IsoleucineIle · I

In brief

The molecule

Isoleucine, leucine and valine are the most hydrophobic of the amino acids, and the three help set the structure of globular proteins and the way membrane proteins sit in the lipid bilayer ¹.

It branches at the β-carbon, as valine does, and like valine it prefers β-sheet, where leucine prefers the α-helix. Swapping one branched-chain residue for another is usually conservative, but not always ¹.

The β-carbon is also a stereocentre, which makes isoleucine one of only two standard amino acids, with threonine, to have two. Its natural diastereomer, L-alloisoleucine, differs only at that second center ¹⁰.

StereochemistryTwo stereocentres, so four stereoisomers; proteins use (2S,3S). L-Alloisoleucine, (2S,3R), is made from it in the body ¹⁰.

Where it comes from

Humans cannot make it; bacteria and plants do, and industry uses Corynebacterium glutamicum ¹⁷.

Unlike most amino acids, the branched chains largely escape metabolism in the gut and liver on first pass ¹.

EssentialHumans cannot make it. The international adult requirement is 20 mg per kilogram per day ¹¹. Isotope studies measured leucine and valine on their own but not isoleucine; for the three together, young men needed 144 mg/kg/day ¹⁸.

In the bottle · fermentedIndustrial branched-chain amino acids are made mainly by fermentation with Corynebacterium glutamicum, in strains engineered for yield ¹⁷.

What the body does with it

Mostly, it is built into protein. The branched-chain amino acids make up about 20 to 25% of most dietary proteins, and their place in nature is explained by protein structure rather than by their metabolic side roles ¹.

It is also a signal in its own right, at least in mice. A low-isoleucine diet increased the liver’s insulin sensitivity and ketone production and raised energy expenditure through the FGF21–UCP1 axis; low valine did something similar but smaller, and low leucine did not ⁴. That mechanism has since been withdrawn by the laboratory that proposed it. A 2025 study across two mouse strains and both sexes confirmed the effect — less dietary isoleucine protects against a Western diet, more impairs metabolic health — and states plainly that the metabolic effects of reduced isoleucine in mice are NOT associated with FGF21, and that in humans plasma FGF21 is likewise not associated with dietary isoleucine ¹⁹. The result stands and the explanation does not, which is the more common shape of progress than the reverse.

Across people, variation in dietary isoleucine helped explain differences in body mass index, an association rather than a test of cause ⁴. The same 2025 analysis of human NHANES data found isoleucine content varies widely across foods, and that people with higher Healthy Eating Index scores tend to eat LESS isoleucine ¹⁹ — a confound built into the association, since the people eating less of it are eating better in other ways too.

The split between the three carries into the brain, in the same model organism. Male and female 3xTg Alzheimer's-model mice were fed diets restricted in leucine, isoleucine or valine for nine months from six months of age: restricting isoleucine or valine, but not leucine, improved metabolic health; restricting any of the three improved short-term memory in males, isoleucine most; and in females the largest cognitive benefit came from restricting valine ²⁰. The sex difference is as interesting as the result, and both are still in mice.

There is one place where isoleucine acts as an immune signal and the work reached patients. A 16-hour fast — brief enough to be tolerated, unlike the regimens usually tested — improved the efficacy of immunotherapy in mice and in humans, and the mechanism runs through isoleucine accumulating INSIDE the tumor, where it reconfigures CD8 T cell epigenetic programs and phospholipid remodeling. In patients having neoadjuvant immunotherapy, short-term fasting enhanced CD8 clonal expansion and cytotoxic programs ²¹. Note the direction: more isoleucine where the T cells are, from eating nothing.

How it is made, moved and broken down

The first two steps are shared with leucine and valine: transamination by BCAT, then oxidative decarboxylation by the branched-chain keto acid dehydrogenase complex, BCKDH ¹.

After that, isoleucine has its own route, through short/branched-chain acyl-CoA dehydrogenase (SBCAD) to tiglyl-CoA, then through HSD10 and β-ketothiolase, two adjacent enzymes, to acetyl-CoA and propionyl-CoA ^12,13.

Most tissues oxidize branched-chain amino acids. In mice the largest amounts went through muscle, brown fat, liver, kidney and heart, and the pancreas took a fifth of its Krebs-cycle carbon from them ²².

Propionyl-CoA is not only fuel. In prostate cancer cells, catabolism of isoleucine and valine is the primary source of intracellular propionyl-CoA, and the propionyl-CoA then acts as a signal — propionylating specific lysines on nuclear SREBP2, raising its transcriptional activity, driving cholesterol synthesis and de novo androgen production, and sustaining androgen receptor signaling under metabolic and therapeutic stress, androgen deprivation included ²³. The chemistry on this page's own pathway diagram turns out to end at a transcription factor.

Running the pathway short does something to chromatin that is not explained by the usual signaling story. Mice fed diets depleted of methionine, leucine or isoleucine gave distinct, amino-acid-specific responses in liver transcriptome, metabolome and histone proteome, NONE of which was reproduced by inhibiting mTORC1 with rapamycin. Isoleucine depletion was the most dramatic of the three: a prominent state of histone H2A and H4 hypoacetylation, with hepatic acetyl-CoA maintained despite β-oxidation being turned down. Multi-omics factor analysis across 14,139 data points separated it from the other two, and the same response appeared in cultured cells, so it is cell-intrinsic ²⁴.

Carbon skeleton · glucogenic and ketogenicIts carbons leave as acetyl-CoA, which is ketogenic, and propionyl-CoA, which becomes succinyl-CoA and can make glucose ^1,8.

Where it matters most

Skeletal muscle

The largest share of branched-chain oxidation in mice ²².

Liver and brown fat

Major oxidisers, and where a low-isoleucine diet changes metabolism ^4,22.

Pancreas

In mice, takes 20% of its Krebs-cycle carbon from the branched chains ²².

Brain

Entry competes with tryptophan and tyrosine at the blood–brain barrier ²⁵.

When it goes wrong

How it is measured

Plasma isoleucine is part of the amino acid panel ²⁹. Newborn screening measures leucine, isoleucine and valine by tandem mass spectrometry, and babies on intravenous nutrition can screen falsely high, one reason the second-tier alloisoleucine test exists ³.

For the isoleucine-only disorders, the useful samples are blood acylcarnitines and urine acylglycines and organic acids: 2-methylbutyrylglycine for SBCAD deficiency, and 2-methyl-3-hydroxybutyrate with tiglylglycine for HSD10 and β-ketothiolase deficiency ^12,13.

The isoleucine-only disorder now has a denominator. Of 693,797 newborns screened for organic acidemias in Quanzhou, China, between 2014 and 2023, 69 were confirmed, an incidence of 1 in 10,055 — and 2-methylbutyryl-CoA dehydrogenase deficiency, the first isoleucine-only step, was tied for the commonest finding at 18 cases, level with 3-methylcrotonyl-CoA carboxylase deficiency and ahead of glutaric acidemia type 1 at 13 ³⁰. It is not a footnote disorder; it is one of the things a screening program most often finds.

For the emergency that actually kills in maple syrup urine disease, the useful early test is not an amino acid. Across 269 emergency department visits by 25 children over nine years, 79 visits (29.4%) were acute metabolic decompensations, carrying far higher intensive care (22% against 2.6%) and hemodialysis rates (19% against 1.6%). Hyponatremia and hyperuricemia marked them; acid–base parameters did NOT discriminate; and serum uric acid was the only clinically meaningful predictor among the primary tests, with a cutoff of 4.65 mg/dL ³¹. Plasma leucine is the right measurement and is frequently too slow to help.

• Plasma alloisoleucineThe pathognomonic marker of maple syrup urine disease ^2,3.Above 5 µmol/L in 2,451 of 2,453 samples from classic MSUD, and not raised in diabetes, ketotic hypoglycemia, phenylketonuria or carrier parents ⁹.Newborn screening by tandem mass spectrometry cannot see it, because it has the same mass as leucine and isoleucine; a second-tier LC-MS/MS test separates them ³.
• Urine 2-methylbutyrylglycineThe hallmark of SBCAD deficiency, the isoleucine-only step after the shared ones; suspected from a raised C5-carnitine on newborn screening ¹².A raised C5-carnitine has to be told apart from isovaleric acidemia and from pivalic acid exposure ¹².About 10% of 162 reported patients had symptoms, so a positive screen is often a biochemical finding rather than a disease ¹².

Food, supplements and the evidence

What is strange about it

Isoleucine, leucine and alloisoleucine weigh exactly the same, so the mass spectrometer at the heart of newborn screening sees them as one ³.

Its twin is always present. A large oral load of isoleucine, 1,527 µmol per kilogram, raised alloisoleucine in three healthy people by only about 5 µmol/L ⁹.

HSD10 and β-ketothiolase sit next to each other on the isoleucine pathway and leave nearly the same urine pattern, yet HSD10 disease went much worse; the enzyme has other jobs, including neurosteroid metabolism ¹³.

When isoleucine runs short, cells quietly build the wrong amino acid into their proteins. On isoleucine restriction, healthy human fibroblasts consistently incorporated VALINE at isoleucine codons, through misacylation of tRNA-Ile with valine by wild-type IARS1 — the synthetase's editing step being bypassed rather than broken. Valine supplementation fully compensated for the shortage and restored translation in healthy cells, and reversed the antiproliferative effect of deprivation, but did neither in IARS1-deficient cells ³⁸. The authors read it as a general trade: translational speed preserved at the cost of translational fidelity.

The most unexpected route by which isoleucine does harm starts in the mouth. Dental plaque from patients with abdominal aortic aneurysm was enriched for Fusobacterium nucleatum, and plasma isoleucine was raised in both human and experimental aneurysm. Applying the bacterium orally worsened angiotensin-II-induced aneurysm in mice; deleting the bacterium's own isoleucine-biosynthesis gene reduced the isoleucine it released and mitigated the disease. The bacterial isoleucine drove ferroptosis in vascular smooth muscle cells through H3K9ac-dependent activation of ACSL4, and dietary isoleucine restriction lowered H3K9ac, suppressed the ferroptosis and slowed progression ³⁹. The authors call the plaque Fusobacterium and the circulating isoleucine exploratory biomarker candidates needing larger independent validation, which is the right description of them.

Where it connects

Sources

39 sources, numbered as they are cited. Every one was checked against PubMed or its publisher before it was cited here; the note under each says what it shows and what it does not.

1. 1
   Brosnan JT, Brosnan ME. Branched-chain amino acids: enzyme and substrate regulation.J Nutr · 2006 · 136(1 Suppl):207S–211Sdoi:10.1093/jn/136.1.207S · PMID 16365084

   Structure, first-pass escape and the shared first steps of breakdown.

2. 2
   Blackburn PR, Gass JM, Vairo FPE, et al. Maple syrup urine disease: mechanisms and management.Appl Clin Genet · 2017 · 10:57–66doi:10.2147/TACG.S125962 · PMID 28919799

   Alloisoleucine as the pathognomonic marker.

3. 3
   Oglesbee D, Sanders KA, Lacey JM, et al. Second-tier test for quantification of alloisoleucine and branched-chain amino acids in dried blood spots to improve newborn screening for maple syrup urine disease (MSUD).Clin Chem · 2008 · 54(3):542–549doi:10.1373/clinchem.2007.098434 · PMID 18178665

   Isobaric amino acids and the second-tier test.

4. 4
   Yu D, Richardson NE, Green CL, et al. The adverse metabolic effects of branched-chain amino acids are mediated by isoleucine and valine.Cell Metab · 2021 · 33(5):905–922.e6doi:10.1016/j.cmet.2021.03.025 · PMID 33887198

   Each branched chain has distinct effects in mice.

5. 5
   Green CL, Trautman ME, Chaiyakul K, et al. Dietary restriction of isoleucine increases healthspan and lifespan of genetically heterogeneous mice.Cell Metab · 2023 · 35(11):1976–1995.e6doi:10.1016/j.cmet.2023.10.005 · PMID 37939658

   UM-HET3 mice, both sexes.

6. 6
   Gurney R, Thomas CM. Mupirocin: biosynthesis, special features and applications of an antibiotic from a gram-negative bacterium.Appl Microbiol Biotechnol · 2011 · 90(1):11–21doi:10.1007/s00253-011-3128-3 · PMID 21336932

   Isoleucyl-tRNA synthetase and resistance.

7. 7
   Manoli I, Myles JG, Sloan JL, et al. A critical reappraisal of dietary practices in methylmalonic acidemia raises concerns about the safety of medical foods. Part 1: isolated methylmalonic acidemias.Genet Med · 2016 · 18(4):386–395doi:10.1038/gim.2015.102 · PMID 26270765

   Leucine-heavy medical foods and growth.

8. 8
   Margoses D, Imbard A, Pontoizeau C, et al. Nutritional management in severe methylmalonic and propionic acidemias: how much medical food is too much?J Inherit Metab Dis · 2026 · 49(1):e70114doi:10.1002/jimd.70114 · PMID 41344680

   71 patients; isoleucine and valine fall on amino acid mixtures.

9. 9
   Schadewaldt P, Bodner-Leidecker A, Hammen HW, et al. Significance of L-alloisoleucine in plasma for diagnosis of maple syrup urine disease.Clin Chem · 1999 · 45(10):1734–1740PMID 10508118

   Reference values and the 5 µmol/L cut-off.

10. 10
    Schadewaldt P, Bodner-Leidecker A, Hammen HW, et al. Formation of L-alloisoleucine in vivo: an L-[13C]isoleucine study in man.Pediatr Res · 2000 · 47(2):271–277doi:10.1203/00006450-200002000-00020 · PMID 10674358

    Alloisoleucine as a by-product of transamination.

11. 11
    Joint WHO/FAO/UNU Expert Consultation. Protein and amino acid requirements in human nutrition.World Health Organ Tech Rep Ser · 2007 · (935):1–265PMID 18330140

    The international requirement figures.

12. 12
    Porta F, Chiesa N, Martinelli D, et al. Clinical, biochemical, and molecular spectrum of short/branched-chain acyl-CoA dehydrogenase deficiency: two new cases and review of literature.J Pediatr Endocrinol Metab · 2019 · 32(2):101–108doi:10.1515/jpem-2018-0311 · PMID 30730842

    162 patients reviewed.

13. 13
    Su L, Li X, Lin R, et al. Clinical and molecular analysis of 6 Chinese patients with isoleucine metabolism defects: identification of 3 novel mutations in the HSD17B10 and ACAT1 gene.Metab Brain Dis · 2017 · 32(6):2063–2071doi:10.1007/s11011-017-0097-y · PMID 28875337

    HSD10 and β-ketothiolase deficiency compared.

14. 14
    Vockley J, Ensenauer R. Isovaleric acidemia: new aspects of genetic and phenotypic heterogeneity.Am J Med Genet C Semin Med Genet · 2006 · 142C(2):95–103doi:10.1002/ajmg.c.30089 · PMID 16602101

    The leucine pathway's best-known disorder.

15. 15
    Peters H, Ferdinandusse S, Ruiter JP, et al. Metabolite studies in HIBCH and ECHS1 defects: implications for screening.Mol Genet Metab · 2015 · 115(4):168–173doi:10.1016/j.ymgme.2015.06.008 · PMID 26163321

    The valine pathway's own disorders.

16. 16
    Calubag MF, Ademi I, Green CL, et al. Lifelong restriction of dietary valine has sex-specific benefits for health and lifespan in mice.Nat Aging · 2026 · 6(8):1611–1630doi:10.1038/s43587-026-01169-0 · PMID 42498891

    Median male lifespan up 23%.

17. 17
    Reifenberg P, Zimmer A. Branched-chain amino acids: physico-chemical properties, industrial synthesis and role in signaling, metabolism and energy production.Amino Acids · 2024 · 56(1):51doi:10.1007/s00726-024-03417-2 · PMID 39198298

    Industrial production with Corynebacterium glutamicum.

18. 18
    Riazi R, Wykes LJ, Ball RO, et al. The total branched-chain amino acid requirement in young healthy adult men determined by indicator amino acid oxidation by use of L-[1-13C]phenylalanine.J Nutr · 2003 · 133(5):1383–1389doi:10.1093/jn/133.5.1383 · PMID 12730426

    144 mg/kg/day for the three together.

19. 19
    Trautman ME, Green CL, MacArthur MR, et al. Dietary isoleucine content modulates the metabolic and molecular response to a Western diet in mice.Mol Metab · 2025 · 101:102248doi:10.1016/j.molmet.2025.102248 · PMID 40939730

    The same laboratory that found the effect, taking the mechanism back. Two strains, both sexes, plus human NHANES data.

20. 20
    Babygirija R, Green CL, Sonsalla MM, et al. Restriction of individual branched-chain amino acids has distinct effects on the development and progression of Alzheimer's disease in 3xTg mice.Adv Sci (Weinh) · 2026 · 13(30):e15220doi:10.1002/advs.202515220 · PMID 41817439

    Nine months of restriction, one amino acid at a time, in both sexes. Still mice.

21. 21
    Chen S, Hu T, Zhu K, et al. 16-h fasting optimizes cancer immunotherapy in mice and humans.Cell Metab · 2026 · 38(5):944–962.e14doi:10.1016/j.cmet.2026.01.015 · PMID 41720105

    A rare dietary mechanism involving isoleucine that was tested in patients and not only in mice.

22. 22
    Neinast MD, Jang C, Hui S, et al. Quantitative analysis of the whole-body metabolic fate of branched-chain amino acids.Cell Metab · 2019 · 29(2):417–429.e4doi:10.1016/j.cmet.2018.10.013 · PMID 30449684

    Isotope tracing of oxidation by tissue, in mice.

23. 23
    Li Z, Liu S, Jin W, et al. Isoleucine and valine promote prostate cancer progression via propionyl-CoA-mediated cholesterol metabolism.Nat Metab · 2026 · 8(8):1772–1790doi:10.1038/s42255-026-01583-z · PMID 42625032

    Propionyl-CoA given a second job: modifying a transcription factor rather than only burning.

24. 24
    Haws SA, Liu Y, Green CL, et al. Depletion of individual dietary amino acids induce distinct metabolic and chromatin states.J Biol Chem · 2025 · 302(2):111074doi:10.1016/j.jbc.2025.111074 · PMID 41419194

    Methionine, leucine and isoleucine removed one at a time, with the histones measured. Rapamycin does not reproduce any of it.

25. 25
    Fernstrom JD. Large neutral amino acids: dietary effects on brain neurochemistry and function.Amino Acids · 2013 · 45(3):419–430doi:10.1007/s00726-012-1330-y · PMID 22677921

    Competition at the blood–brain barrier.

26. 26
    Strauss KA, Carson VJ, Soltys K, et al. Branched-chain α-ketoacid dehydrogenase deficiency (maple syrup urine disease): treatment, biomarkers, and outcomes.Mol Genet Metab · 2020 · 129(3):193–206doi:10.1016/j.ymgme.2020.01.006 · PMID 31980395

    184 patients over three decades.

27. 27
    Bandura A, Chandoga J, Vengríni J, et al. Outlook on ACADSB variants shaping metabolomic patterns and clinical outcomes — experience from a Central European country.Clin Biochem · 2026 · 142:111105doi:10.1016/j.clinbiochem.2026.111105 · PMID 41722717

    Three probands, and a reappraisal of what the condition is. Also the practical point that the confirmatory ratio can read normal.

28. 28
    Shittu AO, Perovic O, Layer-Nicolaou F, et al. Molecular characterization of mupirocin-resistant MRSA from Germany and South Africa.Front Cell Infect Microbiol · 2026 · 16:1865925doi:10.3389/fcimb.2026.1865925 · PMID 42643317

    82 resistant strains, and the two mechanisms separated: a plasmid gene for high-level, a point substitution in the target enzyme for low-level.

29. 29
    Sharer JD, De Biase I, Matern D, et al. Laboratory analysis of amino acids, 2018 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG).Genet Med · 2018 · 20(12):1499–1507doi:10.1038/s41436-018-0328-6 · PMID 30459394

    Methods for amino acid analysis.

30. 30
    Lin Y, Lin C, Zheng Z, Chen Y, Zheng F, Lin W. Large-scale newborn screening for organic acidemias in Quanzhou, China: a 10-year retrospective observational study.Sci Rep · 2025 · 15(1):30592doi:10.1038/s41598-025-15625-1 · PMID 40835664

    693,797 newborns over ten years. The denominator the isoleucine disorders never had.

31. 31
    Oral-Cebeci S, Aktuğlu-Zeybek Ç, Aslan B, et al. Early laboratory indicators of acute metabolic decompensation during emergency presentations in pediatric maple syrup urine disease.Eur J Pediatr · 2026 · 185(6):07081doi:10.1007/s00431-026-07081-4 · PMID 42151652

    269 emergency visits by 25 children, and the cheap test that beat the obvious one.

32. 32
    Aamann L, Deshpande N, Dam G, et al. Branched-chain amino acids for people with cirrhosis and hepatic encephalopathy.Cochrane Database Syst Rev · 2026 · 1(1):CD001939doi:10.1002/14651858.CD001939.pub5 · PMID 41542879

    18 trials, 934 adults, published 1984 to 2023, with GRADE certainty attached to every line. Three of the trials were supported by for-profit organizations in the form of the intervention.

33. 33
    Wolfe RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality?J Int Soc Sports Nutr · 2017 · 14:30doi:10.1186/s12970-017-0184-9 · PMID 28852372

    No human study of oral BCAAs alone.

34. 34
    Aguilera JA, Tinline-Goodfellow CT, Lees MJ, et al. Dileucine-supplemented essential amino acids support whole-body anabolism after resistance exercise and serum-stimulated cell-based anabolism.J Int Soc Sports Nutr · 2025 · 22(1):2590090doi:10.1080/15502783.2025.2590090 · PMID 41321015

    The measurement that was missing in 2017, made. Twelve people, stable isotopes, collagen as the comparator.

35. 35
    Kittiprachakul S, Maung ST, Poorirerngpoom N, et al. Exercise and nutrition interventions for sarcopenia in cirrhosis: a systematic review and network meta-analysis of randomized controlled trials.Eur J Clin Nutr · 2025 · 79(11):1087–1098doi:10.1038/s41430-025-01642-w · PMID 40925921

    23 trials, 1,525 people, in the clinical population where branched-chain supplements are most prescribed.

36. 36
    Liang Z, Su Z, Yang H, et al. Branched-chain amino acids in bone health: from molecular mechanisms to therapeutic potential.Biomed Pharmacother · 2025 · 192:118645 · RETRACTEDdoi:10.1016/j.biopha.2025.118645 · PMID 41075410

    Indexed by PubMed as a Retracted Publication. Cited here only as the thing that was withdrawn.

37. 37
    Liang Z, Su Z, Yang H, et al. Retraction notice to 'Branched-chain amino acids in bone health: from molecular mechanisms to therapeutic potential' [Biomed. Pharmacother. 192 (2025) 118645].Biomed Pharmacother · 2026 · 199:119415doi:10.1016/j.biopha.2026.119415 · PMID 42031570

    The notice. Fourteen months after publication, and it states no reason.

38. 38
    Kok G, Schene IF, Ilcken EF, et al. Isoleucine-to-valine substitutions support cellular physiology during isoleucine deprivation.Nucleic Acids Res · 2025 · 53(1):gkae1184doi:10.1093/nar/gkae1184 · PMID 39657787

    Speed bought with fidelity. Healthy fibroblasts do it; IARS1-deficient ones cannot.

39. 39
    Chen L, Lin L, Wang Z, et al. Fusobacterium nucleatum-derived isoleucine exacerbates aneurysm by inducing ferroptosis in vascular smooth muscle cells.Arterioscler Thromb Vasc Biol · 2026 · 46(8):e324050doi:10.1161/ATVBAHA.126.324050 · PMID 42206370

    Dental plaque metagenomics, a bacterial biosynthesis gene deleted, and authors who call their own biomarker exploratory.