glycine

Glycine is the simplest of the 20 standard proteinogenic amino acids, distinguished by having a single hydrogen atom as its side chain. Chemically termed aminoacetic acid, it has the formula NH2CH2COOH, and is unique among the amino acids in that it is achiral (not optically active), meaning it has no stereoisomers. Glycine is classified as a nonessential amino acid because the human body can synthesize it from other metabolic precursors such as serine, choline, and threonine. However, dietary intake significantly contributes to total glycine availability, particularly in individuals with higher metabolic demands. Glycine is a key component of proteins, and particularly abundant in collagen – the most plentiful protein in the human body, which provides structural support to skin, bone, ligaments, and connective tissues. Collagen is rich in glycine, proline, and hydroxyproline, and approximately one-third of collagen’s amino acid content comprises glycine. Without sufficient glycine, the structural integrity and repair of these tissues can be compromised. Beyond its role as a protein building block, glycine functions within the nervous system as both an inhibitory neurotransmitter and a modulator of excitatory neurotransmission at N‑methyl‑D‑aspartate (NMDA) receptors in the brain. Glycine’s neurotransmitter roles influence neural processing, sleep regulation, and cognitive functions. The body also uses glycine in the synthesis of creatine – a compound vital for rapid energy production in muscle cells – and glutathione, a potent endogenous antioxidant that protects cells against oxidative damage. Because glutathione production decreases with age, glycine’s role becomes more critical for maintaining cellular redox balance and optimal physiologic function. Glycine participates in various metabolic pathways, including heme synthesis, bile acid conjugation (important for fat digestion), and one‑carbon metabolism through interconversion with serine. As research continues, glycine’s functions expand to metabolic regulation, immune modulation, and cellular repair. Despite being nonessential, glycine’s multifaceted roles indicate that adequate dietary sources may support optimal health across the lifespan.

Glycine serves multiple physiological roles beyond its identity as an amino acid. Structurally, it is indispensable for protein assembly, particularly within collagen – the body’s most abundant protein, which requires glycine for its triple helix formation. Collagen’s glycine residues allow tight packing of polypeptide chains, supporting the structural strength of skin, tendons, cartilage, and bone. Glycine’s contribution to connective tissue integrity makes it central to wound healing and musculoskeletal health. In the central nervous system (CNS), glycine functions as an inhibitory neurotransmitter, especially in the brainstem and spinal cord, where it binds to glycine receptors to increase chloride influx, stabilizing neuronal membranes and reducing excitability. Glycine also acts as a co‑agonist at NMDA receptors, which are critical for synaptic plasticity and cognitive processes. Clinical evidence suggests that glycine supplementation may improve aspects of sleep quality; small human studies indicate that 3 grams of glycine taken before bedtime can reduce sleep latency, enhance subjective sleep quality, and improve next‑day alertness, potentially via thermoregulatory and neurotransmitter mechanisms. Glycine is one of three amino acids required for glutathione synthesis, alongside cysteine and glutamate. Glutathione is one of the body’s most potent antioxidants, protecting cells from oxidative stress and supporting immune function. Because glutathione levels decline with age, glycine availability may limit antioxidant capacity in older adults. Research also highlights glycine’s role in creatine biosynthesis. Creatine provides a rapid source of energy for high‑intensity muscle contraction and plays roles in cellular energy metabolism beyond muscle, including neural tissues. Emerging evidence from systematic reviews (2023, GeroScience) suggests that glycine administration may lower systemic inflammation and improve metabolic health markers, although larger, longer studies are needed to confirm these effects in diverse populations. Additionally, glycine may support metabolic flexibility by improving insulin sensitivity and lipid metabolism, with observational associations linking higher circulating glycine levels to lower risks of type 2 diabetes and cardiovascular disease. Glycine’s anti‑inflammatory properties stem partly from its ability to modulate cytokine production and reduce oxidative damage in tissues. In sports nutrition, emerging research (2024) explores glycine’s potential to reduce muscle damage, support recovery, and attenuate proteolytic pathways after exercise.

Unlike essential amino acids, glycine has no formal Recommended Dietary Allowance (RDA) set by the NIH or other major dietary authorities because the human body can synthesize it endogenously. However, typical dietary intake of glycine in omnivorous diets is approximately 2–3 grams per day. In research contexts, supplemental glycine doses often range from 3 to 5 grams taken orally per day, especially in studies examining sleep or metabolic outcomes. Factors that influence glycine requirements include age, metabolic stress, illness, and physical activity level. Individuals with higher collagen turnover – such as older adults or those recovering from injury – may benefit from higher intakes, either through concentrated food sources like bone broth and collagen peptides or through targeted supplementation. Athletes and people with high training demands may have increased needs due to enhanced creatine and glutathione turnover. Pregnant and lactating individuals may also have elevated glycine demands because of increased protein synthesis and fetal growth, though specific guidance is not established. Optimal intake has not been definitively determined but is often extrapolated from clinical studies and metabolic research. Some experts suggest aiming for dietary patterns that supply more than the typical 2–3 grams per day, especially for populations with increased physiological demands. Because glycine is abundant in high‑protein foods, meeting higher intake levels through food often aligns with meeting overall protein requirements for health and recovery. In contrast, strict plant‑based diets may provide less glycine unless intentional food choices are made, such as including legumes, seeds, and soy products. When using supplements, consult a healthcare provider to tailor dose and timing to individual needs.

Because glycine is classified as a nonessential amino acid, outright clinical deficiency due solely to inadequate intake is uncommon in healthy individuals with adequate protein consumption. The human body can synthesize it from serine, choline, and other precursors. However, relative insufficiency can occur when metabolic demands outpace synthesis and intake, such as during chronic illness, aging, intense physical stress, or when dietary glycine is very low. Suboptimal glycine availability may contribute to symptoms related to impaired collagen synthesis, antioxidant production, and neurotransmission. Potential signs of low glycine levels are not specific and often overlap with other conditions. Observational and clinical reports associate relative glycine insufficiency with chronic fatigue, weak muscle strength, and reduced exercise tolerance due to impaired creatine synthesis and energy availability. Cognitive symptoms such as poor concentration, mental fog, and mood disturbances may arise from disruptions in neurotransmitter balance and NMDA receptor modulation. Poor sleep quality and difficulty maintaining restful sleep have also been reported in contexts of low dietary glycine or metabolic depletion. Connective tissue and skin health may be compromised, leading to slower wound healing, reduced skin elasticity, and joint discomfort from suboptimal collagen formation. Oxidative stress may increase due to reduced glutathione synthesis, potentially exacerbating inflammation and cellular damage. While severe deficiency syndromes specific to glycine are rare, metabolic disorders affecting glycine metabolism (such as glycine encephalopathy) demonstrate the critical role of glycine in central nervous system function and development. Clinical diagnosis of glycine insufficiency uses plasma amino acid profiling, but interpretation requires expert evaluation due to interactions with other metabolic pathways.

Dietary glycine comes primarily from protein‑rich foods, with especially high concentrations in collagenous animal parts and processed collagen products. Dry gelatin and collagen peptides provide exceptionally concentrated glycine. Animal proteins, especially those including connective tissue, skin, and bone, supply substantial glycine per serving. Poultry, including chicken and turkey, offers high amounts of glycine in both lean and skin‑on parts. Ground beef, pork, and lamb are also rich sources. Fish varieties including tuna, salmon, and mackerel contribute appreciable glycine as part of their protein profiles. Plant‑based sources contain glycine as part of their protein, though generally in lower concentrations than animal‑based foods. Legumes such as soybeans and lentils provide plant protein along with glycine. Seeds like pumpkin and sesame, along with nuts and whole grains such as buckwheat and quinoa, contribute glycine and complement dietary intake, particularly in vegetarian or vegan diets. Bone broth, prepared by simmering animal bones and connective tissues, releases gelatin and glycine into the broth, yielding a nutrient‑rich liquid. Gelatin powder, derived from collagen, can be incorporated into foods and beverages to boost glycine intake. Combining diverse protein sources ensures a broad spectrum of amino acids, including glycine, to support bodily functions. Food preparation methods can influence amino acid availability, but most protein sources listed provide reliable glycine contributions to the diet.

Glycine from dietary proteins is absorbed in the small intestine via amino acid transporters and appears in the bloodstream to be utilized by tissues. Protein digestion yields free amino acids and small peptides, which are absorbed efficiently; glycine has high bioavailability from both animal and plant proteins. When consumed as free glycine (e.g., as a supplement), absorption is rapid and does not require peptide digestion. Glycine bound within collagen or gelatin is released during digestion and contributes to the circulating amino acid pool. Factors that enhance absorption include co‑consumption of other amino acids and adequate digestive enzyme activity. Conversely, impaired digestion due to conditions like pancreatic insufficiency or small intestinal disorders may diminish amino acid uptake. Because glycine is not dependent on cofactors for absorption, its bioavailability remains robust in varied dietary contexts. However, overall protein intake and digestive health influence the extent of glycine absorbed and utilized. Timing of intake may affect metabolism; for example, consuming glycine‑rich foods or supplements around periods of increased demand, such as post‑exercise or before sleep, may align substrate availability with physiological needs.

Supplementation with glycine may be considered when dietary intake is low or when specific health goals target functions glycine may support. People with sleep difficulties have used glycine supplements at doses around 3 grams before bedtime, with evidence from small clinical studies suggesting improvements in sleep quality and next‑day alertness. Glycine supplements are also explored in athletic populations to support recovery and muscle protein synthesis due to roles in creatine formation and anti‑inflammatory effects. Individuals with metabolic conditions, impaired collagen formation, or high oxidative stress may benefit from supplemental glycine to optimize glutathione production and detoxification pathways. Forms of glycine supplements include pure glycine powder, capsules, and collagen peptide products. Powders allow flexible dosing and incorporation into foods and drinks, while capsules provide convenience. Collagen peptides not only supply glycine but also additional amino acids relevant to connective tissue health. Typical supplemental doses range from 1 gram up to 5 grams daily, though research studies have examined higher doses without serious adverse effects. Always consult a healthcare provider before initiating supplementation, especially for pregnant or lactating individuals or those with underlying health conditions. Supplements should complement, not replace, a nutrient‑rich diet.

There is no established tolerable upper intake level (UL) for glycine due to its nonessential status and the body’s capacity to metabolize it. Clinical studies administering glycine have used doses up to 6 grams daily for several weeks with few adverse effects. Glycine is generally well tolerated, but extremely high doses may result in minor gastrointestinal symptoms such as nausea or diarrhea in some individuals. Because glycine participates in neurotransmitter pathways, very high concentrations may theoretically influence neural excitability, but evidence for clinical toxicity in healthy adults is limited. Individuals with rare metabolic disorders affecting glycine metabolism require careful clinical monitoring, as disrupted glycine catabolism can lead to elevated glycine levels with neurological consequences. As with any supplement, excessive intake without medical guidance is not recommended, especially in populations with impaired kidney or liver function, where amino acid handling may be altered. Because glycine is widely distributed in protein foods and used in normal metabolic processes, toxicity from dietary intake alone is unlikely.

Glycine can interact with certain medications, primarily through effects on neurotransmission and metabolic pathways. One documented interaction involves clozapine, an antipsychotic medication: concurrent glycine intake may reduce clozapine’s effectiveness, possibly due to competitive modulation of NMDA receptor activity. Patients taking clozapine should avoid high‑dose glycine supplementation without medical supervision. Glycine’s influence on neurotransmitter systems means that interactions with CNS‑active drugs should be discussed with clinicians to prevent alterations in drug response or side‑effect profiles. Because glycine participates in amino acid metabolism and glutathione synthesis, it may theoretically interact with drugs affecting hepatic metabolism, though specific interactions are not well documented. Healthcare providers should review all medications and supplements to identify potential interactions. Patients on medications for sleep disorders, psychiatric conditions, or those altering amino acid transport should be monitored closely when initiating glycine supplementation. Always consult a clinician when combining supplements with prescription medications to ensure safety.

Common Forms: glycine powder, capsules, collagen peptides, gelatin

Typical Doses: 3–5 grams daily for sleep or metabolic support

When to Take: evening for sleep, any time for metabolic support

Best Form: glycine powder or collagen peptides

⚠️ Interactions: clozapine