Glycine

    Glycine is a non-essential amino acid that is synthesized from choline, serine, hydroxyproline, and threonine. It has many important physiological functions. It is one of the three amino acids that make up glutathione. Dietary sources of glycine include meat, fish, legumes, and gelatin.

    Glycine is an important component of collagen and elastin, which are the most abundant proteins in the body. Like taurine, it is an amino acid required for bile acid conjugation; therefore, it plays a key role in the digestion and absorption of lipids. Glycine is the precursor of several important metabolites, such as porphyrins, purines, heme, and creatine. It acts both as an inhibitory neurotransmitter in the CNS and as an excitatory neurotransmitter at N-methyl-D-aspartate (NMDA) receptors. Glycine has antioxidant, anti-inflammatory, immunomodulatory, and cytoprotective roles in all tissues. In the folate cycle, glycine and serine are interconverted. These methyltransferase reactions and interconversions are easily reversible, depending on the needs of the folate cycle to synthesize purines.

    Glycine can also be generated from choline, betaine, dimethylglycine, and sarcosine within the methylation cycle itself. Glycine accepts a methyl group from -adenosylmethionine (SAM) to form sarcosine. This conversion functions to control excess SAM.

    Glycine supplementation has been used to improve metabolic disorders in patients with obesity, diabetes, cardiovascular disease, ischemia-reperfusion injury, inflammatory diseases, and cancers. Due to the excitatory effects of glycine on CNS NMDA receptors, research into the treatment of psychiatric disorders such as schizophrenia using glycine transport antagonists has shown great promise.

    Oral glycine may increase tissue glutathione levels, especially with concomitant NAC and/or lipoic acid. Because glutathione levels decline during the aging process, glycine supplementation may impact elderly patients with low protein intake.

 Elevated glycine may be due to dietary intake (i.e., meat, fish, legumes, and gelatin) or supplementation.

Deficiencies in enzyme SNPs or cofactors in the production and metabolism of glycine (vitamin B₆, B₁₂, and folate) may result in abnormal glycine levels.

    Low glycine may be due to reduced intake or malabsorption and GI maldigestion.

    Glycine’s function as an antioxidant plays an important role in disease processes and is incorporated into glutathione, an important antioxidant. Therefore, low levels have significant clinical impact. Antioxidants such as vitamins A and E can help mitigate damage from oxidative stress.

LOW GLYCINE

•Decreased intake

• Potentially overmethylation, excessive methyl supplementation

HIGH GLYCINE

• Dietary intake (i.e. meat,fish, legumes, and gelatins)

• Supplementation

• GNMT SNP or cofactor deficiency2

• SHMT SNP or cofactor deficiency (vitamin B6, iron)