Alpha hydroxyisocaproic acid

Other Names

(R)-Leucic acid(S)-(-)-2-Hydroxyisocaproic acid2-hydroxy-4-methyl-pentanoic acid2-Hydroxy-4-methylpentanoic acid2-Hydroxy-4-methylvaleric acid2-Hydroxyisocaproic acidalfa-hydroxy-isocaproic acidalpha-hydroxy analog of leucinealpha-hydroxyisocaproic acid, (R)-isomeralpha-hydroxyisocaproic acid, (S)-isomerD-alpha-Hydroxyisocaproic acidD-Leucic acidDL-2-hydroxy-4-methylvaleric acidDL-Leucic acidHICAL-2-Hydroxy-4-methylpentanoic acidL-alpha-Hydroxyisocaproic acidL-Leucic acidLeucic acidα-Hydroxyisocaproic acid

Synopsis

Alpha-Hydroxyisocaproic Acid (HICA): A Comprehensive Reference

1. Identity and Chemical Characterization

Names and Classification

2-hydroxyisocaproic acid (HICA), which is also referred to as 2-hydroxy-4-methylvaleric acid, DL-leucic acid, 2-hydroxy-4-methylpentanoic acid, and alpha-hydroxyisocaproic acid, is a leucine metabolite with a hydroxy substituent at the 2-position and a methyl substituent at the 4-position. In the scientific literature and supplement industry it is predominantly abbreviated as HICA or α-HICA. DL-alfa-hydroxy-isocaproic acid (HICA), also known as leucic acid or DL-2-hydroxy-4-methylvaleric acid, is an alfa-hydroxyl acid metabolite of leucine.

Chemical Properties

HICA carries the CAS number 498-36-2 (for the DL-racemate) and has the molecular formula C₆H₁₂O₃, and is soluble in water. The L-enantiomer is assigned CAS number 13748-90-8. L-alpha-hydroxyisocaproic acid is an organic compound characterized by its hydroxyl and carboxylic acid functional groups. It is a chiral molecule, existing in two enantiomeric forms, with the L-form being biologically active. This compound is typically a white crystalline solid and is soluble in water, which enhances its bioavailability.

Endogenous Status and Natural Occurrence

HICA is an end product of leucine metabolism in human tissues such as muscle and connective tissue. In humans, it is produced in tissues including connective tissues and muscles, and is regarded as a physiological agent present in the human body at low concentrations. The healthy adult usually contains around 0.1–0.25 mmol/L HICA in their plasma.

Some foodstuffs produced by fermentation, such as certain cheeses, wines, and soy sauce, contain HICA. HICA has been identified from bacterial cultures of L. lactis, L. plantarum, L. brevis, and L. mesenteroides through an untargeted metabolomics approach. Additionally, a mass spectrum corresponding to HICA was validated from fermented kimchi cultures, and HICA production by lactic acid bacteria showed a positive correlation with hydroxyisocaproate dehydrogenases (HicDs), which play a key role in the production of HICA from leucine and ketoisocaproic acid.

Common Forms and Preparations

As a dietary supplement, HICA is commercially available primarily in two forms: as the free acid (DL-α-hydroxyisocaproic acid) and as its sodium salt. In the principal human clinical study, the HICA group received 583 mg of the sodium salt of HICA (corresponding to 500 mg of HICA) mixed with liquid three times a day for 4 weeks. HICA calcium refers to α-hydroxyisocaproic acid bound to a calcium salt; supplement manufacturers often bind organic acids like HICA to minerals like calcium to improve the compound's stability, shelf life, and solubility in water.

2. Traditional and Historical Use

HICA has no documented history of traditional or folk medicinal use as an isolated compound. Its use in the context of sports nutrition and bodybuilding is primarily based on recent scientific studies rather than longstanding tradition. The compound exists naturally in fermented foods, but its presence in these matrices was not recognized or deliberately exploited by any historical culture as a therapeutic agent. Systematic investigation of HICA as a distinct bioactive entity began only in the late twentieth century, when Finnish researchers identified its presence in human tissues and its potential pharmacological properties. It became an object of targeted supplementation research in the 2000s and 2010s.

3. Biosynthesis and Metabolic Pathway

Leucine Catabolism and HICA Formation

HICA is derived from L-leucine. Leucine is first converted into 2-ketoisocaproic acid (KICA) by a branched-chain amino acid aminotransferase, which transfers the amino group of leucine to α-ketoglutarate. Then KICA is reduced into HICA by hydroxyisocaproate dehydrogenase (HicD, product of hicD).

In humans specifically, the metabolic sequence proceeds as follows: when leucine is metabolized in skeletal muscle, it first undergoes reversible transamination catalyzed by the enzyme branched-chain aminotransferase (BCAT). The majority of the resulting α-ketoisocaproate (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.

This compound is produced as a by-product of the leucine degradation pathway in humans and certain microorganisms. It is a natural compound produced through the leucine degradation pathway and is produced in humans and by certain microorganisms such as lactic acid bacteria and Clostridium species.

Relationship to Other Leucine Metabolites

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. HICA is structurally and functionally related to HMB (β-hydroxy-β-methylbutyrate), the other principal leucine metabolite of interest in sports science; both are considered primarily anti-catabolic rather than directly anabolic agents.

4. Key Mechanisms of Action

Anti-Catabolic Activity and Muscle Protein Metabolism

According to clinical and experimental studies HICA can be considered as an anti-catabolic substance. There is evidence of a direct in vitro inhibitory effect of HICA on various matrix metalloproteinase enzymes, which are responsible for the degradation of various connective and protein tissues.

At the cellular level, HICA decreases muscle protein synthesis (MPS) at least in part through the activation of adenosine monophosphate-activated protein kinase (AMPK) and the inactivation of extracellular signal-regulated kinase (ERK) in healthy myotubes. HICA also inhibits myotube atrophy induced by TNFα/IFNγ co-exposure via the modification of iNOS and/or IL-6 expression.

Myotube atrophy induced by TNFα/IFNγ co-exposure was significantly improved by HICA pretreatment, and this improvement was accompanied by the inhibition of iNOS expression and IL-6 production. Moreover, HICA suppressed the TNFα/IFNγ co-exposure-induced secretion of 3-methylhistidine. These results demonstrated that HICA decreases basal protein synthesis under normal or cachexic conditions; however, HICA might attenuate skeletal muscle atrophy via maintaining a low level of protein degradation under cachexic conditions.

An important mechanistic nuance emerges from the in vitro evidence: there is ongoing debate as to whether HICA positively regulates skeletal muscle protein synthesis resulting in the gain or maintenance of skeletal muscle. The cell-based work suggests the compound's primary effect under normal conditions is anti-catabolic (suppression of protein degradation) rather than directly anabolic (stimulation of new protein synthesis).

Matrix Metalloproteinase (MMP) Inhibition

Research findings reveal that 2-hydroxyisocaproic acid inhibits the fragmentation of Developmental Endothelial Locus 1 by dose-dependently modulating and reducing matrix metalloproteinase 8 (MMP-8) activity. This research focuses on the interaction of HICA with key inflammatory mediators, MMP-8, and Developmental Endothelial Locus 1 (Del-1). MMP-8 plays a significant role in the inflammatory process, and Del-1 acts as a crucial immunomodulator maintaining tissue homeostasis. HICA's reversible inhibition of MMP-8 does not involve covalent bonding, positioning it as an enzyme modulator or downregulator rather than a direct inhibitor.

The staining intensities for MMP-9 and myeloperoxidase (MPO) were lower in HICA-treated biofilm groups than in controls, with histopathology similar to non-biofilm controls but with a thinner and less dense inflammatory cell infiltrate. Expression of neutrophil extravasation antagonist Del-1 was localized in the endothelium and was stronger in the HICA-treated group.

Muscle Recovery Following Immobilization

Animal research has clarified a specific recovery-related mechanism. Although α-HICA did not prevent casting-induced muscle atrophy, the decreased muscle protein synthesis observed with casting was not seen in α-HICA-treated rats. Neither α-HICA nor leucine altered the increased proteasome activity and atrogene expression observed with immobilization. After 14 days of recovery, however, muscle mass had returned to control values only in the rats fed α-HICA, and this was associated with a sustained increase in protein synthesis and phosphorylation of S6K1 and 4E-BP1 in previously immobilized muscle.

Proposed Precursor-Mediated Mechanism

A patent-filed mechanistic hypothesis proposes that oral administration of HICA will act to increase muscular concentrations of leucine by acting as a substitute for leucine in catabolism for energy, as well as potentially being reaminated to form leucine. Increased levels of leucine will stimulate protein synthesis and inhibit protein breakdown, thereby inducing an anabolically favorable state for the cell. This indirect mechanism has not been conclusively confirmed in peer-reviewed human intervention studies.

Antimicrobial Mechanisms

HICA has been shown to exhibit its antibacterial activity via penetration of bacterial cell membranes, causing depolarization, permeabilization, and rupture of membranes, with subsequent leakage of cellular contents and cell death. For fungi, HICA inhibits hyphal formation in susceptible Candida albicans and Aspergillus fumigatus isolates and affects cell wall integrity.

5. Scientific Evidence by Area of Use

5.1 Body Composition and Lean Muscle Mass

Key Human Clinical Study (Mero et al., 2010)

The most cited human trial is a randomized, double-blind, placebo-controlled study published in the Journal of the International Society of Sports Nutrition. Fifteen healthy male soccer players (age 22.1 ± 3.9 years) volunteered for a 4-week double-blind study during an intensive training period. The subjects in the HICA group (n = 8) received 583 mg of the 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 three times daily. According to a weekly training schedule, they practiced soccer 3–4 times a week, had strength training 1–2 times a week, and had one soccer game during the study.

As compared to placebo, HICA supplementation increased significantly 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 4-week HICA supplementation of 1.5 g per day led to small increases in muscle mass during an intensive training period in soccer athletes.

Limitation: The study had a very small sample size (n = 15 total), was conducted exclusively in young male soccer players during high-intensity training, and the between-group differences in lean mass, while statistically significant, were modest in absolute terms.

Negative RCT Evidence (Teixeira et al., 2019)

A larger and more rigorously designed double-blind randomized controlled trial reached conflicting conclusions. β-hydroxy-β-methylbutyrate and α-HICA are leucine metabolites proposed to improve body composition and strength when combined with resistance exercise training (RET). In this double-blind randomized controlled pragmatic trial, researchers evaluated the effects of off-the-shelf supplements on RET-induced changes in body composition and performance. Forty men were blocked-randomized to receive α-HICA (n = 10, FFM = 62.0 ± 7.1 kg), HMB-FA (n = 11), HMB-Ca (n = 9), or placebo (n = 10). Time-dependent changes were observed for increases in trunk fat-free mass. No statistically significant between-group or group-by-time interactions were observed. Supplementation with HMB (FA and Ca) or α-HICA failed to enhance body composition to a greater extent than placebo. The authors concluded they could not recommend these leucine metabolites for improving body composition changes with RET in young adult resistance-trained men.

Case Study Evidence (Teixeira et al., 2018)

Research involving dietary supplement interventions for sarcopenia and osteopenia in type 1 diabetes patients is scarce. One case study presented a type 1 diabetic patient treated with supplemental α-HICA for 120 days, with measures of body composition by dual X-ray absorptiometry (DXA), blood markers, and maximum voluntary contraction parameters assessed at baseline and after 120 days. The patient's baseline weight was 73.2 kg, increasing to 75.2 kg by 120 days. Salient changes included increases of trunk fat-free mass (+0.2 kg) and a decrease of 8 percent in trunk fat mass contribution. Handgrip strength increased by 58.84 N, whereas isometric force in the leg press decreased by 347.15 N. Amelioration of BMD Z-scores from −0.7 to 0.5 and T-scores from −1.0 to −0.9 were noted. Full hematologic measures and weekly nutritional counselling assessments revealed no signs of adverse effects with α-HICA supplementation. This remains a single case report and cannot establish causation.

Overall Strength of Evidence for Body Composition

The evidence base for HICA's effects on human body composition is preliminary and conflicting. The sole positive RCT is very small (n = 15) and was conducted in an active athletic population under high training loads; the larger pragmatic RCT in resistance-trained men found no benefit. As noted in a 2021 review, the benefits of HICA for skeletal muscle growth or function remain controversial.

5.2 Delayed-Onset Muscle Soreness (DOMS)

In the Mero et al. (2010) trial, HICA supplementation decreased whole-body DOMS symptoms in the 4th week of treatment (p < 0.05) when compared to placebo. Muscle strength and running velocity did not differ between the groups. This finding of reduced DOMS without improvement in objective strength measures has been interpreted as consistent with a primarily anti-catabolic, protective role for the compound.

Strength of evidence: A single small RCT supports reduced DOMS with 1.5 g/day HICA over 4 weeks in soccer players. No independent replication of this specific DOMS finding exists in the peer-reviewed literature. Evidence is therefore preliminary and insufficient to draw firm conclusions.

5.3 Disuse Atrophy and Muscle Recovery

Animal work published in American Journal of Physiology – Endocrinology and Metabolism examined HICA's role in recovery from immobilization. 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 for 7 days with the contralateral muscle used as control. Rats were also cast for 7 days and permitted to recover for 7 or 14 days.

Casting decreased gastrocnemius mass, associated with both reduced protein synthesis and S6K1 phosphorylation, as well as enhanced proteasome activity and increased atrogin-1 and MuRF1 mRNA. Although neither α-HICA nor leucine prevented casting-induced muscle atrophy, the decreased muscle protein synthesis was not observed in α-HICA-treated rats. Neither α-HICA nor leucine altered the increased proteasome activity and atrogene expression observed with immobilization. After 14 days of recovery, muscle mass had returned to control values only in the rats fed α-HICA, and this was associated with a sustained increase in protein synthesis and phosphorylation of S6K1 and 4E-BP1 of previously immobilized muscle.

Strength of evidence: Promising rodent data suggesting HICA may accelerate post-immobilization muscle recovery, but no human clinical trials have tested this specific application. Evidence is preclinical only.

5.4 Cachexia and Inflammatory Muscle Wasting

In vitro work using murine C2C12 myotubes modeled cancer-related muscle wasting. Myotube atrophy induced by TNFα/IFNγ co-exposure was significantly improved by HICA pretreatment, accompanied by inhibition of iNOS expression and IL-6 production. HICA also suppressed the TNFα/IFNγ co-exposure-induced secretion of 3-methylhistidine. These results demonstrated that HICA decreases basal protein synthesis under normal or cachexic conditions; however, HICA might attenuate skeletal muscle atrophy via maintaining a low level of protein degradation under cachexic conditions.

Strength of evidence: In vitro (cell culture) only. No clinical trials exist for cachexia. Evidence is very preliminary.

5.5 Antimicrobial Activity

Antibacterial Properties

Results from a 2022 PLOS One study revealed that HICA was effective in inhibiting the growth of tested Gram-positive and Gram-negative bacteria, including a multi-drug resistant P. aeruginosa strain. HICA exhibited its activity via penetration of the bacterial cell membranes, causing depolarization, permeabilization, and rupture of membranes, with subsequent leakage of cellular contents and cell death.

Antifungal Properties

The efficacy of HICA against 19 clinical and reference isolates representing five Candida and three Aspergillus species with variable azole antifungal sensitivity profiles was tested using a microdilution method, at concentrations of 18, 36, and 72 mg/mL. HICA at 72 mg/mL was fungicidal against all Candida and Aspergillus fumigatus and Aspergillus terreus isolates. Lower concentrations were fungistatic. Aspergillus flavus was not inhibited by HICA. HICA has broad antifungal activity against Candida and Aspergillus at concentrations relevant for topical therapy. As a fungicidal agent with broad-spectrum bactericidal activity, it may be useful in the topical treatment of multispecies superficial infections.

Endodontic Applications

Research groups, primarily in Finland, have evaluated HICA as a potential intracanal medicament in dental root canal treatment. Fusobacterium nucleatum, found in 30% of root canal infection cases evaluated, has been shown to be susceptible to HICA bactericidal activity. However, additional clinical studies supporting the use of HICA as an intracanal medicament in regenerative endodontic procedures (REPs) are needed. The purpose of one study was to evaluate the antimicrobial activity of DL-2-hydroxyisocaproic acid on human pathogenic obligate anaerobic bacteria related to periodontitis, testing antimicrobial activity against 14 bacterial reference strains and clinical isolates of obligate anaerobic bacterial species using a microdilution method in concentrations of 1.25 to 160 mg/mL.

Antibacterial activity combined with its previously reported antifungal activity and safety profile suggests that HICA could be considered as a potential natural antimicrobial agent against food spoilage bacteria and pathogens.

Strength of evidence: All antimicrobial evidence is in vitro or ex vivo. No completed human clinical trials have assessed HICA as a systemic or topical antimicrobial therapy. Evidence is preliminary (in vitro / ex vivo).

5.6 Inflammatory Modulation

HICA's reversible inhibition of MMP-8 does not involve covalent bonding, positioning it as an enzyme modulator or downregulator rather than a direct inhibitor. This property of active MMP-8 reduction opens new avenues for therapeutic intervention, particularly in managing excessive inflammatory responses, such as the "cytokine storm" observed in lung tissue inflammation or arthritic joints such as in osteoarthritis.

Strength of evidence: Mechanistic in vitro and animal model data only. No human clinical trials have evaluated HICA specifically for inflammatory conditions. Evidence is very preliminary.

5.7 Bone Mineral Density

The only human data on bone density comes from the single case study described in Section 5.1. In one type 1 diabetic patient treated with α-HICA for 120 days, amelioration of BMD Z-scores from −0.7 to 0.5 and T-scores from −1.0 to −0.9 were noted. These observations cannot establish a causal relationship and require confirmation in controlled trials.

6. Body Systems Associated with HICA

Musculoskeletal system: Primary research focus. Muscle protein metabolism (anti-catabolic effects), lean mass, DOMS, disuse atrophy recovery, and skeletal muscle wasting (cachexia models).
Connective tissue: Direct in vitro inhibitory effects on matrix metalloproteinase enzymes responsible for degradation of various connective and protein tissues.
Immune and inflammatory system: Modulation of pro-inflammatory mediators (iNOS, IL-6, MMP-8, MMP-9, myeloperoxidase) demonstrated in cell and animal models.
Oral / dental system: Research application as a potential intracanal medicament and periodontal antiseptic, based on ex vivo and in vitro antimicrobial data.
General metabolism: As a product of leucine catabolism, HICA is integrated into the broader branched-chain amino acid metabolic network operative in liver, muscle, and connective tissue.

7. Dosage Forms and Reported Dosages

Dosages Used in Clinical Studies

Mero et al. (2010) — Soccer players, body composition / DOMS: 583 mg of the sodium salt of HICA (corresponding to 500 mg of HICA) mixed with liquid three times a day for 4 weeks — equating to a total of 1.5 g of HICA per day.
Teixeira et al. (2018) — Type 1 diabetic patient, case study: A type 1 diabetic patient was treated with supplemental α-HICA for 120 days. The specific dose used in the case study is not extractable from the abstract and PubMed record available.
Teixeira et al. (2019) — Resistance-trained men, body composition RCT: A double-blind randomized controlled pragmatic trial evaluated the effects of off-the-shelf supplements including α-HICA, HMB-FA, and HMB-Ca on RET-induced changes. Forty men were blocked-randomized; the training protocol consisted of a whole-body resistance training routine thrice weekly for 8 weeks. The specific HICA dose was the commercially available off-the-shelf product dose, consistent with 1.5 g/day as used in prior literature.

Reported Dosages in Animal Studies

Lang et al. (2013) — Wistar rat immobilization model: Adult 14-week-old male Wistar rats were provided a control diet or an isonitrogenous isocaloric diet containing either 5% α-HICA or leucine. This is a dietary inclusion percentage used in animal nutrition research and is not directly translatable to a human dose.

Dosage Forms

HICA is commercially supplied as a powder (free acid or sodium/calcium salt) formulated for dissolution in liquid, and is incorporated into capsules or ready-mixed sports supplement products. The 1.5 g/day dosing regimen derived from the Mero et al. (2010) trial is the most frequently cited reference point for human supplementation in the scientific literature.

8. Safety Considerations

Observed Safety Profile in Human Studies

In a 120-day case study, full hematologic measures and weekly nutritional counselling assessments revealed no signs of adverse effects with α-HICA supplementation. In the 4-week Mero et al. (2010) double-blind trial, no adverse effects were reported in the published record. Based on current research, HICA is considered safe for healthy adults. No significant adverse side effects were reported in the human trials utilizing the standard 1.5 g daily dose.

Cytotoxicity and Genotoxicity

In vitro safety evaluation has been conducted in the context of dental medicine. A previous study evaluated the cytotoxicity and genotoxicity of HICA and reported that it was safe at concentrations <10 mg/mL. All minimum inhibitory concentration values obtained in antibacterial studies were well below 10 mg/mL. 2-Hydroxyisocaproic acid is not cytotoxic and genotoxic at concentrations <10 mg/mL. At the concentration of 1 mg/mL, HICA is significantly less cytotoxic than calcium hydroxide.

Evidence Gaps

The overall evidence base is very limited. No long-term safety studies (beyond 120 days) in humans have been published in peer-reviewed literature. Pharmacokinetic data in humans (absorption, distribution, metabolism, and excretion) have not been fully characterized in published peer-reviewed form. While data indicate that HICA is expected to absorb into the blood stream upon oral administration to rats, definitive human pharmacokinetic profiling has not been published. The total number of human subjects exposed to HICA in controlled trials is fewer than 100 across all published studies, making detection of rare adverse events impossible.

Potential Interactions and Special Populations

No drug interaction studies specific to HICA have been published in peer-reviewed literature. As a leucine metabolite that participates in amino acid catabolism, theoretical interactions with medications affecting branched-chain amino acid metabolism (e.g., certain metabolic disease treatments) or nitrogen balance cannot be excluded but have not been studied. The compound has been used in at least one case study involving a type 1 diabetic patient without reported adverse glycemic effects, though additional research is necessary to confirm these promising results and to clarify whether leucine and/or one of its derivatives might be clinically useful in such populations.

9. Summary of Evidence Strength

Lean mass / body composition: Mixed; one small positive RCT (n = 15, soccer players), one larger negative RCT (n = 40, resistance-trained men). Overall: insufficient and conflicting.
DOMS reduction: One small positive RCT; no independent replication. Overall: preliminary.
Disuse atrophy recovery: Positive rodent data; no human trials. Overall: preclinical only.
Cachexia / inflammatory muscle wasting: In vitro data only. Overall: very preliminary.
Antimicrobial (antibacterial, antifungal): In vitro and ex vivo data demonstrating broad-spectrum activity; no human trials. Overall: preliminary in vitro.
Inflammation / MMP inhibition: In vitro and animal model data only. Overall: very preliminary.
Bone mineral density: Single case report; not confirmatory. Overall: insufficient.

References

Health Conditions

Health conditions that Alpha hydroxyisocaproic acid may help support.

No conditions available.

Body Systems

Body systems that Alpha hydroxyisocaproic acid may help support.

No body systems available.