L-histidine

L-Histidine: A Comprehensive Reference

1. Identity and Chemical Characterization

1.1 Names and Classification

Histidine (symbol His or H) is an essential amino acid used in the biosynthesis of proteins. It contains an α-amino group (in the protonated –NH₃⁺ form under biological conditions), a carboxylic acid group (in the deprotonated –COO⁻ form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Its three-letter code is His, its one-letter code is H, and its systematic name is 2-Amino-3-(1H-imidazol-4-yl)propanoic acid (IUPAC-IUB 1983). Its chemical formula is C₆H₉N₃O₂, and it has a molecular weight of 155.16 g/mol.

Histidine is an amino acid having the chemical name 2-amino-3-(3H-imidazol-4-yl)propanoic acid. It exists in two enantiomeric forms, L-histidine and D-histidine. References to a scalemic mixture include the enantiomers in any relative proportions, while a racemic mixture contains the enantiomers in the ratio 50:50. The L-isomer of histidine, which is the only form involved in protein synthesis, is one of the 20 standard amino acids common in animal proteins and required for normal functioning in humans.

Histidine is variously classified as a "conditionally essential" or "essential" amino acid. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also.

1.2 Discovery and History of Isolation

Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896. The name stems from its discovery in tissue, from the Greek ἱστός (histós), meaning "tissue." In 1896, Kossel discovered histidine while working out the classical method for the quantitative separation of the "hexone bases" (the alpha-amino acids arginine, histidine, agmatine, and lysine).

1.3 Natural Sources

Histidine is one of the nine essential amino acids humans must obtain from their diet and is present in most protein-rich foods such as meat, fish, eggs, soy, whole grains, beans, and nuts. Histidine is found in fruits such as bananas and grapes, meat and poultry, and milk and milk products. It is also found in root vegetables and all green vegetables, though in lesser quantities. Average daily intake of HIS is about 800 mg in adult humans.

1.4 Common Supplement Forms

L-histidine is available as a dietary supplement in the form of free-base L-histidine capsules or tablets and as L-histidine monohydrochloride (HCl) monohydrate — the salt form used in both food/feed additives and clinical research. Doses of up to 4 grams per day have been used in research without causing noticeable side effects. The free amino acid form is the naturally occurring enantiomer found in biological systems. L-histidine occurs naturally and is readily obtainable from natural sources. It may also be synthesised from suitable starting materials using standard procedures of organic chemistry, or may be isolated from natural sources using well-known procedures.

2. Traditional and Historical Use

2.1 Historical Research Context

Initially, HIS was shown to treat rheumatoid arthritis and anaemia in patients with chronic renal failure. Currently, HIS and/or HIS-containing dipeptides (HIS-CD) are investigated to prevent fatigue during strenuous exercise and for therapy in ageing-related disorders, metabolic syndrome, atopic dermatitis, ulcers, inflammatory bowel diseases, ocular diseases, and neurological disorders. Early clinical use of histidine as a targeted supplement dates to the 1970s. A double-blind, randomized, placebo-controlled trial of L-histidine in rheumatoid arthritis was published in the Journal of Rheumatology in 1977, and reports of histidine for uraemic anaemia appeared in the British Medical Journal in 1973. These represented the first attempts to translate biochemical understanding into therapeutic supplementation.

The histidine-tryptophan-ketoglutarate (HTK) solution, also known as Bretschneider's solution, was developed in the early 1970s by German physiologist Hans Jürgen Bretschneider at the University of Göttingen in Germany. Bretschneider, a pioneer in cardioplegia research, created HTK as an intracellular-type electrolyte solution specifically designed for myocardial protection during cardiac procedures. The first human application of HTK solution as a cardioplegic agent occurred in 1980 during clinical heart transplantation procedures at German clinics. This initial deployment was followed by rapid integration into European cardiac surgery protocols, with widespread adoption across clinics by the mid-1980s. Commercialized as Custodiol by Köhler Pharma, HTK received regulatory approval in Germany in the late 1980s, facilitating broader distribution and standardization in European transplant centers.

L-histidine does not have a documented history of use in traditional herbal or botanical medicine systems (such as Ayurveda, Traditional Chinese Medicine, or Western herbalism), as it is not a botanical compound but a constituent amino acid identified through 19th-century biochemical research. Its therapeutic exploration is entirely a product of the modern era of nutritional biochemistry.

3. Key Constituents, Active Compounds, and Mechanisms of Action

3.1 Structure and Fundamental Chemical Properties

Histidine's importance to the human body derives from the properties determined by its distinctive structure. The molecule's side chain is composed of a heterocyclic imidazole ring that contains nitrogen atoms at positions 1 (pi) and 3 (tau). It is ionizable and exists in neutral and protonated forms in the body, giving histidine a pK one pH-unit below neutrality, allowing it to be both acid and base at physiologic pH. The imidazole ring of histidine is aromatic, which confers stability and makes it apolar at physiologic pH.

The most notable feature of histidine is its imidazole side chain, which gives histidine unique properties, such as acting as a proton donor or acceptor, making it highly reactive and essential for enzyme function and other biochemical processes. The pKa of histidine's imidazole ring is around 6.0, making it one of the few amino acids with a side chain that can donate or accept protons at physiological pH.

3.2 Proton Buffering

The unique chemical properties of HIS, which are mainly attributed to the imidazole ring, include proton buffering, metal ion chelation, and antioxidant activities. These cytoprotective interactions may involve free HIS, HIS-containing peptides, HIS-containing dipeptides, and HIS residues in proteins. The imidazole side chains and the relatively neutral pKa of histidine (ca 6.0) mean that relatively small shifts in cellular pH will change its charge. For this reason, this amino acid side chain finds considerable use as a coordinating ligand in metalloproteins, and also as a catalytic site in certain enzymes, such as aiding the catalytic functions of chymotrypsin (digestive enzyme) and those enzymes involved with metabolism of proteins and carbohydrates.

3.3 Role in Enzyme Catalysis

Histidine's unique acid/base properties make it a versatile catalytic residue in many enzymes and for those proteins and enzymes that coordinate metal ions. Serine esterases, such as trypsin, chymotrypsin, acetylcholinesterase, and the various enzymes in the blood-clotting cascade provide one excellent example of histidine's involvement as a catalytic residue. In these cases, histidine lies between a catalytic aspartate (which serves as a specific base), pulling a proton from the imidazole group, thereby making the imidazole group a better base to abstract/polarize the hydrogen ion from the active-site serine hydroxyl.

3.4 Metal Ion Chelation

Histidine is also a good chelator of metal ions like copper, zinc, manganese, and related divalent metals. Histidine and carnosine can form complexes with divalent metal ions such as Fe²⁺, potentially providing stability to intracellular labile iron. This chelating capacity is relevant both to histidine's antioxidant function and its role in erythropoiesis.

3.5 Precursor Roles: Histamine, Carnosine, and Other Derivatives

Histidine is a nutritionally essential amino acid precursor for several hormones (e.g., thyrotropin-releasing hormone) and critical metabolites affecting renal function, neurotransmission, gastric secretion, and the immune system. It is the precursor of histamine and important in the synthesis of purines.

Several HIS-rich proteins (e.g., haemoproteins, HIS-rich glycoproteins, histatins, HIS-rich calcium-binding protein, and filaggrin), HIS-containing dipeptides (particularly carnosine), and methyl- and sulphur-containing derivatives of HIS (3-methylhistidine, 1-methylhistidine, and ergothioneine) have specific functions.

HIS is a precursor of several dipeptides, such as carnosine (beta-alanyl-L-histidine) and anserine (beta-alanyl-N1-methylhistidine), which are found almost exclusively in skeletal muscle. In humans, only carnosine is present (both carnosine and anserine are present in rats). These peptides play a significant role in proton buffering and it is believed that an increase in their intramuscular concentration improves high-intensity exercise performance and reduces muscle fatigue.

3.6 Metabolism

Most HIS metabolism is directed to protein turnover and catabolism to glutamate. The minor pathways, such as synthesis of carnosine, histamine, and HIS-rich proteins, make HIS unique among other amino acids. The main pathway of HIS catabolism begins with deamination catalysed by histidase (EC 4.3.1.3), leading to the production of trans-urocanate and ammonia. The enzyme is primarily located in the stratum corneum of the skin and the liver.

Suggested pathogenic mechanisms of high-dose HIS include increased flux of HIS through the HIS degradation pathway (increases in ammonia and glutamate), increased ammonia detoxification to glutamine and exchange of the BCAA with glutamine via L-transporter system in muscles (increase in glutamine and decrease in BCAA), and tetrahydrofolate depletion (decrease in glycine).

4. Body Systems and Health Areas Associated with L-Histidine

L-histidine (HIS) is an essential amino acid with unique roles in proton buffering, metal ion chelation, scavenging of reactive oxygen and nitrogen species, erythropoiesis, and the histaminergic system. The principal health areas in which L-histidine has been studied or proposed for use include:

• Metabolic health: Insulin resistance, metabolic syndrome, obesity, dyslipidaemia
• Skin barrier: Atopic dermatitis / eczema; filaggrin function
• Haematology/nephrology: Uraemic (renal) anaemia
• Rheumatology: Rheumatoid arthritis
• Surgical/transplant medicine: Myocardial protection and organ preservation (HTK solution)
• Gastroenterology: Cholera-associated diarrhoea, ulcers, inflammatory bowel disease
• Neurology: Neurological disorders (histaminergic system modulation)
• Exercise physiology: Muscle buffering via carnosine synthesis, fatigue resistance

5. Scientific Evidence by Area of Use

5.1 Metabolic Syndrome and Insulin Resistance

Serum histidine levels are lower and are negatively associated with inflammation and oxidative stress in obese women. This observation formed the basis for a randomized, controlled intervention.

Key RCT (Feng et al., 2013, Diabetologia): A total of 100 obese women aged 33–51 years with BMI ≥ 28 kg/m² and diagnosed with metabolic syndrome were included in this randomised, double-blinded, placebo-controlled trial. Participants received 4 g/day histidine (n = 50) or identical placebo (n = 50) for 12 weeks. Serum histidine, HOMA-IR, BMI, waist circumference, fat mass, serum NEFA, and variables connected to inflammation and oxidative stress were measured at baseline and 12 weeks. Histidine supplementation improved insulin resistance, reduced BMI, fat mass and NEFA, and suppressed inflammation and oxidative stress in obese women with MetS; histidine was found to improve IR through suppressed pro-inflammatory cytokine expression, possibly by the NF-κB pathway, in adipocytes.

Unfortunately, the articles reporting data from the clinical trials performed to date are rare and have various limitations, particularly because only a small number of subjects were evaluated, and sometimes the results have been presented by only one research group.

Increasing evidence indicates that glycine and histidine supplementation can be a novel therapy for metabolic diseases; particularly, histidine improves both hyperlipidemia and metabolic syndrome.

Evidence strength: One well-designed RCT (n = 100) shows statistically significant improvements in insulin resistance, adiposity, and inflammatory markers at 4 g/day for 12 weeks. Findings have not yet been independently replicated in large multi-centre trials or in male/diverse populations. Evidence is therefore promising but preliminary.

5.2 Atopic Dermatitis (Eczema)

Atopic dermatitis (AD) etiology has been linked to deficiencies in the skin barrier protein, filaggrin. In mammalian skin, L-histidine is rapidly incorporated into filaggrin. Subsequent filaggrin proteolysis releases L-histidine as an important natural moisturizing factor (NMF).

In vitro: In vitro studies demonstrated that L-histidine significantly increased both filaggrin formation and skin barrier function (P<0.01, respectively).

Clinical pilot study (Tan et al., 2017, Clinical, Cosmetic and Investigational Dermatology): In a clinical pilot study, adult subjects (n = 24) with AD took either a placebo or 4 g oral L-histidine daily for 8 weeks. Unlike the placebo, L-histidine reduced AD (34% reduction in SCORing Atopic Dermatitis scores; P<0.003) after 4 weeks. No improvement was noted with the placebo (P>0.32). The clinical effect of oral L-histidine in AD was similar to that of mid-potency topical corticosteroids, and combined with its safety profile suggests that it may be a safe, nonsteroidal approach suitable for long-term use in skin conditions associated with filaggrin deficits such as AD.

Evidence strength: One small pilot RCT (n = 24) and supportive in vitro data. Results are promising and mechanistically coherent, but the small sample size and pilot nature mean findings must be confirmed in larger trials.

5.3 Rheumatoid Arthritis

Proposed therapeutic uses of histidine include management of rheumatoid arthritis. Although individuals with rheumatoid arthritis may exhibit lower histidine levels, research has yet to establish definitive benefits of histidine supplementation for this condition. A randomized, placebo-controlled, double-blind trial (Pinals, Harris, Burnett, and Gerber, Journal of Rheumatology, 1977) tested L-histidine in rheumatoid arthritis patients. Some speculation has been raised that histidine supplements might be a good treatment for this kind of arthritis, but no studies have confirmed this definitively.

Evidence strength: Weak. The trial dates to 1977 and has not been replicated with modern methodology. Epidemiological observation of lower serum histidine in RA patients does not establish therapeutic causality.

5.4 Uraemic Anaemia (Anaemia of Chronic Renal Failure)

HIS has been used in therapy of rheumatoid arthritis and anaemia of patients with chronic renal failure. Interest in L-histidine for uraemic anaemia dates to the early 1970s (Giordano et al., British Medical Journal, 1973). The mechanistic rationale centres on histidine's role in erythropoiesis and iron chelation. Anaemia is a common comorbidity in the late stages of kidney disease, and patients are treated with erythropoiesis-stimulating agents and iron supplementation. Evidence from animal research indicates that supplementing histidine and carnosine can counteract the excessive production of reactive oxygen species and reduce tissue damage associated with iron loading, and that this supplementation plays a role in correcting anaemia in chronic kidney disease and iron-overload related conditions.

Evidence strength: Early clinical reports (1970s) and more recent preclinical data provide a mechanistic rationale, but robust, contemporary human RCTs specifically for uraemic anaemia remain absent. Evidence is preliminary.

5.5 Organ Preservation and Myocardial Protection (HTK Solution)

HTK solution is intended for perfusion and flushing of donor liver, kidney, heart, lung, and pancreas prior to removal from the donor and for preserving these organs during hypothermic storage and transport to the recipient. HTK solution is based on the principle of inactivating organ function by withdrawal of extracellular sodium and calcium, together with intensive buffering of the extracellular space by means of histidine/histidine hydrochloride, so as to prolong the period during which the organs will tolerate interruption of oxygenated blood.

Histidine buffers the ischemia-induced acidosis, therefore improving anaerobic glycolysis. Tryptophan is an effective cell membrane stabilizer. Ketoglutarate is a Krebs cycle intermediate, which enhances energy production and recovery following reperfusion.

Systematic review and meta-analysis (2022): A systematic review and meta-analysis included 12 trials (n = 1,327). HTK solution resulted in significantly shorter intensive care unit stay (MD = −0.09; 95% CI [−0.15, −0.03], p = 0.006), and shorter hospital stay (MD = −0.51; 95% CI [−0.71, −0.31], p < 0.001) compared with multidose cardioplegia.

Several decades of experience have confirmed the effectiveness of HIS as a component of solutions used for organ preservation and myocardial protection in cardiac surgery.

Evidence strength: Strong and well-established in the surgical/transplantation context. Histidine as a buffering agent in HTK solution has meta-analytic support from RCTs and decades of clinical deployment.

5.6 Diarrhoea Associated with Cholera

Drinking a solution containing histidine seems to reduce diarrhoea in people with cholera who are also receiving antibiotics. A double-blind, randomized trial (Rabbani et al., Journal of Infectious Diseases, 2005) examined antidiarrheal effects of L-histidine-supplemented rice-based oral rehydration solution in male adults with severe cholera in Bangladesh.

Evidence strength: Moderate. One randomized trial in a specific, severe infectious disease context. Evidence does not generalize to diarrhoea of other causes.

5.7 Obesity and Appetite Regulation

Clinical relevance of these findings is suggested by evidence that use of prescription antihistamines (H1 blockers), or of antipsychotic drugs that also inhibit H1, is associated with an increased risk for obesity. A recent Chinese cross-sectional epidemiology study correlated dietary histidine inversely with BMI, waist circumference and various markers of metabolic syndrome, in both sexes; this finding remained valid whether daily histidine intake was expressed in absolute terms, or after adjustment for protein and other dietary values. In a cross-sectional study enrolling female Japanese students, daily histidine intake correlated inversely with daily calorie intake after adjustment for other dietary factors, consistent with the possibility that increased brain histidine uptake can aid appetite control in humans, as it does in rodents.

Evidence strength: Preliminary. Evidence comes from cross-sectional epidemiology and animal studies; intervention trials addressing appetite and obesity per se are lacking.

5.8 Neurological Disorders

Histidine and histidine-containing peptides are under scrutiny for their effectiveness in preventing ageing-related disorders such as atherosclerosis, neurological disorders (including Alzheimer's disease), cancer, metabolic syndrome, and obesity. The theoretical basis rests on histidine's role as a precursor to histamine and its function in the central histaminergic system, which is involved in arousal, appetite, and memory. Further studies are needed to elucidate the effects of HIS supplementation on neurological disorders, atopic dermatitis, metabolic syndrome, diabetes, uraemic anaemia, ulcers, inflammatory bowel diseases, malignancies, and muscle performance during strenuous exercise.

Evidence strength: Very weak in humans. Mainly mechanistic rationale and animal/in vitro data. No human clinical trials for neurological indications have been conducted or reported.

5.9 Exercise Performance and Muscle Buffering

Histidine is investigated for therapy of various disorders and as a nutritional supplement to enhance muscle performance. The primary mechanism is via carnosine synthesis in skeletal muscle; carnosine acts as an intramuscular proton buffer during high-intensity exercise. The reports of HIS supplementation on content of HIS-containing dipeptides in skeletal muscle are not entirely conclusive.

Evidence strength: Indirect and inconclusive. Most exercise buffering evidence relates to beta-alanine (the other component of carnosine), not to L-histidine supplementation directly. Whether oral L-histidine supplementation meaningfully raises muscle carnosine in humans is not established.

6. Dosage Forms and Dosages Reported in Studies

The following dosages are as reported in cited sources; they are not recommendations:

• Metabolic syndrome RCT (Feng et al., 2013): Participants received 4 g/day histidine (n = 50) or identical placebo (n = 50) for 12 weeks.
• Atopic dermatitis pilot study (Tan et al., 2017): Adult subjects (n = 24) with AD took either a placebo or 4 g oral L-histidine daily for 8 weeks.
• General research range: Doses of up to 4 grams per day have been used in research without causing noticeable side effects.
• Animal research (Holeček laboratory): Rats consumed HIS in drinking water at a dose of 0.5 g/L (low HIS), 2 g/L (high HIS) or 0 g/L (control) for 4 weeks.

No established tolerable upper intake level (UL) has been formally set by the NIH Office of Dietary Supplements or EFSA specifically for L-histidine in healthy adults, and dosing guidance in human populations remains a gap in the literature.

7. Safety Considerations and Interactions

7.1 General Safety Profile

Signs of toxicity, mutagenic activity, and allergic reactions have not been reported. When taken by mouth, histidine is commonly consumed as part of the diet. Histidine supplements are possibly safe when used short-term.

7.2 Liver Enlargement and Hypercholesterolaemia

Hypercholesterolaemia and liver enlargement have been reported after long-term intake of HIS. A long-term intake of high amounts of HIS is not suitable for people with impaired liver function.

7.3 Ammonia Production and Branched-Chain Amino Acid Depletion

A high HIS concentration increases HIS flux through the HIS degradation pathway, resulting in increased ammonia production and altered concentrations of several amino acids, particularly increased concentrations of glutamate, alanine, and glutamine and decreased branched-chain amino acids (BCAA) concentrations in the blood plasma. Increased ammonia and glutamine and decreased BCAA levels in HIS-treated subjects indicate that HIS supplementation is inappropriate in patients with liver injury.

A high ammonia concentration in HIS-loaded subjects, which might occur after an infusion of HTK solution during cardiac surgery, may exert detrimental effects on the course of the underlying disease, particularly in subjects with impaired hepatic function.

7.4 Effects on Zinc and Copper Metabolism

There is evidence from studies in experimental animals and humans that intakes of high levels of histidine can alter copper and zinc metabolism and cause deficiencies of the free forms of these metal ions due to increased excretion. High dietary histidine levels have been shown to result in potentially serious adverse effects in both animals (e.g., hyperlipidaemia, hypercholesterolaemia, enlarged liver) and humans (e.g., increase in urinary zinc, headache, weakness).

7.5 Interaction with Beta-Alanine Supplementation

A substantial decrease in HIS content (~30%) in muscles and plasma after beta-alanine (BA) supplementation has been reported. Obviously, further studies are needed to determine whether BA supplementation requires a concomitant increase in histidine intake. This is clinically relevant because beta-alanine is widely used in sports nutrition to raise muscle carnosine; it may deplete endogenous histidine stores.

7.6 Folate Metabolism Interaction

If you have folic acid deficiency, don't use histidine. It can cause an unwanted chemical called formiminoglutamic acid (FIGLU) to build up in the body. This interaction arises because the catabolism of histidine depends on tetrahydrofolate (THF) as a cofactor; in folate deficiency, the degradation pathway is impaired and FIGLU accumulates.

It has been suggested that the drain of the cellular pool of THF by dietary HIS supplementation might improve methotrexate efficacy and might enable reduced dosing of this toxic agent. This potential pharmacodynamic interaction with methotrexate (used in rheumatoid arthritis and oncology) has been proposed theoretically but not established in clinical trials.

7.7 Considerations Related to Histamine Precursor Status

Signs of toxicity, mutagenic activity, and allergic reactions or peptic ulcers have not been reported, although HIS is a histamine precursor. Theoretically, supplemental HIS could increase histamine production, with implications for individuals with histamine intolerance or conditions sensitive to histamine. However, this remains a theoretical concern rather than a documented clinical risk at supplemental doses studied to date.

7.8 Histidinemia

A deficiency in histidase (the enzyme that catalyses the first step in histidine catabolism) is present in the rare metabolic disorder histidinemia. In this context, exogenous L-histidine supplementation would be contraindicated.

References

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