Glycine is the simplest amino acid in human nutrition. The variable group on glycine is composed of just a hydrogen atom and as such L-glycine is the only amino acid in human nutrition that does not demonstrate chirality. Glycine is a non-essential amino acid as it is syntesised from serine and threonine, the latter being an essential amino acid. When taken orally, glycine has a particular sweet taste. Glycine is used metabolically to synthesise purine, porphyrins, glyoxylic acid and creatine. With regards to creatine, glycine and arginine react in the synthesis of creatine, which is subsequently phosphorylated to form the ATP buffer creatine phosphate in cells. Supplements of glycine and arginine are able to increase muscle stores of creatine phosphate, in a similar manner to ingestion of creatine monohydrate supplements. However, while creatine monohydrate supplies the creatine directly, glycine and arginine increase endogenous creatine synthesis rates.
Glycine is also involved in the synthesis of collagen, of which it constitutes around 75 % of the amino acids. Much of the remaining amino content of collagen is taken by proline. Glycine is therefore pivotal to correct join function, although most people consume enough protein in their diet and therefore have an adequate supply of glycine for collagen formation. Collagen however is the most abundant protein in the body and this highlights the importance of glycine. High intakes of up to 10 grams of glycine orally may be able to stimulate growth hormone release. Following oral supplements growth hormone levels rise and peak at around 3 hours. However, it is unclear if the increase in growth hormone has beneficial physiological effect. For example, it is not known whether the glycine induced growth hormone release has an anti-catabolic effect on skeletal muscle. Glycine is also converted to dimethylglycine, which has an important function in the formation of steroid hormones.
Glycine is also an important neurotransmitter in the brain, where it functions as an inhibitory neurotransmitter. Its inhibitory effects in the central nervous system may provide beneficial effects against seizures and epilepsy and may be of some clinical use in treating some mental disorders. Glycine appears to have antiinflammatory and immunomodulatory effects because it can act of immune cells such as macrophages to inhibit cytokine release. Studies have shown that supplemental glycine may be of benefits in cases of fatigue, perhaps because if its creatine synthesising effects. Glycine can also help regulate fat metabolism because it aids in the modulation of bile acid formation. Its sweetness means that glycine also possesses some use in treating type 2 diabetes and obesity by acting as a potential sweetener in place of sugar. Because glycine can be manufactured in the body deficiencies are very rare. Glycine is also commonly found in most protein containing foods and so is abundant in the diet of humans.