline microbiota diversity; a high-fermented-food diet steadily raised diversity and lowered inflammatory markers; authors declare company interests.","url":"https://pubmed.ncbi.nlm.nih.gov/34256014/"},{"id":"zhao2018","authors":"Zhao L, Zhang F, Ding X, et al.","title":"Gut bacteria selectively promoted by dietary fibers alleviate type 2 diabetes","journal":"Science","year":2018,"pmid":"29590046","doi":"10.1126/science.aao5774","type":"rct","finding":"In a randomized study of isoenergetic diets in type 2 diabetes with fecal shotgun metagenomics, dietary fibers promoted a select group of short-chain fatty acid producers while most other potential producers diminished or were unchanged; greater diversity and abundance of those producers went with better HbA1c improvement, partly via increased GLP-1, and diminished producers of indole and hydrogen sulfide.","url":"https://pubmed.ncbi.nlm.nih.gov/29590046/"},{"id":"crowe2011","authors":"Crowe FL, Appleby PN, Allen NE, Key TJ","title":"Diet and risk of diverticular disease in Oxford cohort of European Prospective Investigation into Cancer and Nutrition (EPIC): prospective study of British vegetarians and non-vegetarians","journal":"BMJ","year":2011,"pmid":"21771850","doi":"10.1136/bmj.d4131","type":"cohort","finding":"In 47,033 adults followed 11.6 years with 812 cases of diverticular disease, the highest fifth of fiber intake (25.5 g a day or more for women, 26.1 g for men) carried a 41 percent lower risk (0.59, 0.46 to 0.78) than the lowest fifth (under 14 g), and vegetarians a 31 percent lower risk (0.69) than meat eaters, each independent after mutual adjustment.","url":"https://pubmed.ncbi.nlm.nih.gov/21771850/"},{"id":"yang2012","authors":"Yang J, Wang HP, Zhou L, Xu CF","title":"Effect of dietary fiber on constipation: a meta analysis","journal":"World Journal of Gastroenterology","year":2012,"pmid":"23326148","doi":"10.3748/wjg.v18.i48.7378","type":"meta-analysis","finding":"Across five randomized trials in constipation, dietary fiber increased stool frequency against placebo (odds ratio 1.19) with no significant difference in stool consistency, treatment success, laxative use or painful defecation.","url":"https://pubmed.ncbi.nlm.nih.gov/23326148/"},{"id":"moayyedi2014","authors":"Moayyedi P, Quigley EM, Lacy BE, et al.","title":"The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis","journal":"American Journal of Gastroenterology","year":2014,"pmid":"25070054","doi":"10.1038/ajg.2014.195","type":"meta-analysis","finding":"Across 14 randomized trials with 906 patients, fiber reduced the risk of remaining symptomatic in irritable bowel syndrome (relative risk 0.86, number needed to treat 10); the benefit was confined to soluble fiber (0.83, number needed to treat 7) with no effect of bran (0.90) and no evidence of harm from it.","url":"https://pubmed.ncbi.nlm.nih.gov/25070054/"},{"id":"bonnema2010","authors":"Bonnema AL, Kolberg LW, Thomas W, Slavin JL","title":"Gastrointestinal tolerance of chicory inulin products","journal":"Journal of the American Dietetic Association","year":2010,"pmid":"20497775","doi":"10.1016/j.jada.2010.03.025","type":"rct","finding":"In a randomized double-blind crossover of 26 healthy adults, 5 and 10 g doses of oligofructose and native inulin mildly increased gastrointestinal symptoms, flatulence then bloating most often; the 10 g oligofructose dose substantially increased symptoms; up to 10 g a day of native inulin and 5 g of oligofructose were well tolerated.","url":"https://pubmed.ncbi.nlm.nih.gov/20497775/"},{"id":"hurrell2010","authors":"Hurrell R, Egli I","title":"Iron bioavailability and dietary reference values","journal":"American Journal of Clinical Nutrition","year":2010,"pmid":"20200263","doi":"10.3945/ajcn.2010.28674F","type":"review","finding":"Iron bioavailability is estimated at 14 to 18 percent for mixed diets and 5 to 12 percent for vegetarian diets in people with no iron stores; phytate, polyphenols, calcium, ascorbic acid and muscle tissue repeatedly alter absorption in single-meal studies, with more modest effects in varied multimeal diets; the influence of nondigestible carbohydrates is unresolved; iron status generally outweighs diet composition.","url":"https://pubmed.ncbi.nlm.nih.gov/20200263/"},{"id":"camilleri2022","authors":"Camilleri M, Kuo B, Nguyen L, et al.","title":"ACG Clinical Guideline: Gastroparesis","journal":"American Journal of Gastroenterology","year":2022,"pmid":"35926490","doi":"10.14309/ajg.0000000000001874","type":"guideline","finding":"The American College of Gastroenterology's practice recommendations for gastroparesis; dietary management should include a small particle diet to increase the likelihood of symptom relief and enhance gastric emptying (read from the guideline's full text).","url":"https://pubmed.ncbi.nlm.nih.gov/35926490/"},{"id":"eaton1985","authors":"Eaton SB, Konner M","title":"Paleolithic nutrition. A consideration of its nature and current implications","journal":"New England Journal of Medicine","year":1985,"pmid":"2981409","doi":"10.1056/NEJM198501313120505","type":"review","finding":"The founding statement of the evolutionary discordance hypothesis, that departures from the diet and activity of hunter-gatherer ancestors contribute to modern chronic disease; the article has no abstract on PubMed and sits behind a wall, so its content is characterized through Konner and Eaton 2010, cited alongside it.","url":"https://pubmed.ncbi.nlm.nih.gov/2981409/"},{"id":"konner2010","authors":"Konner M, Eaton SB","title":"Paleolithic nutrition: twenty-five years later","journal":"Nutrition in Clinical Practice","year":2010,"pmid":"21139123","doi":"10.1177/0884533610385702","type":"review","finding":"Twenty-five years after the evolutionary discordance hypothesis was first published, anthropological evidence continues to indicate ancestral diets with much lower refined carbohydrate and sodium, much higher fiber and protein, and comparable fat and cholesterol, with much higher physical activity; official recommendations have moved closer to hunter-gatherer values; the authors call the model heuristic rather than proven.","url":"https://pubmed.ncbi.nlm.nih.gov/21139123/"},{"id":"schnorr2014","authors":"Schnorr SL, Candela M, Rampelli S, et al.","title":"Gut microbiome of the Hadza hunter-gatherers","journal":"Nature Communications","year":2014,"pmid":"24736369","doi":"10.1038/ncomms4654","type":"cross-sectional","finding":"The Hadza of Tanzania have higher gut microbial richness and biodiversity than Italian urban controls, lack Bifidobacterium, differ between the sexes in a way that probably reflects the division of foraging labor, and are enriched in Prevotella, Treponema and unclassified Bacteroidetes that may help them extract nutrition from fibrous plant foods; full text notes that fibrous tuber quids are chewed and expectorated.","url":"https://pubmed.ncbi.nlm.nih.gov/24736369/"},{"id":"smits2017","authors":"Smits SA, Leach J, Sonnenburg ED, et al.","title":"Seasonal cycling in the gut microbiome of the Hadza hunter-gatherers of Tanzania","journal":"Science","year":2017,"pmid":"28839072","doi":"10.1126/science.aan4834","type":"cross-sectional","finding":"In 350 Hadza stool samples collected over more than a year, the microbiome reconfigured in an annual cycle with some taxa becoming undetectable and reappearing; compared with 18 populations in 16 countries, the taxa most seasonally volatile in the Hadza are the ones that differentiate industrialized from traditional populations and have decreased in prevalence and abundance with modernization.","url":"https://pubmed.ncbi.nlm.nih.gov/28839072/"},{"id":"sonnenburg2016","authors":"Sonnenburg ED, Smits SA, Tikhonov M, Higginbottom SK, Wingreen NS, Sonnenburg JL","title":"Diet-induced extinctions in the gut microbiota compound over generations","journal":"Nature","year":2016,"pmid":"26762459","doi":"10.1038/nature16504","type":"laboratory","finding":"In mice carrying a human microbiota, a diet low in microbiota-accessible carbohydrates caused changes largely reversible within one generation but a progressive, unrecoverable loss of diversity over several generations, because low-abundance taxa were inefficiently transferred to offspring and went extinct; restoration required both the missing taxa and dietary fiber.","url":"https://pubmed.ncbi.nlm.nih.gov/26762459/"},{"id":"desai2016","authors":"Desai MS, Seekatz AM, Koropatkin NM, et al.","title":"A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility","journal":"Cell","year":2016,"pmid":"27863247","doi":"10.1016/j.cell.2016.10.043","type":"laboratory","finding":"In gnotobiotic mice colonized with a synthetic human gut microbiota, chronic or intermittent fiber deprivation drove the bacteria to consume host mucus glycoproteins, eroding the colonic mucus barrier and promoting epithelial access and lethal colitis by the mucosal pathogen Citrobacter rodentium.","url":"https://pubmed.ncbi.nlm.nih.gov/27863247/"}],"written":"2026-10-23","updated":"2026-10-23","reviewed":true,"reviewedOn":"2026-10-24","related":{"essays":[],"glossary":["Microbiome","Insulin resistance","ApoB"],"symptoms":["digestive-issues","high-cholesterol","weight-loss-resistance"],"entries":["apob","ldl-cholesterol","non-hdl-cholesterol","fasting-insulin","fasting-glucose","hba1c","homa-ir","triglycerides","hs-crp","ferritin"]},"url":"https://taggehealth.com/labs/fiber","author":"Daniel Tagge, MD","publisher":"Tagge Health","license":"Free to read and cite with attribution to the URL above.","inPractice":"I ask about fiber before I change a lipid plan, because it is one of the few things that moves LDL and ApoB in weeks without a prescription, and I would rather know what a person eats before I decide what a number means. I read it through the markers, not on its own: one marker is a clue, three are a story. An ApoB that is high beside a fasting insulin that is also high and a bowel that moves every other day is a different story from an ApoB that is high in someone who already eats lentils and oats and has a low fasting insulin; the first person has a diet lever to pull first, the second may be looking at genetics and a medication conversation sooner. I use the same reading for glucose: a fasting glucose at the top of its range beside a high insulin and high triglycerides is an insulin story, and fiber is part of the answer alongside sleep, muscle and what the carbohydrate is attached to. For hs-CRP, I expect fiber to help a little, and I do not expect it to carry the whole load. Food comes before anything isolated: the cohorts that found lower mortality and less heart disease, diabetes and colorectal cancer were built on people eating plants, and I take the UK committee at its word that the isolated fibers have not been shown to do all of that. I do use psyllium, because it has the cleanest ApoB evidence of any fiber and it works within a month, and I treat it as an addition to a plate that already has beans, oats, vegetables and fruit on it, not as a substitute. I raise fiber slowly, over weeks, with water, I ask about bowel habit as a marker in its own right, and I check ferritin again in anyone who has moved to a mostly plant diet. The guideline figures are the only numbers I quote: the Adequate Intake of 25 g for women and 38 g for men, the UK's 30 g, and the 25 to 29 g band where the WHO series found the most benefit. I do not set a target of my own, because the evidence does not hand me one and the right amount for a person depends on their gut, their iron and what they can keep doing. I will say plainly where I am ahead of the evidence: the trials show fiber moves LDL, ApoB, glucose, insulin and C-reactive protein within weeks, and the cohorts show that people who eat more of it live longer with less disease, but no trial has yet proved that raising fiber in an adult prevents a heart attack or a cancer. I act on the mechanism and the association together, and I say so."}