Amino Acids

L-Glutamin

L-Glutamine

L-Glutamine (CAS 56-85-9; molecular formula C₅H₁₀N₂O₃; MW 146.15 g/mol) is the most abundant free amino acid in human blood and skeletal muscle, constituting up to 60% of the total free amino-acid pool. Classified as conditionally essential, it becomes critically depleted during surgery, trauma, severe infection, and intense exercise. First isolated in 1883 by Albrecht Kossel from wheat gluten, glutamine is today recognized as a metabolic keystone: it fuels intestinal enterocytes and immune lymphocytes, donates nitrogen for nucleotide and glutathione synthesis, and serves as an anaplerotic substrate feeding the TCA cycle. Oral supplementation at 2–30 g/day has been studied for gut mucosal protection, chemotherapy-induced mucositis reduction, immune support in critical illness, and sports recovery. Clinical evidence is strongest for oral mucositis prophylaxis in oncology and gut-barrier maintenance in surgical patients. High-dose intravenous glutamine in ICU settings has shown potential harm in specific subgroups, warranting specialist supervision. Regulated as a dietary supplement under DSHEA in the US, L-glutamine powder is widely available from reputable brands (NOW Foods, Thorne, Jarrow) at $10–100/month depending on dose and form. This guide synthesizes current biochemistry, pharmacokinetics, evidence-based benefits, safety, and practical selection criteria for the US consumer.

N-Acetyl-Cystein

N-Acetyl-L-cysteine

N-Acetyl Cysteine (NAC) is a sulfur-containing amino acid derivative with a molecular formula of C₅H₉NO₃S and a molar mass of 163.19 g/mol, synthesized by N-acetylation of L-cysteine. First developed in the 1960s as a mucolytic agent, NAC has since evolved into one of the most clinically versatile nutraceuticals and pharmaceuticals available. Its defining biochemical role is dual: it acts as a direct thiol antioxidant — scavenging reactive oxygen species through its free sulfhydryl group — and as the rate-limiting precursor for intracellular glutathione biosynthesis, the body's most abundant endogenous antioxidant. These properties underpin NAC's established use as the gold-standard antidote for acetaminophen overdose, its mucolytic applications in COPD and chronic bronchitis, and a rapidly expanding body of research spanning psychiatric disorders, male fertility, renal protection, and respiratory infections. Oral bioavailability of intact NAC is low (approximately 4–10%), yet effective glutathione augmentation occurs through rapid deacetylation to cysteine. Available in oral capsule, effervescent, intravenous, and nebulized forms, NAC is generally well tolerated at supplement doses of 600–1,200 mg/day, with gastrointestinal upset being the most common adverse effect. In the US market, NAC occupies a unique and contested regulatory space between pharmaceutical drug and dietary supplement under DSHEA.

5-HTP

5-Hydroxytryptophan

<strong>5-Hydroxytryptophan (5-HTP)</strong> is a naturally occurring amino acid and direct precursor to serotonin, the neurotransmitter crucial for mood, sleep, and appetite regulation. Unlike L-tryptophan, <strong>5-HTP crosses the blood-brain barrier efficiently</strong> and converts directly to serotonin without competing with other amino acids. Extracted primarily from <em>Griffonia simplicifolia</em> seeds, this compound has gained significant attention in modern neuroscience for its therapeutic potential. <mark>Clinical studies demonstrate its efficacy in treating depression, improving sleep quality, supporting weight management, and alleviating fibromyalgia symptoms.</mark> With over <strong>200 published studies</strong> and a generally favorable safety profile, 5-HTP represents a bridge between traditional herbal medicine and modern neuropharmacology. The FDA recognizes it as a dietary supplement under DSHEA, making it accessible to millions of Americans seeking natural alternatives to prescription medications for mood and sleep disorders.

L-Theanin

N-Ethyl-L-glutamine

L-Theanine is a non-proteinogenic amino acid — specifically the gamma-ethylamide of L-glutamic acid — found almost exclusively in the leaves of <em>Camellia sinensis</em> (tea plant), first isolated by Japanese researchers in 1949. With the molecular formula C₇H₁₄N₂O₃ and a molar mass of 174.2 g/mol, it represents one of the most studied psychoactive nutraceuticals in the modern supplement market. L-Theanine crosses the blood–brain barrier via L-type amino acid transporters, elevating GABA levels, modulating glutamatergic neurotransmission, and increasing alpha-band EEG activity to produce a state of calm alertness — without sedation. Clinically, it is best known for reducing stress and anxiety, improving sleep quality, and synergizing with caffeine (commonly at a 2:1 ratio) to sharpen focus while blunting jitteriness. Typical supplement doses range from 100–400 mg/day, with an excellent safety profile at these levels. Sold as a dietary supplement under DSHEA in the United States, L-Theanine is available from reputable brands such as Thorne, NOW Foods, and Pure Encapsulations, often standardized to the high-purity Suntheanine® ingredient. It represents a uniquely evidence-backed option for individuals seeking relaxation, cognitive enhancement, and sleep support without pharmaceutical sedation.

BCAA

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs) — comprising L-leucine, L-isoleucine, and L-valine — are three of the nine essential amino acids that humans cannot synthesize and must obtain through diet or supplementation. Together they account for approximately 35–40% of the essential amino acids in muscle protein, making them uniquely critical for athletic performance, recovery, and clinical nutrition. Unlike most amino acids, BCAAs are preferentially catabolized in skeletal muscle rather than the liver, a metabolic distinction with profound physiological implications. Leucine, the most anabolically potent of the trio, functions not merely as a building block but as a powerful signaling molecule that directly activates the mTORC1 pathway — the master regulator of muscle protein synthesis. Commercially produced via microbial fermentation and sold as powders, capsules, and clinical formulations, BCAAs are among the best-selling sports supplements in the United States. Evidence supports their use for stimulating muscle protein synthesis, reducing delayed-onset muscle soreness, preserving lean mass during caloric restriction, and managing hepatic encephalopathy in clinical settings. This guide delivers a thorough, science-based review of BCAA biochemistry, pharmacokinetics, dosing, safety, drug interactions, and quality-selection criteria — written to the standard of a physician-researcher.

L-Leucin

L-Leucine

L-Leucine is an essential, branched-chain amino acid (BCAA) with the molecular formula C₆H₁₃NO₂ (CAS: 61-90-5) that the human body cannot synthesize — making dietary and supplemental intake obligatory for survival, growth, and muscle maintenance. First isolated in 1819 from protein hydrolysates, leucine has since emerged as one of the most pharmacologically significant proteinogenic amino acids, functioning simultaneously as a building block for skeletal muscle proteins and as a potent intracellular signaling molecule. Its defining characteristic is the ability to directly activate mTORC1 (mechanistic target of rapamycin complex 1), the master regulator of muscle protein synthesis (MPS), via dedicated leucine-sensing proteins including Sestrin2 and leucyl-tRNA synthetase. Clinical research consistently demonstrates that achieving a per-meal leucine "threshold" of approximately 2.5–3.0 g is necessary to maximally stimulate MPS — a threshold particularly critical for older adults experiencing anabolic resistance. Available as free-form powder, capsules, BCAA blends, and leucine-enriched proteins, L-leucine is widely used by athletes, aging adults combating sarcopenia, and clinical patients in catabolic states. In the US market, it is regulated as a dietary supplement under DSHEA (1994), with no pre-market FDA approval required. This comprehensive guide covers its biochemistry, pharmacokinetics, clinical evidence, dosing, safety, and US-market considerations.

L-Isoleucin

L-Isoleucine

L-Isoleucine is one of three essential branched-chain amino acids (BCAAs) — alongside leucine and valine — that the human body cannot synthesize and must obtain entirely from diet or supplementation. Classified chemically as (2S,3S)-2-amino-3-methylpentanoic acid (CAS 73-32-5), it carries the molecular formula C₆H₁₃NO₂ and a molar mass of 131.17 g/mol. Unlike most amino acids, isoleucine is preferentially metabolized in skeletal muscle rather than the liver, making it uniquely relevant to exercise physiology, recovery, and lean mass preservation. Its documented roles span muscle protein synthesis support, glucose uptake enhancement via PI3K/Akt and AMPK signaling, modulation of mTORC1 activity, and competition at the blood–brain barrier for large neutral amino acid transporters — indirectly influencing central serotonin production. Clinically, isoleucine is included in BCAA formulations for liver disease, trauma recovery, sarcopenia management, and sport nutrition. Commercially produced via microbial fermentation using engineered Corynebacterium glutamicum strains, it is widely available in the US as free-form powder, capsules, and BCAA blends regulated under DSHEA. Typical supplemental doses range from 0.5 to 3 g/day, with a well-established safety profile at these amounts and specific precautions for patients on levodopa or hypoglycemic medications.

L-Valin

L-Valine

L-Valine is an essential branched-chain amino acid (BCAA) — one of only three BCAAs alongside leucine and isoleucine — that the human body cannot synthesize and must obtain entirely from diet or supplementation. With the molecular formula C₅H₁₁NO₂ and CAS number 72-18-4, L-Valine is a proteinogenic amino acid characterized by its aliphatic isopropyl side chain and (S)-configuration at the alpha carbon. First isolated by Emil Fischer's laboratory in 1901, valine is metabolized primarily in skeletal muscle via the branched-chain aminotransferase (BCAT) and branched-chain alpha-ketoacid dehydrogenase (BCKD) enzyme systems — a metabolic distinction that makes it uniquely important for muscle physiology. Clinical evidence supports its role in muscle protein synthesis, nitrogen balance maintenance, hepatic encephalopathy management, and parenteral/enteral clinical nutrition. Emerging metabolomic research identifies elevated circulating valine as a biomarker of insulin resistance and cardiometabolic risk. In the US market, L-Valine is regulated as a dietary ingredient under DSHEA, widely available in BCAA blends and standalone powder or capsule form at prices ranging from $10–$100+ depending on certification grade. Estimated physiological requirement for adults is approximately 24 mg/kg/day (~1.7 g/day for a 70 kg adult).

L-Arginin

L-Arginine

L-Arginine is a conditionally essential, proteinogenic amino acid (CAS 74-79-3; molecular formula C₆H₁₄N₄O₂; MW 174.20 g/mol) that serves as the sole physiological substrate for nitric oxide synthase (NOS) enzymes — making it indispensable for vascular health, erectile function, immune defense, and metabolic signaling. Discovered in 1886 by Ernst Schulze and Ernst Steiger from lupin seeds, arginine gained transformative scientific relevance in 1998 when the Nobel Prize in Physiology or Medicine was awarded for nitric oxide biology. In the body, arginine is converted by eNOS, nNOS, and iNOS into nitric oxide (NO), a gaseous signaling molecule that relaxes blood vessel walls, reduces platelet aggregation, and supports erectile physiology. Arginine also fuels the urea cycle, donates the guanidino group for creatine biosynthesis, and activates the mTORC1 nutrient-sensing complex via lysosomal sensors. Clinical evidence supports its use for endothelial dysfunction, mild-to-moderate hypertension, erectile dysfunction, wound healing, and growth hormone stimulation. Standard supplemental doses range from 2–6 g/day. Oral bioavailability is moderate (20–45%) due to first-pass arginase metabolism, which is why L-citrulline is often preferred for sustained plasma arginine elevation. Available in multiple forms including free base, HCl salt, AAKG, and stabilized arginine silicate complexes, it is sold as a dietary supplement under FDA/DSHEA regulation in the United States.

L-Arginin-Hydrochlorid

L-Arginine hydrochloride

L-Arginine HCL (monohydrochloride) is the hydrochloride salt form of L-arginine — a conditionally essential amino acid and the sole biological precursor to nitric oxide (NO) in humans. With a molecular formula of C₆H₁₅ClN₄O₂ and a molar mass of approximately 210.66 g/mol, this white crystalline powder delivers roughly 83% elemental arginine by weight. First isolated in 1886 by Ernst Schulze and Ernst Klebs, arginine gained clinical prominence through its central role in the urea cycle, endothelial function, and mTOR anabolic signaling. Oral supplementation at 2–6 g/day supports endothelial-dependent vasodilation, modestly reduces blood pressure, assists with mild-to-moderate erectile dysfunction, and enhances perioperative wound healing. The HCl salt form offers superior crystalline stability and manufacturing consistency compared to the free base. Bioavailability ranges from 20–60% orally due to significant first-pass splanchnic metabolism. Key drug interactions include dangerous potentiation of nitrate-induced hypotension and caution with PDE5 inhibitors. In the US market, L-Arginine HCL is regulated as a dietary supplement under DSHEA and widely available at retailers from Amazon to GNC, priced between $10–$100/month depending on grade and brand.

L-Citrullin

L-Citrulline

L-Citrulline is a non-proteinogenic amino acid — CAS 372-75-8, molecular formula C₆H₁₃N₃O₃ — first isolated from watermelon in 1914 and now recognized as one of the most effective oral precursors to nitric oxide (NO) in the human body. Unlike L-arginine, citrulline bypasses hepatic first-pass arginase catabolism, reliably elevating plasma arginine and downstream NO production to support endothelial vasodilation, blood pressure regulation, and exercise performance. Synthesized endogenously in intestinal enterocytes as a urea cycle intermediate, citrulline is converted to arginine in renal proximal tubule cells via argininosuccinate synthase and lyase. Clinically, supplemental L-citrulline (3–6 g/day) has demonstrated benefits for borderline hypertension, mild-to-moderate erectile dysfunction, exercise capacity, muscle recovery, and ammonia clearance. In specialized pediatric metabolic medicine, it is a cornerstone therapy for select urea cycle disorders. Available as pure powder or citrulline malate blends, it is generally well-tolerated with mild GI side effects at standard doses. This comprehensive guide covers its biochemistry, pharmacokinetics, evidence-based benefits, optimal dosing, drug interactions, safety profile, and practical consumer guidance for the US market.

Citrullin-Malat

L-Citrulline DL-malate

Citrulline malate (CM) is a dietary supplement combining L-citrulline — a non-proteinogenic amino acid first isolated from watermelon in 1914 — with DL-malic acid, a natural Krebs-cycle intermediate found in apples and other fruits. Available in 1:1 or 2:1 (citrulline:malate) stoichiometries, CM is classified under DSHEA as a dietary supplement in the United States. Its primary mechanism of action is dual: L-citrulline is converted in the kidneys to L-arginine via argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL), raising systemic arginine levels more effectively than oral L-arginine itself, thereby fueling endothelial nitric oxide synthase (eNOS) and boosting nitric oxide (NO) production. Meanwhile, malate feeds the tricarboxylic acid (TCA/Krebs) cycle, potentially enhancing aerobic ATP turnover and attenuating metabolic fatigue. Clinically, CM at 6–8 g pre-workout has been studied for improvements in resistance exercise performance, reduction of delayed-onset muscle soreness (DOMS), blood pressure modulation, and adjunctive erectile function support. It is generally well tolerated in healthy adults, with the primary adverse effects being gastrointestinal at higher doses. Athletes should seek NSF Certified for Sport products; all consumers should verify stoichiometry on the Certificate of Analysis.

L-Lysin

L-Lysine

L-Lysine is an essential, positively charged (basic) amino acid with the molecular formula C₆H₁₄N₂O₂ and CAS number 56-87-1 that the human body cannot synthesize — making adequate dietary or supplemental intake non-negotiable for survival. As one of the 20 proteinogenic amino acids, lysine serves as an irreplaceable building block for all body proteins, plays a critical structural role in collagen cross-linking via lysyl oxidase, and regulates gene expression through histone post-translational modifications including acetylation, methylation, and ubiquitination. Its most widely recognized nutraceutical application in the US market is the prevention and management of recurrent Herpes Simplex Virus type 1 (HSV-1) cold sores, achieved by competitively inhibiting arginine — a nutrient essential for viral replication. Clinically studied at doses between 500 mg and 3,000 mg per day, L-lysine supplements are safe, well-tolerated, and broadly available as capsules, tablets, and powders from third-party-tested brands. Additional evidence supports its roles in supporting bone health, wound healing, muscle protein synthesis, and potentially anxiolytic effects via serotonergic modulation. This comprehensive, research-based guide covers L-lysine's biochemistry, pharmacokinetics, clinical benefits, drug interactions, safety profile, and how to select a high-quality supplement in the US market.

L-Tyrosin

L-Tyrosine

L-Tyrosine is a conditionally essential aromatic amino acid (IUPAC: (S)-2-amino-3-(4-hydroxyphenyl)propanoic acid; CAS: 60-18-4; molecular formula C₉H₁₁NO₃; MW 181.19 g/mol) that serves as the biochemical gateway to dopamine, norepinephrine, epinephrine, and thyroid hormones T3 and T4. First isolated from casein in 1846 by German chemist Justus von Liebig — the name derives from the Greek word for cheese, tyros — tyrosine is now one of the most scientifically studied amino acid supplements in the US market. Unlike most amino acids, tyrosine's cognitive and stress-resilience benefits are acutely demonstrable: multiple controlled human trials show that a single oral dose (100–150 mg/kg, or 500–2,000 mg in consumer settings) taken 30–120 minutes before a stressor can measurably counteract catecholamine depletion caused by sleep deprivation, cold exposure, or intense cognitive load. The US supplement market offers free-form L-tyrosine powder and capsules at $10–$80, regulated under DSHEA. It is especially critical in phenylketonuria (PKU), where impaired phenylalanine-to-tyrosine conversion makes dietary tyrosine conditionally essential. This guide covers chemistry, pharmacokinetics, mechanisms, clinical evidence, dosing, safety, and US-specific regulatory context.

N-Acetyl-L-Tyrosin

N-Acetyl-L-tyrosine

N-Acetyl L-Tyrosine (NALT) is an acetylated derivative of the amino acid L-tyrosine, with the molecular formula C₁₁H₁₃NO₄ and a molar mass of 223.23 g/mol. Classified as a nutraceutical dietary supplement under US DSHEA regulation, NALT is chemically synthesized by acetylating L-tyrosine's alpha-amino group, producing a compound that is more water-soluble than its parent amino acid. After oral ingestion, NALT undergoes enzymatic deacetylation — primarily via aminoacylase 1 (ACY1) in the gut mucosa and liver — regenerating free L-tyrosine, which then serves as the direct precursor for the catecholamine neurotransmitters dopamine, norepinephrine, and epinephrine via the tyrosine hydroxylase pathway. Clinically, NALT is marketed as a cognitive-support ingredient, particularly for acute stress resilience, working memory under sleep deprivation, and mental performance during high-demand situations. Typical supplemental doses range from 300–500 mg/day. While mechanistic evidence is robust, high-quality randomized controlled trials specifically isolating NALT (rather than L-tyrosine) remain limited. Significant drug interactions exist with MAOIs, sympathomimetics, and L-DOPA. NALT is regulated as a dietary supplement by the FDA and is available across major US retailers including Amazon, iHerb, and GNC.

L-Tryptophan

L-Tryptophan

L-Tryptophan (CAS 73-22-3) is an essential aromatic amino acid — one of only nine that humans cannot synthesize and must obtain from diet or supplementation. With a molecular formula of C₁₁H₁₂N₂O₂ and a molar mass of 204.23 g/mol, it serves as the biochemical gateway to serotonin, melatonin, kynurenine, and NAD⁺. Discovered in 1901 by Frederick Gowland Hopkins, tryptophan occupies a unique metabolic crossroads: it regulates sleep architecture via melatonin, stabilizes mood through serotonergic neurotransmission, modulates immune tolerance via the kynurenine–IDO pathway, and contributes to NAD⁺ biosynthesis. Brain uptake depends not on absolute plasma levels but on the competitive ratio of free tryptophan to other large neutral amino acids (LNAAs) at the blood–brain barrier LAT1 transporter. Clinical evidence supports supplemental doses of 300–1,000 mg/day for sleep onset, mood augmentation, and PMS/PMDD symptom relief. Safety is generally excellent at recommended doses; however, combination with SSRIs, MAOIs, or other serotonergic agents carries a serious risk of serotonin syndrome. Quality sourcing with third-party certification (USP, NSF) is essential given the historical 1989 eosinophilia–myalgia syndrome (EMS) outbreak linked to contaminated product from a single manufacturer.

L-Carnitin

L-Carnitine

L-Carnitine is a naturally occurring quaternary ammonium compound synthesized endogenously in the human kidney and liver from the amino acids lysine and methionine — with vitamin C, iron, vitamin B6, and niacin as obligate cofactors. Chemically designated as (3R)-3-hydroxy-4-(trimethylazaniumyl)butanoate, with the molecular formula C₇H₁₅NO₃ (MW 161.20 g/mol), it functions as the irreplaceable molecular shuttle that transports long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation and ATP generation. First isolated from muscle tissue in 1905 and named from the Latin carnis (meat), L-carnitine is concentrated in red meat and dairy and is now recognized as conditionally essential in genetic deficiency states and certain clinical conditions. As a dietary supplement sold under DSHEA in the US market, it exists in several key forms — plain L-carnitine, acetyl-L-carnitine (ALC), and propionyl-L-carnitine — each with distinct tissue penetration profiles and clinical applications spanning male fertility, exercise recovery, metabolic liver disease, peripheral neuropathy, and cognitive support. Oral bioavailability ranges from approximately 5–60% depending on dose and form. Generally well tolerated at ≤2 g/day, with dose-dependent gastrointestinal side effects at higher intakes. An emerging area of concern involves gut-microbial conversion to trimethylamine N-oxide (TMAO) and its debated cardiovascular implications.

Acetyl-L-Carnitin

Acetyl-L-carnitine

Acetyl-L-Carnitine (ALCAR), CAS 5080-50-2, is a naturally occurring amino acid derivative and acetylated form of L-carnitine that uniquely crosses the blood–brain barrier — a capability that makes it far more relevant for neurological and cognitive applications than its parent compound. With a molecular formula of C₉H₁₇NO₄ and a molar mass of 203.24 g/mol, ALCAR functions at the intersection of mitochondrial energy metabolism and cholinergic neurotransmission. It donates acetyl groups that fuel acetylcholine synthesis and replenish mitochondrial acetyl-CoA pools, supporting neuronal energy and synaptic communication simultaneously. Clinical research spanning more than four decades supports its use in age-related cognitive decline, peripheral neuropathy (including diabetic), male infertility, depressive symptoms in the elderly, and exercise recovery. Typical supplemental doses range from 500 mg to 2,000 mg per day, with clinical trials most commonly employing 1,000–2,000 mg daily in divided doses. Available as capsules, tablets, powder, and oral solutions, ALCAR is regulated as a dietary supplement under DSHEA in the United States. Its safety profile is well established at standard doses, with mild gastrointestinal effects being the most common adverse reactions. When combined with alpha-lipoic acid or CoQ10, ALCAR demonstrates notable synergistic mitochondrial benefits.

L-Carnitin-L-Tartrat

L-Carnitine L-tartrate

L-Carnitine L-Tartrate (LCLT) is a highly bioavailable salt form of L-carnitine — the conditionally essential amino acid derivative responsible for shuttling long-chain fatty acids into the mitochondrial matrix for energy production. Unlike free-form L-carnitine, LCLT combines L-carnitine with L-tartaric acid in a 2:1 molar ratio, yielding a molecular weight of approximately 537.5 g/mol and superior aqueous solubility that translates into faster absorption kinetics. Used extensively in sports nutrition, cardiovascular support, and healthy aging protocols, LCLT is particularly prized for its role in reducing exercise-induced muscle damage, accelerating recovery, and optimizing androgen receptor upregulation. Clinical research demonstrates that 2 grams of LCLT daily can significantly reduce markers of oxidative stress and muscle soreness post-exercise. The compound is classified as Generally Recognized As Safe (GRAS) by the FDA and is available without a prescription across major US retailers including Amazon, GNC, and Vitamin Shoppe, typically priced between $15 and $45 per month's supply. This encyclopedia-level guide covers LCLT's chemistry, pharmacokinetics, molecular mechanisms, science-backed benefits, drug interactions, safety profile, and practical usage guidance for US consumers seeking maximum evidence-based value.

Glycin

Glycine

Glycine (CAS 56-40-6; IUPAC: 2-aminoacetic acid) is the smallest and only achiral proteinogenic amino acid, classified as non-essential and glucogenic, with the molecular formula C₂H₅NO₂ and a molar mass of 75.07 g/mol. First isolated in 1820 by French chemist Henri Braconnot from gelatin hydrolysis, glycine has evolved from a basic biochemical building block into a recognized inhibitory neurotransmitter and obligatory NMDA receptor co-agonist. It constitutes approximately one-third of all amino acid residues in collagen (in the repeating Gly-X-Y motif), making it indispensable for connective tissue integrity. Clinically, glycine supplementation at doses of 2–3 g taken 30–60 minutes before bedtime has demonstrated improvements in subjective sleep quality and reduced sleep onset latency. Additional research supports roles in metabolic health, hepatoprotection, anti-inflammatory signaling via glycine receptors (GlyR) on macrophages, and adjunctive neuropsychiatric therapy. Plasma glycine in healthy adults typically ranges from 150–350 µM. Oral bioavailability is estimated at 70–100%. Glycine is well tolerated up to 5–6 g/day, is classified as a dietary supplement ingredient under DSHEA in the United States, and is available in powder, capsule, and chelated forms from brands such as Thorne, NOW Foods, and Pure Encapsulations.

L-Methionin

L-Methionine

L-Methionine is an essential sulfur-containing amino acid that the human body cannot synthesize — it must be obtained entirely from diet or supplementation. As one of the 20 standard proteinogenic amino acids, it serves as the universal initiator of protein synthesis and the primary methyl donor in the body through its conversion to S-adenosylmethionine (SAMe). L-Methionine is critical for liver detoxification, glutathione production, creatine synthesis, epigenetic regulation, and healthy hair, skin, and nail formation. It plays a central role in the one-carbon metabolism cycle, directly influencing DNA methylation and gene expression. Adults require approximately 10.4 mg/kg/day according to WHO/FAO reference values, with therapeutic supplement doses typically ranging from 500 mg to 3,000 mg daily. Research from 2020–2026 continues to illuminate its role in non-alcoholic fatty liver disease, cognitive aging, cancer metabolism, and cardiovascular health. Available in capsule, tablet, and powder form from major US retailers including Amazon, iHerb, and Vitamin Shoppe at prices ranging from $12–$45 per month, L-Methionine supplements vary widely in quality — making third-party certifications (USP, NSF, ConsumerLab) essential selection criteria for US consumers.

Taurin

2-Aminoethanesulfonic acid

Taurine (2-aminoethanesulfonic acid; CAS 107-35-7) is one of the most abundant free amino acids in the human body — not incorporated into proteins, yet essential for heart, retina, brain, and muscle function. Discovered in ox bile in 1827 by Tiedemann and Gmelin, taurine is a sulfonic acid with the molecular formula C₂H₇NO₃S that acts simultaneously as an osmolyte, membrane stabilizer, calcium modulator, antioxidant, and inhibitory neuromodulator. Concentrated in cardiomyocytes, skeletal muscle, photoreceptors, and leukocytes, it supports cardiovascular health, exercise recovery, glycemic control, neuroprotection, and retinal integrity. Taurine is conditionally essential in cats — dietary deficiency causes fatal cardiomyopathy and blindness — and is present in human breast milk at higher concentrations than cow's milk. In the U.S. market it is regulated as a dietary ingredient under DSHEA, is FDA-recognized in infant formula, and is commercially available from approximately $15–$100/month depending on grade and certification. Typical supplemental doses range from 500 mg to 3,000 mg/day, with an excellent safety profile at these levels. Its pleiotropic mechanisms, low cost, and robust tissue specificity make it one of the most scientifically compelling amino acid supplements available today.

L-Phenylalanin

L-Phenylalanine

L-Phenylalanine is an essential aromatic amino acid — one of 9 amino acids the human body cannot synthesize — carrying the IUPAC name (S)-2-amino-3-phenylpropanoic acid (CAS 63-91-2) and the molecular formula C₉H₁₁NO₂ (MW 165.19 g/mol). As the biochemical gateway to tyrosine, dopamine, norepinephrine, epinephrine, and melanin, it sits at the crossroads of protein nutrition and neurotransmitter chemistry. Every adult requires approximately 14 mg/kg/day of combined phenylalanine plus tyrosine to maintain nitrogen balance, per established amino-acid requirement models. Dietary sources include meat, eggs, dairy, soy, and legumes. Supplement forms range from the pure L-isomer (physiologic, proteinogenic) to racemic DL-phenylalanine (DLPA), which is explored for mood and pain applications via D-isomer–mediated enkephalinase inhibition. Clinically, phenylalanine is most critically important in phenylketonuria (PKU), an inborn error of metabolism where phenylalanine hydroxylase deficiency causes neurotoxic phenylalanine accumulation. In healthy individuals, L-phenylalanine supplements (250 mg–4 g/day) are used as adjuncts for mood, cognitive support, appetite modulation, and vitiligo phototherapy protocols. Regulated as a dietary supplement under DSHEA in the United States, it is widely available from brands such as NOW Foods, Thorne Research, and Jarrow Formulas at prices ranging from $10 to $100+ per supply cycle.

DL-Phenylalanin

DL-Phenylalanine

DL-Phenylalanine (DLPA) is a racemic mixture of two mirror-image forms of the amino acid phenylalanine — the biologically essential L-enantiomer and the synthetic D-enantiomer — with the molecular formula C₉H₁₁NO₂ and a molar mass of 165.19 g/mol. The L-form serves as an essential dietary amino acid, a direct precursor to tyrosine, and ultimately to the catecholamine neurotransmitters dopamine, norepinephrine, and epinephrine. The D-form, absent from natural proteins, is proposed to inhibit enkephalin-degrading enzymes (enkephalinases/neprilysin), potentially prolonging the action of endogenous opioid peptides and contributing to analgesic effects. Together, these dual mechanisms make DLPA a uniquely positioned nutraceutical for mood support, chronic pain adjunct therapy, cognitive focus, and appetite modulation. Sold legally in the US as a dietary supplement under DSHEA, typical doses range from 250–500 mg taken 1–2 times daily on an empty stomach to maximize blood-brain barrier transport. Absolute contraindications include phenylketonuria (PKU) and concurrent MAOI use. While mechanistic rationale is well-established, high-quality randomized controlled trial evidence remains limited, and DLPA should be viewed as an adjunctive nutraceutical — not a first-line therapy — used under informed clinical guidance.

Beta-Alanin

β-Alanine

Beta-Alanine (IUPAC: 3-Aminopropanoic acid; CAS 107-95-9) is a non-proteinogenic beta-amino acid and the rate-limiting precursor for intramuscular carnosine synthesis — the primary mechanism underlying its well-documented ergogenic effects. Unlike its structural isomer alpha-alanine, beta-alanine carries its amino group on the beta carbon, making it biochemically unique. Taken at 3.2–6.4 g/day in divided doses for 4–12 weeks, beta-alanine reliably increases skeletal muscle carnosine by 40–80%, enhancing intracellular pH buffering during high-intensity exercise. Multiple meta-analyses confirm statistically significant improvements in exercise performance for efforts lasting 60–240 seconds — the glycolytic "sweet spot" where intracellular acidosis is most performance-limiting. The characteristic side effect is dose-dependent paresthesia (tingling/flushing), affecting up to 55–70% of users with immediate-release bolus doses but reduced to approximately 10–12% with sustained-release formulations. Regulated as a dietary supplement under DSHEA in the United States, beta-alanine is one of the most rigorously researched ergogenic aids in sports nutrition, with clear applications for competitive athletes, recreational exercisers, and emerging research in older adults for functional capacity support.

L-Histidin

L-Histidine

L-Histidine is a proteinogenic, conditionally essential amino acid — essential for infants and children, and conditionally essential in adults — identified in 1896 by Kossel and Hedin, with the molecular formula C₆H₉N₃O₂ and CAS number 71-00-1. Its defining structural feature is an imidazole side chain with a pKa of approximately 6.0–6.5, uniquely positioned near physiological pH, enabling L-histidine to act as a biological proton donor/acceptor, enzymatic catalyst, and metal-binding ligand in hundreds of metalloproteins. In the body, it serves three critical biochemical roles: (1) a universal substrate for protein biosynthesis, (2) the sole precursor for histamine synthesis via histidine decarboxylase — influencing immune responses, gastric acid secretion, and CNS neurotransmission — and (3) a one-carbon metabolism contributor through the FIGLU-folate pathway. Emerging clinical research explores L-histidine supplementation for atopic dermatitis, metabolic inflammation in obesity, and histaminergic cognitive modulation. Standard supplemental doses range from 200 mg to 4 g/day. L-histidine is regulated in the US as a dietary supplement under DSHEA, with no established FDA Tolerable Upper Intake Level. Well-tolerated at modest doses, it requires caution in histamine-sensitive individuals and those with renal impairment.

L-Ornithin

L-Ornithine

L-Ornithine is a non-proteinogenic, basic amino acid with the molecular formula C₅H₁₂N₂O₂ (CAS 70-26-8) that serves as the biochemical crossroads of at least three major metabolic pathways: the urea cycle, polyamine biosynthesis, and the proline/glutamate axis. Produced endogenously by the arginase reaction (arginine → ornithine + urea), it is not encoded in the genetic code for ribosomal protein synthesis, yet it is indispensable for nitrogen homeostasis in every mammalian cell. As a dietary supplement in the US market, L-ornithine is regulated under DSHEA and is marketed in several forms — free base, hydrochloride salt, L-ornithine L-aspartate (LOLA), and ornithine alpha-ketoglutarate (OKG) — each with distinct clinical applications ranging from adjunctive treatment of hepatic encephalopathy (LOLA) to postoperative anabolic support (OKG) to consumer-grade uses targeting sleep quality, stress reduction, and exercise recovery. Evidence strength varies considerably by indication: highest for LOLA in hyperammonemia management (multiple RCTs and meta-analyses), moderate for OKG in clinical nutrition, and low-to-moderate for general wellness endpoints. Typical supplemental doses range from 500 mg to 2,000 mg per day, with GI discomfort as the primary dose-dependent adverse effect. Absolute contraindications include ornithine aminotransferase deficiency (gyrate atrophy) and known hypersensitivity.

L-Prolin

L-Proline

L-Proline (CAS 147-85-3; molecular formula C₅H₉NO₂) is a proteinogenic, conditionally essential imino acid uniquely distinguished by its cyclic pyrrolidine ring structure — making it the only standard amino acid whose side chain forms a secondary amine bonded back to the alpha nitrogen. Constituting approximately 17% of all residues in collagen when combined with hydroxyproline, proline is the structural backbone of skin, tendons, cartilage, and bone extracellular matrix. First chemically characterized in the early 20th century during Emil Fischer's pioneering era of amino-acid biochemistry, L-proline has since been recognized as far more than a simple collagen building block: it orchestrates cellular redox balance through the mitochondrial proline-P5C cycle, modulates mTOR and HIF-1 signaling, and represents a metabolic vulnerability in multiple cancer phenotypes. Industrially produced via engineered microbial fermentation (Corynebacterium glutamicum or Escherichia coli), L-proline supplements are commercially available as free-form crystalline powder, capsules, and within collagen peptide blends. Common supplemental doses range from 250–2,000 mg/day. While single-ingredient RCT data remain limited, robust mechanistic biochemistry and collagen peptide clinical trials support targeted use for skin health, wound repair, joint integrity, and sports recovery — especially when combined with vitamin C, glycine, and adequate trace minerals.

L-Serin

L-Serine

L-Serine is a proteinogenic, conditionally essential amino acid (molecular formula C₃H₇NO₃; CAS 56-45-1) that occupies a uniquely central position in human biochemistry. Unlike many other amino acids, L-serine simultaneously functions as a structural building block of proteins, a rate-limiting substrate for sphingolipid biosynthesis, a precursor to the neuromodulator D-serine, and a key donor of one-carbon units for nucleotide synthesis and methylation reactions. First isolated from silk protein sericin in 1865, serine has evolved from a simple dietary constituent into a targeted nutraceutical of considerable clinical interest. In hereditary sensory and autonomic neuropathy type 1 (HSAN1), supplemental L-serine corrects a toxic imbalance in serine palmitoyltransferase (SPT) substrate usage, reducing the accumulation of neurotoxic deoxysphingolipids. In the central nervous system, it feeds the serine racemase pathway that produces D-serine — an obligatory co-agonist at NMDA receptors governing synaptic plasticity and cognition. Available as a highly water-soluble free amino acid powder or in capsules, typical supplemental doses range from 200 mg to 6 g/day. L-serine is generally well tolerated, regulated under DSHEA in the US, and considered GRAS at dietary levels. This guide provides a fully referenced, clinically grounded review of its biochemistry, pharmacokinetics, benefits, safety, and US market landscape.

L-Threonin

L-Threonine

L-Threonine is an essential, proteinogenic alpha-amino acid — one of the nine amino acids humans cannot synthesize de novo — carrying the IUPAC name (2S,3R)-2-amino-3-hydroxybutanoic acid (CAS 72-19-5) and the molecular formula C₄H₉NO₃ (MW 119.12 g/mol). First isolated in 1936 by William C. Rose, threonine is unique among common amino acids in possessing two stereocenters, yielding four possible stereoisomers, of which only the (2S,3R)-L-form is incorporated into human proteins. Beyond its structural role in polypeptides, threonine is the dominant amino acid backbone of intestinal mucin glycoproteins, where it provides the O-glycosylation sites critical for gut barrier integrity. The WHO/FAO/UNU estimated safe dietary intake for adults is approximately 15 mg/kg/day (~1,050 mg/day for a 70 kg adult). Supplemental doses typically range from 500–3,000 mg/day. Clinical evidence supports its role in mucin production, mucosal IgA synthesis, protein anabolism, and parenteral/enteral clinical nutrition. Produced industrially via microbial fermentation using engineered Corynebacterium glutamicum or Escherichia coli strains, L-threonine is commercially available as free-form crystalline powder, capsules, tablets, and balanced EAA blends. This article provides a comprehensive, evidence-based reference for clinicians, researchers, and educated consumers navigating the US dietary supplement market.