{"slug":"tca-cycle-intermediates","name":"TCA-cycle intermediates","aliases":["Krebs cycle intermediates","Citric acid cycle intermediates","Citrate, cis-aconitate, isocitrate, alpha-ketoglutarate, succinate, fumarate, malate","Urine organic acids, citric acid cycle markers"],"kind":"biomarker","system":"mitochondria","unit":"mmol per mol creatinine","unitNote":"Urine panels divide each intermediate by creatinine to correct a spot sample for how dilute it is; plasma results are reported in µmol/L. A spot sample reports the last few hours and a 24-hour collection the whole day, and the one clinical cutoff in this family, urine citrate in kidney stone disease, is defined on a 24-hour collection in mg per day. A recent meal, fruit or juice, fasting, exercise and acid-base state all move these values within hours. No reference interval for any intermediate on a wellness panel is outcome-derived, and even the urine citrate cutoff is a convention that the stone trials used rather than a threshold drawn from outcomes.","answer":"TCA-cycle intermediates are the small acids of the mitochondrial Krebs cycle, and on a urine panel they are a snapshot of a few hours of fuel traffic, not a readout of mitochondrial health. Three uses are validated: a 24-hour urine citrate under 320 mg a day defines hypocitraturia in kidney stone disease, where potassium citrate cut new stones in randomized trials; 2-hydroxyglutarate, a close relative of the cycle, marks IDH-mutant cancers; and blood lactate and pyruvate, with urine organic acids, belong to the workup of suspected mitochondrial disease. Reading a single high or low urinary intermediate as a block in the cycle or a cofactor deficiency has no outcome evidence behind it.","summary":"The intermediates of the Krebs cycle are better understood as a hub than a circle: they flow in from amino acids and out to sugar and fat, and some of them, succinate, fumarate and alpha-ketoglutarate above all, act as signals in inflammation, cancer and the epigenetic machinery. Clinically the measurement earns its keep in three narrow places: urine citrate in stone disease, 2-hydroxyglutarate in IDH-mutant tumors, and lactate, pyruvate and organic acids in suspected mitochondrial disease. Outside those, a single urinary value mostly reports yesterday's food, exercise and collection.","whatItIs":[{"text":"The cycle is a ring of reactions in the mitochondrial matrix. It oxidizes acetyl-CoA and hands the electrons to NADH and FADH2, which feed the electron transport chain; complexes I and II regenerate NAD+ and FAD so the cycle can keep turning, and each turn yields 3 NADH and 1 FADH2 beside the ATP. The current view of mitochondria is wider than the powerhouse: they also generate building blocks for macromolecules, keep compartments of metabolites for redox balance, and act as hubs for metabolic waste management. The seven molecules a panel reports, citrate, cis-aconitate, isocitrate, alpha-ketoglutarate, succinate, fumarate and malate, are the stations on that ring.","cites":["martinezreyes2020","spinelli2018"],"grade":"C"},{"text":"What gets measured is what spills out of cells. The intermediates are present in blood at micromolar concentrations, set by respiration, metabolism and the kidney's reabsorption and secretion. The urine organic acid test that reports them was built to diagnose inborn errors of metabolism: the American College of Medical Genetics technical standard describes it as a first-tier test for a broad spectrum of those disorders, performed by gas chromatography-mass spectrometry. Interpretation has always been hampered by ambiguity about where a given urinary acid came from, which is why a Belgian group compiled a comprehensive table of its metabolic, nutritional, drug-related and artifactual sources.","cites":["he2004","gallagher2018","kumps2002"],"grade":"B"},{"text":"The research field is alive because these molecules turned out to be signals. Reviews from the Chandel, O'Neill and Frezza groups describe intermediates once thought of as only biosynthetic as controllers of chromatin modification, DNA methylation, the hypoxic response and immunity, and state that succinate, fumarate, itaconate, the 2-hydroxyglutarate isomers and acetyl-CoA now have extensive evidence for signaling functions in immunity and in the initiation of cancer. Succinate stabilizes the transcription factor HIF-1 alpha in some tumors and in activated macrophages and stimulates dendritic cells through its receptor; a previously orphan receptor, GPR91, was shown in 2004 to respond to succinate and a close relative, GPR99, to alpha-ketoglutarate, and succinate raised blood pressure in animals through the renin-angiotensin system. This is cell and animal biology; it explains why the molecules matter, not what a urine value means.","cites":["martinezreyes2020","ryan2019","mills2014","he2004"],"grade":"C"}],"origins":[{"text":"Hans Krebs and William Johnson described the role of citric acid in intermediate metabolism in animal tissues, the paper that founded the cycle, and FEBS Letters reprinted it in 1980. The machinery is older than animals. The favored model holds that mitochondria descend from a bacterial endosymbiont related to the alpha-Proteobacteria, and the gene sequence data place the organelle in the common ancestor of all living eukaryotes, possibly arising together with the nucleus rather than after it. Whether the cycle's chemistry is older still is a live hypothesis: in a purely chemical network promoted by ferrous iron, pyruvate and glyoxylate built 9 of the 11 intermediates of the Krebs cycle, including all 5 of the universal metabolic precursors, and broke them back down to carbon dioxide, which its authors read as a possible prebiotic precursor of the pathway.","cites":["krebs1980","gray1999","muchowska2019"],"grade":"C"},{"text":"The drawing of a closed circle misleads, because the cycle is a hub with traffic in both directions. Carbon enters by anaplerosis, the refilling of the pool, chiefly from pyruvate carboxylated to oxaloacetate and from glutamine converted to glutamate and then alpha-ketoglutarate; in activated macrophages glutamine-dependent anaplerosis is the main source of succinate. When the electron transport chain is impaired, the cycle can partly reverse, making citrate from glutamine-derived alpha-ketoglutarate through NADPH-dependent isocitrate dehydrogenase and aconitase; tumor cells with defective mitochondria use this reductive carboxylation as their major route to citrate. Carbon leaves by cataplerosis: mitochondrial citrate is exported by a dedicated antiporter and supplies acetyl-CoA for fat synthesis, and oxaloacetate made from pyruvate leaves as malate for the remaining steps of gluconeogenesis. A urinary malate or citrate is therefore downstream of amino acids, sugar and fat synthesis as much as of oxidation.","cites":["martinezreyes2020","spinelli2018","mullen2011","tannahill2013"],"grade":"C"},{"text":"The cycle's enzymes lean on cofactors, and this is where the wellness panels make their leap. Thiamine diphosphate is the cofactor of pyruvate dehydrogenase, which feeds acetyl-CoA into the cycle, and of alpha-ketoglutarate dehydrogenase, the step reported to be most sensitive to thiamine deficiency; pyruvate and alpha-ketoglutarate rise in the blood of thiamine-deficient people, and thiamine deficiency has been described as an acquired mitochondrial disease. The cycle runs on NAD+ and FAD, regenerated by complexes I and II; the alpha-ketoglutarate-dependent dioxygenases need ferrous iron; aconitase and the chain's iron-sulfur clusters are iron-dependent and sensitive to oxidative damage; magnesium stabilizes many ATP-generating reactions; and lipoic acid, riboflavin and coenzyme Q10 are among the agents physicians give in diagnosed mitochondrial disease. Each of those facts is real. The step from a raised urinary alpha-ketoglutarate or succinate to a named vitamin deficiency is an inference, not a measurement: the accepted biomarkers of thiamine status are blood thiamine diphosphate and the red-cell transketolase assay, and the global review notes the lack of a readily accessible, agreed biomarker even for thiamine.","cites":["whitfield2018","martinezreyes2020","spinelli2018","jahnen2012","parikh2015"],"grade":"C"}],"labRange":{"typical":"No common reference interval: each laboratory prints its own population interval for each intermediate, in mmol per mol creatinine for urine. The one clinical cutoff is for 24-hour urine citrate, where hypocitraturia is generally defined as under 320 mg (1.67 mmol) a day in adults","claims":[{"text":"Urine citrate is the one intermediate with a clinical definition. Hypocitraturia is generally defined as a urinary citrate excretion under 320 mg (1.67 mmol) a day in adults, and it is found in 20 to 60 percent of stone formers; in complete distal renal tubular acidosis citrate falls under 100 mg a day. The definition is a convention, not a threshold drawn from outcomes. When 24-hour urines were collected from 807 people with a history of stones and 239 without in the Nurses' Health Studies and the Health Professionals Follow-up Study, urine citrate did not differ between cases and controls, a substantial share of controls met the accepted definitions of abnormal for other factors, and the authors concluded that the traditional definitions of normal need to be reassessed and that risk may be continuous rather than dichotomous.","cites":["zuckerman2009","curhan2001"],"grade":"B"},{"text":"For the other six intermediates, the printed interval is a laboratory's reference population, and the test was built to catch inborn errors of metabolism, which produce values far outside any healthy range. Urine is also the most diet-sensitive fluid in the body: in 30 healthy volunteers sampled on four mornings, within-person and between-person variation in the urinary metabolic profile were both considerable, standardizing the previous day's food reduced the between-person variation in urine but not in plasma or saliva, and the authors concluded that urine reflects acute dietary intake. A value inside or outside a wellness panel's interval describes where the sample sits in a distribution, not a risk.","cites":["gallagher2018","walsh2006"],"grade":"C"}]},"evidence":{"headline":"The firm evidence is narrow. In stone disease a 24-hour urine citrate under 320 mg a day defines hypocitraturia, and potassium citrate cut the stone rate from 1.1 to 0.1 per patient-year in one randomized trial and new stones from 63.6 to 12.9 percent in another. In IDH-mutant acute myeloid leukemia, serum 2-hydroxyglutarate ran at a median of 3,004 ng/mL against 61 in wild-type disease. In suspected mitochondrial disease the consensus workup measures lactate, pyruvate and urine organic acids, with a properly collected lactate above 3 mmol/L pointing to mitochondrial dysfunction. Plasma intermediates tracked atrial fibrillation, heart failure and mortality in cohorts as associations. No study tests the wellness reading of a single urinary intermediate as a block in the cycle or a cofactor deficiency.","scale":{"min":0,"max":1,"ticks":[]},"bands":[{"label":"No outcome-derived interval for any intermediate on a urine organic acid panel; each laboratory prints its own population interval. The one clinical cutoff, 24-hour urine citrate under 320 mg a day in stone disease, is reported in another unit and stated in the text.","low":0,"tone":"lab","source":"Gallagher 2018; Zuckerman 2009; Curhan 2001"}],"claims":[{"text":"Low urine citrate in a person who forms calcium stones is the one place a measured intermediate changes treatment, and the evidence is guideline and trial grade. Citrate inhibits stone formation by binding calcium in the urine and blocking crystal nucleation, growth and clumping. The American Urological Association guideline, built on a federal systematic review and 46 studies, sets out a screening evaluation for stone formers, metabolic testing in selected patients, and medication or diet chosen from that testing. In a double-blind trial of 57 patients with active stone disease and hypocitraturia, the 18 who took potassium citrate for three years went from 1.2 to 0.1 stones per patient-year, with remission in 72 percent, while the 20 on placebo stayed at 1.1, and only the citrate group's urinary citrate and pH rose. In a second double-blind trial of 64 people with recurrent calcium oxalate stones, not selected for low citrate, new stones formed in 63.6 percent on placebo and 12.9 percent on potassium-magnesium citrate over up to three years (relative risk 0.16, 0.05 to 0.46).","cites":["zuckerman2009","pearle2014","barcelo1993","ettinger1997"],"grade":"A"},{"text":"2-hydroxyglutarate is the oncology example, and it is a near relative of the cycle rather than one of its stations. Mutations in isocitrate dehydrogenase 1, common in a major subset of brain cancers, give the enzyme a new ability to reduce alpha-ketoglutarate to 2-hydroxyglutarate, which was markedly raised in IDH1-mutant gliomas; 2-hydroxyglutarate competitively inhibits the alpha-ketoglutarate-dependent dioxygenases, including histone demethylases and the TET enzymes that edit DNA methylation. In sera from 223 people with newly diagnosed acute myeloid leukemia, 2-hydroxyglutarate ran at a median of 3,004 ng/mL with IDH mutations against 61 without, a level of 700 ng/mL best separated the two groups, 9 of 13 apparently wild-type samples above it turned out to carry IDH mutations on further testing, and an IDH-mutant patient with a level above 200 ng/mL in remission had shorter overall survival (hazard ratio 3.9). This is a cancer test ordered by oncologists; it is not on a wellness panel and should not be.","cites":["dang2009","xu2011","dinardo2013"],"grade":"B"},{"text":"Succinate and fumarate are the original oncometabolites. Succinate dehydrogenase and fumarate hydratase, two enzymes of the cycle, act as tumor suppressors: inherited mutations in fumarate hydratase predispose to leiomyomas and kidney cancer, and mutations in succinate dehydrogenase to paragangliomas and pheochromocytomas. The tumors accumulate fumarate or succinate, which inhibit the prolyl hydroxylases that normally destroy HIF-1 alpha, so the cell behaves as if it were starved of oxygen; the same two molecules competitively inhibit alpha-ketoglutarate-dependent histone and DNA demethylases, altering methylation across the genome. These are rare genetic syndromes diagnosed by genetic testing and pathology; the mechanism is solid in cells and tumors, and it does not make a mildly raised urinary succinate in a healthy person a cancer signal.","cites":["selak2005","pollard2005","xiao2012"],"grade":"C"},{"text":"In suspected mitochondrial disease the measurement belongs to a consensus workup, and the consensus is cautious about the intermediates themselves. The Mitochondrial Medicine Society recommends an initial blood evaluation that includes lactate and pyruvate, amino acids and acylcarnitines, along with urinary organic acids. A markedly raised plasma lactate, above 3 mmol/L in a properly collected sample, suggests mitochondrial dysfunction, primary or secondary; truly raised lactate has a sensitivity of 34 to 62 percent and a specificity of 83 to 100 percent for primary mitochondrial disease; and the lactate-to-pyruvate ratio is of value only when lactate is raised. Urinary organic acids often change in these patients, but in a retrospective analysis of 67 people with mitochondrial disease against 21 with organic acidemias, only malate and fumarate correlated well, and the other cycle intermediates and lactate correlated poorly. In 46 children with lactic acidosis, the lactate-to-pyruvate ratio at the recommended cut point of about 25 separated pyruvate dehydrogenase deficiency from other mitochondrial disease with 77 percent sensitivity and 91 percent specificity, and when lactate was under 2.5 mmol/L it did no better than chance.","cites":["parikh2015","debray2007"],"grade":"A"},{"text":"The signaling biology that makes the field exciting is mostly in cells and mice. Bacterial lipopolysaccharide strongly raised succinate in mouse macrophages, chiefly by glutamine-dependent anaplerosis, and the succinate stabilized HIF-1 alpha and drove the inflammatory cytokine interleukin-1 beta. Across a range of tissues, selective accumulation of succinate was a universal metabolic signature of ischemia; it built up because succinate dehydrogenase ran in reverse, driven by fumarate overflow, and when blood flow returned its rapid reoxidation drove reactive oxygen species through reverse electron transport at complex I, while blocking the accumulation reduced injury in mouse models of heart attack and stroke. These findings explain why a tissue's succinate matters; none of them was a study of urine or of healthy people.","cites":["tannahill2013","chouchani2014"],"grade":"C"},{"text":"In people, plasma intermediates keep company with cardiometabolic disease as associations. In the PREDIMED trial population, two nested case-control studies over about 10 years compared 512 people who developed atrial fibrillation and 334 who developed heart failure with matched controls: the highest against the lowest quarter of plasma succinate, malate, citrate and 2-hydroxyglutarate carried odds ratios for atrial fibrillation of 1.80, 2.13, 1.87 and 1.95, and malate an odds ratio of 2.15 for heart failure. In 9,842 people of the Estonian Biobank, each standard deviation of non-fasting plasma citrate carried a 1.33-fold hazard of death (1.21 to 1.45), replicated in 7,503 Finns; in a later pooled analysis of 44,168 people, citrate dropped out of the final 14-marker mortality model because it added little once the others were counted. Circulating succinate was higher in obesity, tracked specific succinate-producing and succinate-consuming gut bacteria, and fell with weight loss. Each is a single design in one population, and none tested whether changing the level changes the outcome.","cites":["bullo2021","fischer2014","deelen2019","serena2018"],"grade":"B"},{"text":"Alpha-ketoglutarate is sold as a longevity supplement on the strength of one mouse study. Delivered as a calcium salt, it extended lifespan and compressed late-life morbidity in one strain of mice, with lower systemic inflammatory cytokines, and its authors declared commercial interests in the compound. The first human randomized trial, giving 1 g of sustained-release calcium alpha-ketoglutarate or placebo for six months to 120 healthy adults aged 40 to 60 whose methylation clocks ran older than their age, has so far published only how it recruited. There is no human outcome evidence yet, and a urinary alpha-ketoglutarate is not a test of whether anyone needs it.","cites":["asadi2020","lim2025"],"grade":"C"},{"text":"The best proof that an intermediate's chemistry can be therapeutic comes from a drug, not a supplement. Dimethyl fumarate, a fumaric acid ester, activated the antioxidant transcription factor Nrf2 and protected myelin, axons and neurons in a mouse model of multiple sclerosis, with its metabolite directly modifying the Nrf2 inhibitor KEAP1, and the benefit vanished in mice lacking Nrf2. In a phase 3 trial in relapsing-remitting multiple sclerosis, 27 and 26 percent of people on two doses relapsed within two years against 46 percent on placebo, with flushing, gut symptoms and lower lymphocyte counts as side effects. That shows fumarate esters signal; it does not show that dietary fumarate, or any intermediate taken by mouth, does the same.","cites":["linker2011","gold2012"],"grade":"B"},{"text":"The wellness reading of a urine panel has no outcome evidence. No study found in this search has tested whether treating a single high or low urinary intermediate, read as a block at one step of the cycle or as a deficiency of the cofactor that step uses, changes symptoms, fitness or any outcome. What the literature does show argues for caution: urinary organic acids have nutritional, drug-related and artifactual sources that have to be ruled out, urine tracks the last day's food, in mitochondrial disease itself most cycle intermediates correlated poorly with the diagnosis, and the cofactors in question are assessed by their own blood tests, not by a urinary acid. This is a practice judgment drawn from the absence of evidence, stated as one.","cites":["kumps2002","walsh2006","parikh2015","whitfield2018"],"grade":"D"}]},"levers":[{"name":"Exercise","effect":"raises plasma succinate, malate and fumarate within a session; lowers urine citrate a little","claim":{"text":"Exercise expands the cycle's pool and spills it into the blood. In a mass-spectrometry study of more than 200 plasma metabolites before and after exercise, fumarate, malate and succinate rose 2- to 4-fold, citrate and isocitrate more modestly and alpha-ketoglutarate not at all, and the fittest people, by peak oxygen uptake and by marathon time, expanded the pool most. Working muscle in mice and humans releases succinate into the surrounding tissue and the circulation as the cell acidifies, where it signals through its receptor to drive muscle remodeling. In urine, exercise causes a mild fall in citrate excretion. A sample taken within hours of a hard session reports the session.","cites":["lewis2010","reddy2020","zuckerman2009"],"grade":"C"}},{"name":"Fasting, ketosis and the last meal","effect":"fasting and ketogenic diets lower urine citrate; any recent meal shifts the urine profile","claim":{"text":"Urine reflects the last day's food more than plasma does, and standardizing the previous day's diet reduced between-person variation in urine in the dietary-standardization study. Starvation increases the kidney's reabsorption of citrate and promotes acidosis, and severely carbohydrate-restricted, animal protein-rich diets deepen the acid load through ketones, both lowering urinary citrate. The mouse study of alpha-ketoglutarate begins from the observation that its levels change with fasting, exercise and age. A panel collected after an overnight fast, a long fast or a week of ketosis is not comparable with one collected after an ordinary day.","cites":["walsh2006","zuckerman2009","asadi2020"],"grade":"C"}},{"name":"Fruit, citrus juice and alkali","effect":"raise urine citrate; potassium citrate more than juice","claim":{"text":"Low fruit and vegetable intake is a listed cause of hypocitraturia, and the stone literature's dietary advice is high fluid and citrus fruit intake. Lemonade made from concentrated lemon juice raised urinary citrate in hypocitraturic patients, but potassium citrate raised urinary citrate and pH more than lemonade in a direct comparison, and orange juice raised citrate without lowering calcium oxalate saturation, as potassium citrate did. In the potassium citrate trial, only the treated group's urinary citrate, pH and potassium rose. A high urinary citrate after a day of fruit is the fruit.","cites":["zuckerman2009","barcelo1993"],"grade":"B"}},{"name":"Acid load: animal protein and salt","effect":"lowers urine citrate","claim":{"text":"The kidney reabsorbs and burns more citrate when the body is acid. In 15 young adults fed three 12-day diets matched for protein and minerals but rising in sulfur, the animal-protein diet raised urinary calcium from 103 to 150 mg a day, lowered urine pH and reduced citrate excretion because of the acid load. The physiology is old: acidosis decreases renal citrate excretion and alkalosis increases it, through the pH gradient across the inner mitochondrial membrane of kidney cells, so that citrate excretion has been called a window on renal metabolism. High-sodium diets also decrease urinary citrate. A low urinary citrate is first a question about the acid load of the diet.","cites":["breslau1988","simpson1983","zuckerman2009"],"grade":"B"}},{"name":"Alcohol","effect":"heavy use can raise pyruvate and alpha-ketoglutarate through thiamine deficiency","claim":{"text":"In higher-income countries most thiamine deficiency outside bariatric surgery and advanced HIV is in people who drink heavily, because chronic alcohol use impairs thiamine absorption and raises its requirement. Thiamine is the cofactor of the enzymes on either side of the cycle's entry and its alpha-ketoglutarate step, and pyruvate and alpha-ketoglutarate rise in blood in deficiency. That is the one well-described route from a drinking pattern to a shifted intermediate; the cited sources do not describe the effect of a single recent drink on a urine panel, so the honest use of the information is to ask about alcohol before reading the panel, and to test thiamine properly when the history points that way.","cites":["whitfield2018","kumps2002"],"grade":"C"}},{"name":"Fiber and gut bacteria","effect":"supply succinate from the colon; circulating succinate tracks the microbiome","claim":{"text":"Host cells are not the only source. In mice fed a fiber-rich diet, succinate was the most abundant carboxylic acid in the cecum, produced by bacterial fermentation, and it served as a substrate for glucose production by the gut. In people, circulating succinate in obesity tracked the balance of succinate-producing and succinate-consuming gut bacteria and fell with weight loss. A urinary or plasma succinate therefore carries a bacterial share that no panel separates from the mitochondrial one.","cites":["devadder2016","serena2018"],"grade":"C"}},{"name":"B vitamins, lipoic acid, coenzyme Q10 and other cofactors","effect":"untested as a treatment for a panel result","claim":{"text":"The cofactor question is where the evidence is thinnest. In diagnosed primary mitochondrial disease, the consensus statement notes a general lack of agreement on which agents to use, reports that most physicians prescribe coenzyme Q10, L-carnitine, creatine, alpha-lipoic acid and certain B vitamins, finds that no randomized trials have tested the B-vitamin combinations commonly recommended, and still advises offering alpha-lipoic acid and riboflavin to those patients. No trial has tested any cofactor against a urinary intermediate in people without mitochondrial disease. Where a deficiency is suspected, it has its own test: thiamine status, for example, is measured by blood thiamine diphosphate or the red-cell transketolase assay.","cites":["parikh2015","whitfield2018"],"grade":"D"}}],"caveats":[{"text":"Collection decides the number. Urine organic acids have physiological, nutritional, drug-related and artifactual sources, and the comprehensive source table was compiled because preanalytical issues lead to misinterpretation. A first-morning spot, a random spot and a 24-hour collection report different windows; creatinine correction assumes ordinary muscle mass and kidney function; and the previous day's food shifts the urinary profile. Two panels a month apart, on different days of eating and training, are not a trend.","cites":["kumps2002","walsh2006"],"grade":"C"},{"text":"Lactate and pyruvate, the blood tests that sit beside these intermediates in a mitochondrial workup, are easy to spoil. A tourniquet or a struggle during the draw can raise venous lactate spuriously, pyruvate is very unstable, post-meal lactate is more sensitive than fasting but small post-meal rises should not be overinterpreted, and the lactate-to-pyruvate ratio means something only when lactate is high.","cites":["parikh2015"],"grade":"A"},{"text":"Blood and urine can point in opposite directions. Higher non-fasting plasma citrate went with higher mortality in two Nordic cohorts, while low urinary citrate is the stone risk; urinary citrate largely reports how much the kidney's own cells reabsorb and burn, set by acid-base state, not how much the body makes. A single word, citrate, covers two different measurements.","cites":["fischer2014","simpson1983"],"grade":"C"},{"text":"The diseases these tests were built for are rare and serious. Urine organic acid analysis exists to find inborn errors of metabolism and to follow up abnormal newborn screens, and the cycle's own enzyme defects, in fumarate hydratase and succinate dehydrogenase, are inherited tumor syndromes. A healthy adult's panel is being read with a tool designed to find large, unmistakable abnormalities, and small departures from a laboratory interval carry little of that tool's validity.","cites":["gallagher2018","pollard2005"],"grade":"B"},{"text":"The research is ahead of the clinic, and the supplement market is ahead of the research. Succinate as an inflammatory signal, fumarate and 2-hydroxyglutarate as oncometabolites, and alpha-ketoglutarate as an epigenetic cofactor are well supported in cells and animals, and alpha-ketoglutarate extended life in mice; none of that has yet been turned into a validated reading of a urinary value in a healthy person, and the one human alpha-ketoglutarate trial has not reported an outcome.","cites":["ryan2019","xiao2012","asadi2020","lim2025"],"grade":"C"}],"seeAPhysician":["A kidney stone, especially a recurrent calcium stone, where a 24-hour urine collection that includes citrate belongs in the evaluation and a low citrate is treated.","A urinary citrate under 320 mg a day on a 24-hour collection, which is the guideline definition of hypocitraturia and worth reading beside urine pH, potassium and kidney function, because distal renal tubular acidosis, chronic diarrhea and some medicines lower it.","Suspected mitochondrial disease: exercise intolerance or muscle weakness out of proportion to training, with problems in more than one organ such as hearing, vision, the heart, the gut or the brain, or a family history of the same, which calls for the consensus workup of lactate, pyruvate, amino acids, acylcarnitines and urine organic acids rather than a wellness panel.","Unexplained lactic acidosis, or a properly collected blood lactate above 3 mmol/L without a clear cause such as hard exercise, infection or a medicine, which the consensus statement treats as a sign of mitochondrial dysfunction until shown otherwise.","Heavy drinking, recent bariatric surgery or a very restricted diet together with confusion, unsteady walking, abnormal eye movements, numbness or breathlessness, which can be thiamine deficiency and needs treatment the same day, not a urine panel.","A personal or family history of uterine fibroids with kidney cancer at a young age, or of paraganglioma or pheochromocytoma, which points to the inherited fumarate hydratase and succinate dehydrogenase syndromes and to genetic evaluation.","A panel result that has been used to recommend a list of supplements, which deserves a second reading by a physician who knows when the sample was collected and what was eaten the day before."],"openQuestions":["No study has tested whether acting on a single high or low urinary intermediate in a person without mitochondrial disease, whether by supplementing a cofactor or by changing training or diet, changes any outcome.","Whether a pattern across several intermediates, repeated under standardized collection, carries information about mitochondrial capacity in healthy adults beyond what fitness testing and ordinary blood work already give is unknown.","The plasma associations with atrial fibrillation, heart failure, diabetes and mortality come from single cohorts; whether the intermediates mark a cause, a consequence of disease or simply fuel traffic is untested, and no Mendelian randomization study was found for these metabolites.","How much of a urinary succinate or citrate comes from gut bacteria and food rather than human mitochondria has not been measured in people, and no panel separates the sources.","Whether the 320 mg a day citrate convention is the right threshold, or whether stone risk rises continuously as citrate falls, is open; the Harvard cohorts did not find a case-control difference in citrate.","Alpha-ketoglutarate extended life in one strain of mice; whether it does anything measurable in humans awaits the first randomized trial's results.","Within-person variation of the urinary intermediates over days and weeks under standardized conditions, the number a physician would need to call a change real, has not been established."],"questions":[{"q":"My urine organic acid test shows a high succinate. Does that mean my mitochondria are blocked?","a":"Not on that evidence. Succinate is a real signal in cells, it rises in inflamed and oxygen-starved tissue, and inherited defects in succinate dehydrogenase cause rare tumors, but none of that has been turned into a validated reading of a mildly raised urinary value in a healthy person. A single urinary succinate also carries a share made by gut bacteria from fiber, rises in the blood after hard exercise, and moves with the previous day's food and the way the sample was collected. In diagnosed mitochondrial disease, most cycle intermediates correlated poorly with the diagnosis; malate and fumarate did better. The useful questions are what was eaten and done the day before, whether the pattern repeats on a standardized collection, and how it sits beside fitness, lactate, magnesium, iron and the rest of the blood work."},{"q":"Is a low urine citrate a problem if I have never had a kidney stone?","a":"The clinical definition, a 24-hour urine citrate under 320 mg a day, was built for people who form stones, and the trials that showed potassium citrate prevents new stones enrolled stone formers. In the Harvard cohorts, urine citrate did not differ between people with and without a stone history. A low value on a spot urine panel in someone who has never had a stone is mostly a question about acid load, the amount of fruit and vegetables eaten, fasting or ketosis, exercise and how the sample was collected. It is worth a 24-hour collection and a conversation if there is a family history of stones, chronic diarrhea, or a reason to suspect renal tubular acidosis."},{"q":"Should I take alpha-ketoglutarate?","a":"The case rests on one mouse study, in which the calcium salt extended lifespan and reduced frailty in one strain, with authors who declared commercial interests. The first human randomized trial, in 120 middle-aged adults with older-running methylation clocks, has so far reported only its recruitment. No human study has shown that it improves an outcome, and a urinary alpha-ketoglutarate does not say who would benefit. Whether to take it is a decision to make with a physician, knowing that it is untested in people."},{"q":"Do the intermediates show whether I need B vitamins?","a":"Not directly. Thiamine is a cofactor of the enzyme that converts alpha-ketoglutarate, and pyruvate and alpha-ketoglutarate rise in the blood in true thiamine deficiency, but the accepted tests of thiamine status are blood thiamine diphosphate and the red-cell transketolase assay, and even for thiamine an easily available, agreed biomarker is lacking. The other links, riboflavin to FAD, niacin to NAD+, lipoic acid and coenzyme Q10 to the cycle and the chain, are real biochemistry, but no trial has tested supplementing them against a urinary intermediate in people without mitochondrial disease. If a deficiency is suspected, the deficiency has its own test."}],"citations":[{"id":"martinezreyes2020","authors":"Martínez-Reyes I, Chandel NS","title":"Mitochondrial TCA cycle metabolites control physiology and disease","journal":"Nature Communications","year":2020,"pmid":"31900386","doi":"10.1038/s41467-019-13668-3","type":"review","finding":"TCA cycle intermediates, long seen as biosynthetic, act as signals controlling chromatin modification, DNA methylation, the hypoxic response and immunity; the cycle yields 3 NADH and 1 FADH2 per turn, anaplerosis refills it from pyruvate and glutamine, and with an impaired electron transport chain it partly reverses by reductive carboxylation; alpha-ketoglutarate-dependent dioxygenases require ferrous iron.","url":"https://pubmed.ncbi.nlm.nih.gov/31900386/"},{"id":"spinelli2018","authors":"Spinelli JB, Haigis MC","title":"The multifaceted contributions of mitochondria to cellular metabolism","journal":"Nature Cell Biology","year":2018,"pmid":"29950572","doi":"10.1038/s41556-018-0124-1","type":"review","finding":"Mitochondria catabolize nutrients, generate biosynthetic precursors, compartmentalize metabolites for redox balance and manage metabolic waste; glutamine and pyruvate carboxylase supply anaplerosis, mitochondrial citrate is exported by SLC25A1, oxaloacetate leaves as malate for gluconeogenesis, and iron-sulfur clusters and aconitase need protection from reactive oxygen species.","url":"https://pubmed.ncbi.nlm.nih.gov/29950572/"},{"id":"ryan2019","authors":"Ryan DG, Murphy MP, Frezza C, Prag HA, Chouchani ET, O'Neill LA, Mills EL","title":"Coupling Krebs cycle metabolites to signalling in immunity and cancer","journal":"Nature Metabolism","year":2019,"pmid":"31032474","doi":"10.1038/s42255-018-0014-7","type":"review","finding":"Succinate, fumarate, itaconate, the D- and L-2-hydroxyglutarate isomers and acetyl-CoA have extensive evidence for non-metabolic signaling functions in immunity and in disease, including the initiation of carcinogenesis.","url":"https://pubmed.ncbi.nlm.nih.gov/31032474/"},{"id":"mills2014","authors":"Mills E, O'Neill LA","title":"Succinate: a metabolic signal in inflammation","journal":"Trends in Cell Biology","year":2014,"pmid":"24361092","doi":"10.1016/j.tcb.2013.11.008","type":"review","finding":"Succinate stabilizes HIF-1 alpha in specific tumors and activated macrophages, stimulates dendritic cells through succinate receptor 1, and post-translationally modifies proteins.","url":"https://pubmed.ncbi.nlm.nih.gov/24361092/"},{"id":"he2004","authors":"He W, Miao FJ, Lin DC, Schwandner RT, Wang Z, Gao J, Chen JL, Tian H, Ling L","title":"Citric acid cycle intermediates as ligands for orphan G-protein-coupled receptors","journal":"Nature","year":2004,"pmid":"15141213","doi":"10.1038/nature02488","type":"laboratory","finding":"Cycle intermediates circulate at micromolar concentrations regulated by respiration, metabolism and renal handling; GPR91 is a receptor for succinate and GPR99 responds to alpha-ketoglutarate; succinate raised blood pressure in animals through the renin-angiotensin system, abolished in GPR91-deficient mice.","url":"https://pubmed.ncbi.nlm.nih.gov/15141213/"},{"id":"gallagher2018","authors":"Gallagher RC, Pollard L, Scott AI, Huguenin S, Goodman S, Sun Q; ACMG Biochemical Genetics Subcommittee","title":"Laboratory analysis of organic acids, 2018 update: a technical standard of the American College of Medical Genetics and Genomics (ACMG)","journal":"Genetics in Medicine","year":2018,"pmid":"29543224","doi":"10.1038/gim.2018.45","type":"guideline","finding":"Urine organic acid analysis, typically by gas chromatography-mass spectrometry, is a crucial first-tier test for a broad spectrum of inborn errors of metabolism and follow-up of positive newborn screens; the standard guides analysis, interpretation and reporting.","url":"https://pubmed.ncbi.nlm.nih.gov/29543224/"},{"id":"kumps2002","authors":"Kumps A, Duez P, Mardens Y","title":"Metabolic, nutritional, iatrogenic, and artifactual sources of urinary organic acids: a comprehensive table","journal":"Clinical Chemistry","year":2002,"pmid":"11978597","type":"review","finding":"Interpretation of urinary organic acid profiles is hindered by ambiguity in their sources; a comprehensive table lists inborn-error, other pathological, physiological, nutritional, iatrogenic and artifactual sources, and reviews preanalytical issues and possible misinterpretations.","url":"https://pubmed.ncbi.nlm.nih.gov/11978597/"},{"id":"krebs1980","authors":"Krebs HA, Johnson WA","title":"The role of citric acid in intermediate metabolism in animal tissues","journal":"FEBS Letters","year":1980,"pmid":"6998725","doi":"10.4159/harvard.9780674366701.c143","type":"laboratory","finding":"The founding description of the citric acid cycle, reprinted by FEBS Letters in 1980.","url":"https://pubmed.ncbi.nlm.nih.gov/6998725/"},{"id":"gray1999","authors":"Gray MW, Burger G, Lang BF","title":"Mitochondrial evolution","journal":"Science","year":1999,"pmid":"10066161","doi":"10.1126/science.283.5407.1476","type":"review","finding":"Gene sequence data strongly support a single origin of mitochondria from a eubacterial ancestor shared with a subgroup of the alpha-Proteobacteria; the organelle arose in the common ancestor of all extant eukaryotes, possibly at the same time as the nuclear component of the cell.","url":"https://pubmed.ncbi.nlm.nih.gov/10066161/"},{"id":"muchowska2019","authors":"Muchowska KB, Varma SJ, Moran J","title":"Synthesis and breakdown of universal metabolic precursors promoted by iron","journal":"Nature","year":2019,"pmid":"31043728","doi":"10.1038/s41586-019-1151-1","type":"laboratory","finding":"In a purely chemical network promoted by ferrous iron, pyruvate and glyoxylate built 9 of the 11 intermediates of the Krebs cycle, including all 5 universal metabolic precursors, which simultaneously broke down to carbon dioxide; the network may represent a prebiotic precursor of the Krebs and glyoxylate cycles.","url":"https://pubmed.ncbi.nlm.nih.gov/31043728/"},{"id":"mullen2011","authors":"Mullen AR, Wheaton WW, Jin ES, Chen PH, Sullivan LB, Cheng T, Yang Y, et al.","title":"Reductive carboxylation supports growth in tumour cells with defective mitochondria","journal":"Nature","year":2011,"pmid":"22101431","doi":"10.1038/nature10642","type":"laboratory","finding":"Tumor cells with defective mitochondria use glutamine-dependent reductive carboxylation, through NADP-dependent isocitrate dehydrogenase, as the major route to citrate, which supplies acetyl-CoA for lipid synthesis and the four-carbon intermediates; the pathway reverses many reactions of the canonical cycle.","url":"https://pubmed.ncbi.nlm.nih.gov/22101431/"},{"id":"tannahill2013","authors":"Tannahill GM, Curtis AM, Adamik J, Palsson-McDermott EM, McGettrick AF, Goel G, Frezza C, et al.","title":"Succinate is an inflammatory signal that induces IL-1β through HIF-1α","journal":"Nature","year":2013,"pmid":"23535595","doi":"10.1038/nature11986","type":"laboratory","finding":"Lipopolysaccharide strongly increased succinate in mouse macrophages, chiefly by glutamine-dependent anaplerosis with a contribution from the GABA shunt; the succinate stabilized HIF-1 alpha and enhanced interleukin-1 beta production.","url":"https://pubmed.ncbi.nlm.nih.gov/23535595/"},{"id":"whitfield2018","authors":"Whitfield KC, Bourassa MW, Adamolekun B, Bergeron G, Bettendorff L, Brown KH, Cox L, et al.","title":"Thiamine deficiency disorders: diagnosis, prevalence, and a roadmap for global control programs","journal":"Annals of the New York Academy of Sciences","year":2018,"pmid":"30151974","doi":"10.1111/nyas.13919","type":"review","finding":"Thiamine diphosphate is the cofactor of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, the latter most sensitive to deficiency; pyruvate and alpha-ketoglutarate rise in blood in deficiency; status is assessed by blood thiamine diphosphate or the red-cell transketolase assay, with no readily accessible agreed biomarker; in higher-income countries deficiency is most common with chronic alcoholism.","url":"https://pubmed.ncbi.nlm.nih.gov/30151974/"},{"id":"jahnen2012","authors":"Jahnen-Dechent W, Ketteler M","title":"Magnesium basics","journal":"Clinical Kidney Journal","year":2012,"pmid":"26069819","doi":"10.1093/ndtplus/sfr163","type":"review","finding":"Magnesium is a cofactor in numerous enzymatic reactions, acts as the counter ion for ATP and nucleic acids, and critically stabilizes many ATP-generating reactions.","url":"https://pubmed.ncbi.nlm.nih.gov/26069819/"},{"id":"parikh2015","authors":"Parikh S, Goldstein A, Koenig MK, Scaglia F, Enns GM, Saneto R, Anselm I, et al.","title":"Diagnosis and management of mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society","journal":"Genetics in Medicine","year":2015,"pmid":"25503498","doi":"10.1038/gim.2014.177","type":"consensus","finding":"Initial blood evaluation includes lactate and pyruvate, amino acids and acylcarnitines with urine organic acids; lactate above 3 mmol/L in a properly collected sample suggests mitochondrial dysfunction (sensitivity 34 to 62 percent, specificity 83 to 100 percent); the lactate-to-pyruvate ratio helps only when lactate is high; urinary malate and fumarate best correlated with disease in 67 patients against 21 with organic acidemias while other cycle intermediates correlated poorly; no randomized trials support B-vitamin combinations, and lipoic acid and riboflavin should be offered to patients.","url":"https://pubmed.ncbi.nlm.nih.gov/25503498/"},{"id":"zuckerman2009","authors":"Zuckerman JM, Assimos DG","title":"Hypocitraturia: pathophysiology and medical management","journal":"Reviews in Urology","year":2009,"pmid":"19918339","type":"review","finding":"Hypocitraturia, generally defined as urinary citrate under 320 mg (1.67 mmol) a day in adults, occurs in 20 to 60 percent of stone formers; causes include distal renal tubular acidosis (under 100 mg a day in its complete form), hypokalemia, bowel disease, animal protein, high sodium, ketosis-promoting diets, low fruit and vegetable intake, starvation and exercise (a mild reduction); management is diet, oral alkali and possibly lemonade or citrus juice, with potassium citrate raising citrate more than lemonade.","url":"https://pubmed.ncbi.nlm.nih.gov/19918339/"},{"id":"curhan2001","authors":"Curhan GC, Willett WC, Speizer FE, Stampfer MJ","title":"Twenty-four-hour urine chemistries and the risk of kidney stones among women and men","journal":"Kidney International","year":2001,"pmid":"11380833","doi":"10.1046/j.1523-1755.2001.00746.x","type":"cross-sectional","finding":"In 24-hour urines from 807 people with and 239 without a history of stones in NHS I, NHS II and HPFS, urine calcium was higher and volume lower in cases while urine oxalate and citrate did not differ; the authors concluded the traditional definitions of normal 24-hour values need reassessment and risk may be continuous rather than dichotomous.","url":"https://pubmed.ncbi.nlm.nih.gov/11380833/"},{"id":"pearle2014","authors":"Pearle MS, Goldfarb DS, Assimos DG, Curhan G, Denu-Ciocca CJ, Matlaga BR, Monga M, et al.; American Urological Association","title":"Medical management of kidney stones: AUA guideline","journal":"Journal of Urology","year":2014,"pmid":"24857648","doi":"10.1016/j.juro.2014.05.006","type":"guideline","finding":"Built on the AHRQ systematic review and 46 studies, the guideline sets out screening evaluation for first-time and recurrent stone formers, metabolic evaluation in selected patients, and medication or dietary therapy based on metabolic testing and close follow-up to reduce recurrence.","url":"https://pubmed.ncbi.nlm.nih.gov/24857648/"},{"id":"barcelo1993","authors":"Barcelo P, Wuhl O, Servitge E, Rousaud A, Pak CY","title":"Randomized double-blind study of potassium citrate in idiopathic hypocitraturic calcium nephrolithiasis","journal":"Journal of Urology","year":1993,"pmid":"8230497","doi":"10.1016/s0022-5347(17)35888-3","type":"rct","finding":"Of 57 randomized patients with active stone disease and hypocitraturia, 18 on potassium citrate for 3 years fell from 1.2 to 0.1 stones per patient-year (72 percent in remission) while 20 on placebo stayed at 1.1; only potassium citrate raised urinary citrate, pH and potassium.","url":"https://pubmed.ncbi.nlm.nih.gov/8230497/"},{"id":"ettinger1997","authors":"Ettinger B, Pak CY, Citron JT, Thomas C, Adams-Huet B, Vangessel A","title":"Potassium-magnesium citrate is an effective prophylaxis against recurrent calcium oxalate nephrolithiasis","journal":"Journal of Urology","year":1997,"pmid":"9366314","doi":"10.1016/s0022-5347(01)68155-2","type":"rct","finding":"In 64 patients with recurrent calcium oxalate stones randomized for up to 3 years, new stones formed in 63.6 percent on placebo and 12.9 percent on potassium-magnesium citrate (relative risk 0.16, 0.05 to 0.46).","url":"https://pubmed.ncbi.nlm.nih.gov/9366314/"},{"id":"dang2009","authors":"Dang L, White DW, Gross S, Bennett BD, Bittinger MA, Driggers EM, Fantin VR, et al.","title":"Cancer-associated IDH1 mutations produce 2-hydroxyglutarate","journal":"Nature","year":2009,"pmid":"19935646","doi":"10.1038/nature08617","type":"laboratory","finding":"Cancer-associated IDH1 mutations give the enzyme a new NADPH-dependent ability to reduce alpha-ketoglutarate to R(-)-2-hydroxyglutarate, which was markedly elevated in human gliomas carrying the mutations.","url":"https://pubmed.ncbi.nlm.nih.gov/19935646/"},{"id":"xu2011","authors":"Xu W, Yang H, Liu Y, Yang Y, Wang P, Kim SH, Ito S, et al.","title":"Oncometabolite 2-hydroxyglutarate is a competitive inhibitor of α-ketoglutarate-dependent dioxygenases","journal":"Cancer Cell","year":2011,"pmid":"21251613","doi":"10.1016/j.ccr.2010.12.014","type":"laboratory","finding":"2-hydroxyglutarate competitively inhibits multiple alpha-ketoglutarate-dependent dioxygenases, including histone demethylases and the TET 5-methylcytosine hydroxylases, producing genome-wide histone and DNA methylation changes.","url":"https://pubmed.ncbi.nlm.nih.gov/21251613/"},{"id":"dinardo2013","authors":"DiNardo CD, Propert KJ, Loren AW, Paietta E, Sun Z, Levine RL, Straley KS, et al.","title":"Serum 2-hydroxyglutarate levels predict isocitrate dehydrogenase mutations and clinical outcome in acute myeloid leukemia","journal":"Blood","year":2013,"pmid":"23641016","doi":"10.1182/blood-2013-03-493197","type":"cohort","finding":"In sera of 223 people with de novo acute myeloid leukemia, 2-hydroxyglutarate ran at a median of 3,004 ng/mL with IDH mutations against 61 ng/mL without; 700 ng/mL optimally separated the groups, 9 of 13 wild-type samples above it carried IDH mutations on further analysis, and a level above 200 ng/mL in remission predicted shorter survival (hazard ratio 3.9).","url":"https://pubmed.ncbi.nlm.nih.gov/23641016/"},{"id":"selak2005","authors":"Selak MA, Armour SM, MacKenzie ED, Boulahbel H, Watson DG, Mansfield KD, Pan Y, et al.","title":"Succinate links TCA cycle dysfunction to oncogenesis by inhibiting HIF-alpha prolyl hydroxylase","journal":"Cancer Cell","year":2005,"pmid":"15652751","doi":"10.1016/j.ccr.2004.11.022","type":"laboratory","finding":"Succinate dehydrogenase and fumarate hydratase are tumor suppressors; succinate that accumulates with succinate dehydrogenase inhibition inhibits HIF-alpha prolyl hydroxylases, stabilizing and activating HIF-1 alpha.","url":"https://pubmed.ncbi.nlm.nih.gov/15652751/"},{"id":"pollard2005","authors":"Pollard PJ, Brière JJ, Alam NA, Barwell J, Barclay E, Wortham NC, Hunt T, et al.","title":"Accumulation of Krebs cycle intermediates and over-expression of HIF1alpha in tumours which result from germline FH and SDH mutations","journal":"Human Molecular Genetics","year":2005,"pmid":"15987702","doi":"10.1093/hmg/ddi227","type":"laboratory","finding":"Germline fumarate hydratase mutations predispose to leiomyomas and renal cell cancer and succinate dehydrogenase mutations to paragangliomas and pheochromocytomas; the tumors accumulate fumarate or succinate and overexpress HIF-1 alpha, a pseudo-hypoxic drive.","url":"https://pubmed.ncbi.nlm.nih.gov/15987702/"},{"id":"xiao2012","authors":"Xiao M, Yang H, Xu W, Ma S, Lin H, Zhu H, Liu L, et al.","title":"Inhibition of α-KG-dependent histone and DNA demethylases by fumarate and succinate that are accumulated in mutations of FH and SDH tumor suppressors","journal":"Genes and Development","year":2012,"pmid":"22677546","doi":"10.1101/gad.191056.112","type":"laboratory","finding":"Fumarate and succinate competitively inhibit multiple alpha-ketoglutarate-dependent dioxygenases, including histone demethylases, prolyl hydroxylases, collagen prolyl-4-hydroxylases and the TET enzymes, altering genome-wide histone and DNA methylation.","url":"https://pubmed.ncbi.nlm.nih.gov/22677546/"},{"id":"debray2007","authors":"Debray FG, Mitchell GA, Allard P, Robinson BH, Hanley JA, Lambert M","title":"Diagnostic accuracy of blood lactate-to-pyruvate molar ratio in the differential diagnosis of congenital lactic acidosis","journal":"Clinical Chemistry","year":2007,"pmid":"17384007","doi":"10.1373/clinchem.2006.081166","type":"cohort","finding":"At the recommended cut point of about 25, the lactate-to-pyruvate ratio separated pyruvate dehydrogenase deficiency (11 patients) from other mitochondrial disease (35) with 77 percent sensitivity and 91 percent specificity; with lactate under 2.5 mmol/L it performed no better than chance, and accuracy improved at higher lactate.","url":"https://pubmed.ncbi.nlm.nih.gov/17384007/"},{"id":"chouchani2014","authors":"Chouchani ET, Pell VR, Gaude E, Aksentijević D, Sundier SY, Robb EL, Logan A, et al.","title":"Ischaemic accumulation of succinate controls reperfusion injury through mitochondrial ROS","journal":"Nature","year":2014,"pmid":"25383517","doi":"10.1038/nature13909","type":"laboratory","finding":"Selective succinate accumulation is a universal metabolic signature of ischemia across tissues, arising from reversal of succinate dehydrogenase driven by fumarate overflow; its rapid reoxidation on reperfusion drives reactive oxygen species by reverse electron transport at complex I, and blocking the accumulation reduced injury in mouse models of heart attack and stroke.","url":"https://pubmed.ncbi.nlm.nih.gov/25383517/"},{"id":"bullo2021","authors":"Bulló M, Papandreou C, García-Gavilán J, Ruiz-Canela M, Li J, Guasch-Ferré M, Toledo E, et al.","title":"Tricarboxylic acid cycle related-metabolites and risk of atrial fibrillation and heart failure","journal":"Metabolism","year":2021,"pmid":"34678258","doi":"10.1016/j.metabol.2021.154915","type":"cohort","finding":"In two nested case-control studies within PREDIMED over about 10 years (512 atrial fibrillation and 334 heart failure cases), the top against the bottom quarter of plasma succinate, malate, citrate and 2-hydroxyglutarate carried odds ratios for atrial fibrillation of 1.80, 2.13, 1.87 and 1.95, and malate 2.15 for heart failure.","url":"https://pubmed.ncbi.nlm.nih.gov/34678258/"},{"id":"fischer2014","authors":"Fischer K, Kettunen J, Würtz P, Haller T, Havulinna AS, Kangas AJ, Soininen P, et al.","title":"Biomarker profiling by nuclear magnetic resonance spectroscopy for the prediction of all-cause mortality: an observational study of 17,345 persons","journal":"PLoS Medicine","year":2014,"pmid":"24586121","doi":"10.1371/journal.pmed.1001606","type":"cohort","finding":"In 9,842 Estonian Biobank participants with non-fasting plasma measured by NMR (508 deaths over a median 5.4 years), citrate predicted all-cause mortality (hazard ratio 1.33 per standard deviation, 1.21 to 1.45), replicated in 7,503 Finns.","url":"https://pubmed.ncbi.nlm.nih.gov/24586121/"},{"id":"deelen2019","authors":"Deelen J, Kettunen J, Fischer K, van der Spek A, Trompet S, Kastenmüller G, Boyd A, et al.","title":"A metabolic profile of all-cause mortality risk identified in an observational study of 44,168 individuals","journal":"Nature Communications","year":2019,"pmid":"31431621","doi":"10.1038/s41467-019-11311-9","type":"cohort","finding":"In 44,168 people with 5,512 deaths, 14 circulating biomarkers independently predicted mortality; citrate, previously identified, was not selected in the fully adjusted model because of its limited additional contribution.","url":"https://pubmed.ncbi.nlm.nih.gov/31431621/"},{"id":"serena2018","authors":"Serena C, Ceperuelo-Mallafré V, Keiran N, Queipo-Ortuño MI, Bernal R, Gomez-Huelgas R, Urpi-Sarda M, et al.","title":"Elevated circulating levels of succinate in human obesity are linked to specific gut microbiota","journal":"ISME Journal","year":2018,"pmid":"29434314","doi":"10.1038/s41396-018-0068-2","type":"cross-sectional","finding":"In Spanish cross-sectional and prospective cohorts, obesity was associated with higher circulating succinate and impaired glucose metabolism, determined largely by the ratio of succinate-producing to succinate-consuming gut bacteria; weight loss lowered the ratio and plasma succinate.","url":"https://pubmed.ncbi.nlm.nih.gov/29434314/"},{"id":"asadi2020","authors":"Asadi Shahmirzadi A, Edgar D, Liao CY, Hsu YM, Lucanic M, Asadi Shahmirzadi A, Wiley CD, et al.","title":"Alpha-Ketoglutarate, an Endogenous Metabolite, Extends Lifespan and Compresses Morbidity in Aging Mice","journal":"Cell Metabolism","year":2020,"pmid":"32877690","doi":"10.1016/j.cmet.2020.08.004","type":"laboratory","finding":"Alpha-ketoglutarate, whose levels change with fasting, exercise and aging, given as a calcium salt to C57BL/6 mice promoted a longer, healthier life with lower systemic inflammatory cytokines; several authors declared commercial interests and a patent application.","url":"https://pubmed.ncbi.nlm.nih.gov/32877690/"},{"id":"lim2025","authors":"Lim ZM, Chew YE, Guan L, Wang W, Mahadzir MDA, Dorajoo R, Kennedy B, Sandalova E, Maier AB","title":"Recruitment evaluation of a gerotherapeutic randomized controlled trial testing alpha-ketoglutarate in biologically older, middle-aged adults (ABLE)","journal":"Experimental Gerontology","year":2025,"pmid":"40819772","doi":"10.1016/j.exger.2025.112867","type":"rct","finding":"A double-blind trial of 1 g sustained-release calcium alpha-ketoglutarate or placebo for 6 months in 120 generally healthy adults aged 40 to 60 with methylation-clock age above chronological age; this report covers recruitment feasibility only, not outcomes.","url":"https://pubmed.ncbi.nlm.nih.gov/40819772/"},{"id":"linker2011","authors":"Linker RA, Lee DH, Ryan S, van Dam AM, Conrad R, Bista P, Zeng W, et al.","title":"Fumaric acid esters exert neuroprotective effects in neuroinflammation via activation of the Nrf2 antioxidant pathway","journal":"Brain","year":2011,"pmid":"21354971","doi":"10.1093/brain/awq386","type":"laboratory","finding":"In a mouse model of chronic multiple sclerosis, dimethyl fumarate improved the disease course and preserved myelin, axons and neurons by activating Nrf2, with monomethyl fumarate modifying KEAP1 at cysteine 151; the benefit was almost abolished in Nrf2-deficient mice.","url":"https://pubmed.ncbi.nlm.nih.gov/21354971/"},{"id":"gold2012","authors":"Gold R, Kappos L, Arnold DL, Bar-Or A, Giovannoni G, Selmaj K, Tornatore C, et al.","title":"Placebo-controlled phase 3 study of oral BG-12 for relapsing multiple sclerosis","journal":"New England Journal of Medicine","year":2012,"pmid":"22992073","doi":"10.1056/NEJMoa1114287","type":"rct","finding":"In relapsing-remitting multiple sclerosis, oral dimethyl fumarate twice or three times daily reduced the proportion relapsing by 2 years to 27 and 26 percent against 46 percent on placebo, with flushing, gastrointestinal events and lower lymphocyte counts as adverse effects.","url":"https://pubmed.ncbi.nlm.nih.gov/22992073/"},{"id":"walsh2006","authors":"Walsh MC, Brennan L, Malthouse JP, Roche HM, Gibney MJ","title":"Effect of acute dietary standardization on the urinary, plasma, and salivary metabolomic profiles of healthy humans","journal":"American Journal of Clinical Nutrition","year":2006,"pmid":"16960166","doi":"10.1093/ajcn/84.3.531","type":"laboratory","finding":"In 30 healthy volunteers sampled on 4 mornings, within- and between-person variation was considerable in every fluid; standardizing the previous day's diet reduced between-person variation in urine but not plasma or saliva, and urine reflected acute dietary intake.","url":"https://pubmed.ncbi.nlm.nih.gov/16960166/"},{"id":"lewis2010","authors":"Lewis GD, Farrell L, Wood MJ, Martinovic M, Arany Z, Rowe GC, Souza A, et al.","title":"Metabolic signatures of exercise in human plasma","journal":"Science Translational Medicine","year":2010,"pmid":"20505214","doi":"10.1126/scitranslmed.3001006","type":"laboratory","finding":"Measuring more than 200 plasma metabolites before and after exercise, succinate, malate and fumarate rose 2- to 4-fold, citrate and isocitrate more modestly and alpha-ketoglutarate not at all; fitter people and faster marathoners expanded the pool more.","url":"https://pubmed.ncbi.nlm.nih.gov/20505214/"},{"id":"reddy2020","authors":"Reddy A, Bozi LHM, Yaghi OK, Mills EL, Xiao H, Nicholson HE, Paschini M, et al.","title":"pH-Gated Succinate Secretion Regulates Muscle Remodeling in Response to Exercise","journal":"Cell","year":2020,"pmid":"32946811","doi":"10.1016/j.cell.2020.08.039","type":"laboratory","finding":"In mice and humans, exercising muscle releases succinate into the interstitium and circulation through pH-gated transport as the cell acidifies; succinate signals through SUCNR1 to control muscle remodeling, innervation and strength gains with training.","url":"https://pubmed.ncbi.nlm.nih.gov/32946811/"},{"id":"devadder2016","authors":"De Vadder F, Kovatcheva-Datchary P, Zitoun C, Duchampt A, Bäckhed F, Mithieux G","title":"Microbiota-Produced Succinate Improves Glucose Homeostasis via Intestinal Gluconeogenesis","journal":"Cell Metabolism","year":2016,"pmid":"27411015","doi":"10.1016/j.cmet.2016.06.013","type":"laboratory","finding":"In mice fed a fiber-rich diet, succinate produced by bacterial fermentation was the most abundant carboxylic acid in the cecum and served as a substrate for intestinal gluconeogenesis, improving glucose and insulin tolerance.","url":"https://pubmed.ncbi.nlm.nih.gov/27411015/"},{"id":"breslau1988","authors":"Breslau NA, Brinkley L, Hill KD, Pak CY","title":"Relationship of animal protein-rich diet to kidney stone formation and calcium metabolism","journal":"Journal of Clinical Endocrinology and Metabolism","year":1988,"pmid":"2826524","doi":"10.1210/jcem-66-1-140","type":"laboratory","finding":"In 15 young adults fed three 12-day diets matched for protein and minerals but rising in sulfur, the animal-protein diet raised urinary calcium from 103 to 150 mg a day, lowered urine pH and reduced citrate excretion because of the acid load.","url":"https://pubmed.ncbi.nlm.nih.gov/2826524/"},{"id":"simpson1983","authors":"Simpson DP","title":"Citrate excretion: a window on renal metabolism","journal":"American Journal of Physiology","year":1983,"pmid":"6338740","doi":"10.1152/ajprenal.1983.244.3.F223","type":"review","finding":"Urinary citrate is set largely by intracellular citrate metabolism in the kidney; acid-base changes cause striking changes in citrate clearance, with alkalosis raising and acidosis lowering it, mediated by the pH gradient across the inner mitochondrial membrane.","url":"https://pubmed.ncbi.nlm.nih.gov/6338740/"}],"written":"2026-10-23","updated":"2026-10-23","reviewed":true,"reviewedOn":"2026-10-24","related":{"essays":[],"glossary":["TCA cycle (Krebs cycle)","Mitochondria","Metabolomics","Reference range"],"symptoms":["fatigue","athletic-recovery"],"entries":["organic-acids","amino-acids","magnesium","aerobic-fitness","fasting-insulin","ferritin","uric-acid","vitamin-b12","alcohol","protein-intake","fiber"]},"url":"https://taggehealth.com/labs/tca-cycle-intermediates","author":"Daniel Tagge, MD","publisher":"Tagge Health","license":"Free to read and cite with attribution to the URL above.","inPractice":"I love the Krebs cycle as a teaching picture. It is the clearest way I know to show someone how food becomes energy, why fitness, sleep and metabolic health are one conversation, and why the cell's chemistry is also its signaling system. I read the measured intermediates with more humility than the picture invites. One marker is a clue, three are a story: a pattern across several intermediates, seen again on a repeat collection, and read beside lactate, pyruvate, magnesium, ferritin, the B vitamins, fasting insulin and the fitness number, is a fair starting point for a conversation about mitochondrial capacity and fuel. A single high or low line is first a question about yesterday: what was eaten, how much fruit, a hard workout, alcohol, a fast, and how the urine was collected. I do not sell cofactors against a single line on a panel. The one place the number changes what I do is low urine citrate in someone who has had a stone, where the guideline definition is a 24-hour citrate under 320 mg a day and the trials support treating it. I will say plainly where I am ahead of the evidence: I believe the pattern can tell me something about how well someone's mitochondria are handling fuel, especially when it moves in the right direction as their fitness improves, but no study has validated that reading in healthy people, and I treat it as a hypothesis I am testing with the person, not a diagnosis."}