Leucic Acid (α-Hydroxyisocaproic Acid / HICA): A Comprehensive Reference
1. Identity and Chemical Characterization
Names and Synonyms
Leucic acid — formally DL-α-hydroxy-isocaproic acid (HICA), also known as DL-2-hydroxy-4-methylvaleric acid — is an end product of leucine metabolism in human tissues such as muscle and connective tissue. It is catalogued under a substantial number of systematic and colloquial names in chemical databases. These include: (2S)-2-hydroxy-4-methylpentanoic acid (IUPAC), 2-hydroxyisocaproic acid, α-hydroxyisocaproic acid, 2-hydroxy-4-methylvaleric acid, and L-(+)-leucic acid for the L-enantiomer; the corresponding R-enantiomer is designated D-(−)-leucic acid.
Stereoisomers and CAS Registry Numbers
The L-form, designated (S)-leucic acid, carries CAS registry number 13748-90-8. The D-form, designated (R)-leucic acid or D-(−)-leucic acid, carries CAS registry number 20312-37-2. The racemic mixture, DL-leucic acid, is registered under CAS 498-36-2 and is the form most commonly found in commercial supplement preparations.
Molecular Formula and Structure
Leucic acid has the molecular formula C₆H₁₂O₃ and a molecular weight of 132.16 g/mol. Structurally, it is a 2-hydroxycarboxylic acid (an alpha-hydroxy acid) derived from the six-carbon branched-chain skeleton of leucine, bearing a hydroxyl group at the alpha (C-2) position and an isobutyl side chain. It is catalogued in PubChem under CID 83697 (L-form) and in ChEBI under ID 44510.
Physical Properties
Like many alpha-hydroxy acids of its class, leucic acid is a white crystalline solid that is water-soluble. It may be formulated as the free acid or as a salt (most commonly the sodium or calcium salt) for supplemental use. The calcium salt of DL-leucic acid — DL-leucic acid calcium (C₁₂H₂₂CaO₆) — is a recognized chemical entity used in some research contexts.
2. Natural Sources and Occurrence
Endogenous Metabolite in Human and Animal Tissues
HICA is an end product of leucine metabolism in human tissues such as muscle and connective tissue. HICA is a typical constituent of human plasma, naturally circulating at a concentration of 0.25 ± 0.02 mmol/L, and is found in muscle, where it is generally considered to have anti-catabolic activity. HICA is also detectable in urine and other biological fluids.
Fermented Foods
HICA is an end-product of the microbial metabolism of leucine and is therefore found in many fermented foods, including wine, soy sauce, certain cheeses, kimchi, and yogurt. The variability of HICA levels in commercial yogurts may reflect the diversity of activities of hydroxyisocaproate dehydrogenase enzymes in the diverse Lactobacillus strains used in fermentation.
In a scientific analysis, the culture broth from Leuconostoc lactis, a lactic acid bacterium isolated from kimchi, was analysed by liquid chromatography-tandem mass spectrometry. HICA production ranged from 153.1 to 526 μg/mL in L. lactis, L. plantarum, L. brevis, and L. mesenteroides; HICA was identified from bacterial cultures of these four species through an untargeted metabolomics approach. Research on fermented whole-grain rye foods also found notably higher levels of HICA compared to versions made without sourdough or bacterial fermentation.
Microbial Production in the Gut
Metabolomic analysis of specific gut anaerobes revealed that they biosynthesise leucic acid (α-hydroxyisocaproic acid) via conserved 2-hydroxyisocaproate dehydrogenases (HicDHs). HICA production by lactic acid bacteria shows a positive correlation with hydroxyisocaproate dehydrogenases (HicDs), which play a key role in the production of HICA from leucine and ketoisocaproic acid; HICA is formed by the transamination of leucine to 2-ketoisocaproic acid (KICA), followed by a reduction reaction of 2-KICA to 2-HICA, which is the end product of the leucine catabolism pathway.
3. Traditional and Historical Use
Leucic acid as an isolated, characterized molecule has no documented history of traditional or ethnobotanical use as such. It is an endogenous human metabolite and a microbial fermentation product, neither of which was knowable to pre-modern practitioners. Its investigation as a distinct compound is entirely a product of twentieth- and twenty-first-century biochemistry and nutritional science. Foods in which it is found incidentally — fermented cheeses, wines, soy sauces, vinegars, kimchi — have long histories of consumption across numerous cultures (Korean, Japanese, European), but leucic acid's presence in these foods was not identified or attributed with any specific therapeutic role in traditional systems of medicine. The scientific isolation and characterization of leucic acid as a discrete metabolite of leucine emerged from studies on branched-chain amino acid catabolism, beginning in earnest in the latter half of the twentieth century.
4. Biosynthesis and Metabolic Origin
Leucine Catabolism Pathway
The vast majority of L-leucine metabolism is initially catalyzed by the branched-chain amino acid aminotransferase enzyme, producing α-ketoisocaproate (α-KIC). The majority of KIC is irreversibly oxidatively decarboxylated by the branched-chain alpha-keto acid dehydrogenase (BCKDH) complex within the mitochondria, eventually yielding acetyl-CoA and acetoacetate for energy production; alternatively, KIC can be metabolized in the cytosol.
A small percentage of KIC is oxidized by the enzyme KIC dioxygenase to form beta-hydroxy-beta-methylbutyrate (HMB). Another fraction of KIC is reduced by lactate dehydrogenase (LDH) or a specific KIC reductase to form α-hydroxyisocaproic acid (HICA), also known as leucic acid. Thus, leucic acid represents one of several terminal products of leucine catabolism, produced in parallel with the more extensively studied metabolite HMB.
Relationship to Leucine
Of all nutrients, the single amino acid leucine possesses the most marked anabolic characteristics in acting as a trigger element for the initiation of protein synthesis. As a branched-chain amino acid, leucine can be catabolized within muscle, thus posing the possibility that metabolites of leucine could be involved in mediating its anabolic effects. According to clinical and experimental studies, HICA can be considered as an anticatabolic substance that may also increase muscle protein synthesis in some situations. Although leucine has a unique role as a promoter of protein synthesis, metabolites of leucine may be more effective in preventing breakdown of proteins, particularly muscle proteins.
Microbial Biosynthesis
The amino acid derivative HICA (leucic acid) is a protein-fermentation product of bacteria, such as lactobacilli. HICA cannot be degraded by many bacterial species, and its production may thus represent a survival strategy for Lactobacillus, because HICA displays antibacterial activity. Mammalian cells can also metabolize HICA and use the metabolite for protein synthesis.
5. Key Constituents, Active Compounds, and Mechanisms of Action
Anti-Catabolic Actions — Protease and MMP Inhibition
There is evidence of a direct inhibitory effect of HICA on various matrix metalloproteinase (MMP) enzymes, which are responsible for degradation of various connective and protein tissues. This is considered a primary mechanism by which leucic acid may protect skeletal muscle and connective tissue from exercise-induced or inflammation-induced breakdown. HICA can inhibit various matrix metalloproteinase enzymes that are responsible for degrading connective and protein tissues.
Suppression of Inflammatory Cytokines
Skeletal muscle immobilization induces skeletal muscle atrophy via increasing oxidative stress and inflammation. Oxidative stress mediated by inducible nitric oxide synthase (iNOS) and its product nitric oxide (NO) has been shown to be a critical factor in immobilization-evoked skeletal muscle atrophy in mouse. Additionally, immobilization induces expression of the inflammatory cytokine interleukin-6 (IL-6), and the inhibition of IL-6 expression and/or IL-6 signaling in skeletal muscle ameliorates skeletal muscle atrophy. Alpha-hydroxyisocaproic acid attenuates TNFα/IFNγ co-exposure-induced protein degradation and myotube atrophy via suppression of iNOS and IL-6 in murine C2C12 myotube cells.
HCAR2 Receptor Signaling and Metabolic Effects
Metabolomic studies have identified leucic acid as a natural ligand for the membrane receptor hydroxycarboxylic acid receptor 2 (HCAR2), which signals to suppress adipose lipid anabolism and promote catabolism. Administration of leucic acid reduces adiposity and enhances lean mass and exercise capacity by suppressing lipid synthesis and promoting catabolism, effects dependent on HCAR2 signaling. This receptor-mediated pathway represents a distinct mechanism of action from the MMP-inhibitory and anti-inflammatory pathways and is an area of active research interest.
AMPK and ERK Pathways
Leucic acid alters AMPK and ERK1/2 phosphorylation status, suppresses lipid synthesis, and promotes catabolism. These kinase-mediated effects place leucic acid in the category of metabolic signaling molecules capable of influencing cellular energy sensing, though the precise hierarchy and physiological significance of these effects in humans remain to be fully characterized.
Role in Protein Synthesis and Degradation
It may be that HICA impacts muscle protein synthesis (MPS) directly in the healthy state, but it may also ameliorate the decrease of MPS on the way to skeletal muscle atrophy, at least as induced by immobilization. The 2021 Sumi et al. cell-based study found that while HICA reduced basal protein synthesis rates in unstressed C2C12 myotubes, it substantially attenuated the protein degradation and myotube atrophy induced by combined TNFα/IFNγ cytokine exposure. HICA is found in muscle and is generally considered to have anticatabolic actions. Because a dietary substitution of α-HICA for leucine supports normal growth, it is possible that at least part of the anabolic effect of leucine is mediated via this metabolite.
6. Scientific Evidence by Area of Application
6.1 Skeletal Muscle — Body Composition and Lean Mass
Human / Clinical Evidence
The pivotal human clinical study on leucic acid as a dietary supplement is the 2010 randomized controlled trial by Mero et al., published in the Journal of the International Society of Sports Nutrition. Fifteen healthy male soccer players (mean age 22.1 ± 3.9 years) volunteered for the 4-week double-blind study during an intensive training period; subjects in the HICA group (n=8) received 583 mg of sodium salt of HICA (corresponding to 500 mg of HICA) mixed with liquid three times a day for 4 weeks, and those in the placebo group (n=7) received 650 mg of maltodextrin mixed with liquid three times a day for the same period.
Body composition was evaluated with dual-energy X-ray absorptiometry (DXA) before and after the 4-week period. As compared to placebo, HICA supplementation significantly increased body weight (p < 0.005) and whole lean body mass (p < 0.05) while fat mass remained constant. The lean body mass of lower extremities increased by 400 g in the HICA group but decreased by 150 g in the placebo group during the study, a difference between groups that was statistically significant (p < 0.01).
The increase in lean mass (approximately 0.5 kg) was more pronounced in lower extremities, and the soccer players had milder delayed-onset muscular soreness (DOMS) symptoms after using HICA than placebo subjects. There were no changes in physical performance of athletes even though the muscle mass of lower extremities increased significantly.
Evidence strength assessment: The human clinical data is currently categorized as "limited." The cornerstone of HICA research is the single 2010 double-blind, randomized controlled trial conducted by Mero et al. The study enrolled only 15 participants, was short in duration (four weeks), involved an exclusively male, athlete-specific population, and has not been independently replicated in a large-scale trial. In humans the available evidence suggests that HICA may relieve DOMS symptoms and can increase lean mass during training; however, more long-term and mechanistic studies are needed. A relevant conflict of interest is noted: two co-authors of the Mero et al. study, Dr. Tuomo Karila and Dr. Timo Seppälä, are inventors of the HICA patent for a "Nutrient Supplement and use of the same" and are also partners at Oy Elmomed Ltd.
Animal / Preclinical Evidence
One animal study addressed the hypothesis that a diet containing 5% α-hydroxyisocaproic acid (α-HICA), a leucine metabolite, will slow the loss and/or improve recovery of muscle mass in response to disuse. Adult 14-week-old male Wistar rats were provided a control diet or an isonitrogenous isocaloric diet containing either 5% α-HICA or leucine; disuse atrophy was produced by unilateral hindlimb immobilization ("casting") for 7 days. Casting decreased gastrocnemius mass, which was associated with both a reduction in protein synthesis and S6K1 phosphorylation as well as enhanced proteasome activity and increased atrogin-1 and MuRF1 mRNA. The study (Tuvdendorj et al., 2013, published in the American Journal of Physiology — Endocrinology and Metabolism) found that chronic α-HICA treatment improved muscle recovery after immobilization-induced atrophy in this preclinical model.
6.2 Delayed-Onset Muscle Soreness (DOMS)
In the Mero et al. 2010 study, HICA supplementation significantly decreased whole-body DOMS symptoms in the 4th week of the treatment (p < 0.05) compared to placebo. Muscle strength and running velocity did not differ between groups. The reduction in DOMS is consistent with leucic acid's proposed mechanism of inhibiting MMPs and suppressing inflammatory mediators such as IL-6 and iNOS. However, no large-scale, independently replicated human trials have been published to confirm this outcome across diverse populations or training modalities.
6.3 Inflammatory Modulation
In Vitro (Cell-Based) Evidence
A 2021 study published in Nutrients (Sumi et al.) examined HICA in murine C2C12 myotube cells. α-Hydroxyisocaproic acid (HICA, leucic acid) is an end-product of the microbial metabolism of leucine. The investigators hypothesized that HICA would reduce both the decrease of muscle protein synthesis (MPS) and the atrophy of skeletal muscles evoked by the iNOS- and IL-6-mediated pathways. The study found that HICA attenuated the cytokine-induced myotube atrophy, with effects on iNOS and IL-6 expression. There remains ongoing debate as to whether or not α-hydroxyisocaproic acid positively regulates skeletal muscle protein synthesis resulting in the gain or maintenance of skeletal muscle.
A 2024 study in the Journal of Microbiology and Biotechnology (Lee M. et al.) investigated leucic acid derived from kimchi lactic acid bacteria. The study found that metabolites of kimchi lactic acid bacteria — including indole-3-lactic acid, phenyllactic acid, and leucic acid — inhibit obesity-related inflammation in human mesenchymal stem cells. This was an in vitro investigation and does not constitute human clinical evidence.
6.4 Obesity, Adiposity, and Metabolic Regulation
Through systematic culturing of cecal content from an obesity-prone mouse model (Aida−/−), researchers isolated two previously unidentified anaerobes whose administration attenuated diet-induced obesity. Metabolomic analysis revealed that both strains biosynthesise leucic acid via conserved 2-hydroxyisocaproate dehydrogenases (HicDHs), which were responsible for the anti-obesity effects. By combining computational prediction with transcriptomic and human genetic evidence, the investigators identified HCAR2 as a candidate receptor for leucic acid and confirmed this by demonstrating direct binding and showing that HCAR2 is required for metabolic effects of leucic acid in vitro and in adipose tissue in vivo.
This research (Lin SY et al., 2025, Research Square preprint) is preclinical in nature. No human clinical trials have evaluated leucic acid for obesity or metabolic syndrome outcomes. The evidence is preliminary and comes entirely from animal models and cell culture.
6.5 Antimicrobial Properties
HICA cannot be degraded by many bacterial species, and its production may represent a survival strategy for Lactobacillus, because HICA displays antibacterial activity. HICA has also been reported to have fungicidal properties. A publication indexed on ResearchGate (Roth et al., 2012) identifies HICA as a potential topical antibacterial agent, and it has been explored as an intracanal medicament in regenerative endodontic procedures. 2-Hydroxyisocaproic acid (HICA) has been recommended as an intracanal medicament in regenerative endodontic treatments (RETs). These applications remain in the investigational stage, with limited controlled clinical evidence.
7. Body Systems and Health Areas Associated with Leucic Acid
- Skeletal muscle system: Anti-catabolic effects on muscle protein balance, reduction of exercise-induced DOMS, potential support for lean mass during intensive training.
- Connective tissue: Inhibition of matrix metalloproteinase enzymes responsible for degrading connective and protein tissues.
- Immune and inflammatory system: Suppression of pro-inflammatory cytokines (IL-6, iNOS) in preclinical models; inhibition of obesity-related inflammation in human mesenchymal stem cells in vitro.
- Metabolic/adipose system: Preclinical evidence of HCAR2-mediated suppression of lipid synthesis and promotion of fat catabolism; AMPK and ERK pathway modulation.
- Gut microbiome: Endogenous production of leucic acid by specific gut bacterial species (Clostridiales/Lachnospiraceae lineages) identified as potentially relevant to host metabolic health.
- Oral/dental health: Investigational use as an intracanal medicament based on antibacterial properties.
- Clinical biomarker utility: Elevated levels of HICA have been reported in the urine of patients with dihydrolipoyl dehydrogenase deficiency, making urinary leucic acid a potential indicator of this metabolic disorder.
8. Dosage Forms and Reported Dosages
Forms Available
Leucic acid is commercially available as a dietary supplement primarily in powder and capsule forms. In the pivotal Mero et al. (2010) study, the active supplement was administered as 583 mg of the sodium salt of HICA (corresponding to 500 mg of HICA free acid) mixed with liquid, three times daily. Calcium salt formulations (DL-leucic acid calcium) also exist as distinct chemical preparations.
Dosages Reported in Published Research
- Mero et al. 2010 (human RCT, soccer players): 583 mg of sodium salt of HICA (equivalent to 500 mg HICA) three times daily for 4 weeks, totalling 1.5 g of HICA per day; this 4-week supplementation of 1.5 g per day led to small increases in muscle mass during an intensive training period.
- Tuvdendorj et al. 2013 (rat model): A diet containing 5% α-hydroxyisocaproic acid was administered to adult 14-week-old male Wistar rats over a 7-day immobilization period and subsequent recovery periods of 7 or 14 days.
The clinical trials that have been conducted used 1.5 grams per day over four to eight weeks and did not report significant adverse effects. However, the total number of people studied is small, and there are no long-term safety trials. No dose-ranging studies have been published in humans to establish an optimal or maximal effective dose.
9. Safety Considerations and Interactions
General Safety Profile
HICA is a substance the body produces naturally, and it occurs in fermented foods, which gives it a reasonable baseline safety profile. The clinical trials conducted at 1.5 grams per day over four to eight weeks did not report significant adverse effects; however, the total number of people studied is small, and there are no long-term safety trials.
Conflict of Interest in the Core Human Study
Two co-authors of the foundational Mero et al. (2010) study — Dr. Tuomo Karila and Dr. Timo Seppälä — are inventors of the HICA patent for a "Nutrient Supplement and use of the same" and are partners at Oy Elmomed Ltd., though the study leader Dr. Mero and the other co-authors are stated to have no relationships to any studied substances. This conflict-of-interest context is pertinent to interpreting the sole published human RCT.
Absence of Regulatory Classification
As of the date of this article, no safety monograph for leucic acid as a dietary supplement has been published by the NIH Office of Dietary Supplements, NCCIH, EFSA, EMA, or the German Commission E. There is no USP or European Pharmacopoeia monograph for leucic acid as a standalone supplement ingredient. Its occurrence as a naturally occurring food metabolite (in fermented foods) and as an endogenous human metabolite provides contextual support for its general safety, but formal regulatory safety evaluations are absent.
Known Biochemical Safety Signals
Elevated levels of HICA have been reported in the urine of patients with dihydrolipoyl dehydrogenase deficiency, a rare inborn error of metabolism — indicating that abnormally high endogenous accumulation of leucic acid is a disease marker rather than a health indicator in that context. This does not directly predict harm from exogenous supplementation but highlights the importance of understanding the compound's metabolic handling.
Limitations of Available Safety Data
The only human intervention study enrolled 15 subjects over four weeks, making it underpowered to detect rare adverse events or long-term effects. The use of HICA for supporting the muscular system is primarily justified by scientific evidence, though the quality and quantity of studies are moderate; several small-scale clinical trials and animal studies suggest that HICA supplementation may help reduce muscle soreness and decrease muscle breakdown. No pharmacokinetic studies in humans have been published that characterize absorption, half-life, or metabolic fate following exogenous oral administration.
10. Evidence Summary and Research Gaps
Leucic acid (HICA) is a well-characterized endogenous metabolite with a clearly defined biochemical identity and natural occurrence across multiple food matrices and within human tissues. Its proposed mechanisms of action — including MMP inhibition, suppression of inflammatory cytokines (iNOS, IL-6), HCAR2-mediated metabolic signaling, and modulation of AMPK/ERK pathways — are supported by in vitro and animal data of varying quality. However, the human clinical evidence base is narrow: it rests primarily on a single double-blind RCT of only 15 male soccer players over four weeks (Mero et al., 2010), with a noted conflict of interest among co-authors. No independent replication of this trial has been published. Preclinical work (particularly the 2013 rat immobilization model and the 2025 gut bacteria/HCAR2 study) is mechanistically interesting but cannot be directly translated to human efficacy. The anti-obesity and anti-inflammatory findings from cell culture and mouse models represent early-stage science. Large, independent, well-powered human trials across diverse populations are needed before the efficacy and optimal dosing of leucic acid can be established with confidence.
References
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