Glycine is a nonessential amino acid that is synthesized from choline, serine, hydroxyproline, and threonine. It has many important physiologic functions. It is one of three amino acids that make up glutathione. Glycine’s dietary sources include meat, fish, legumes, and gelatins. Glycine is a major collagen and elastin component, which are the most abundant proteins in the body. Like taurine, it is an amino acid necessary for bile acid conjugation; therefore, it plays a key role in lipid digestion and absorption.
Glycine is the precursor to various important metabolites such as porphyrins, purines, heme, and creatine. It acts both as an inhibitory neurotransmitter in the CNS and as an excitatory neurotransmitter on N-methyl-D-aspartate (NMDA) receptors.
Glycine has anti-oxidant, anti-inflammatory, immunomodulatory, and cytoprotective roles in all tissues. In the folate cycle, glycine and serine are interconverted. These methyltransferase reactions and interconversions are readily 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 S-adenosylmethionine (SAM) to form sarcosine. This conversion functions to control SAM excess.
Supplementation with glycine has been used to ameliorate metabolic disorders in patients with obesity, diabetes, cardiovascular disease, ischemia-reperfusion injuries, inflammatory diseases, and cancers.
Because of glycine’s excitatory effects on CNS NMDA receptors, research regarding the treatment of psychiatric disorders, such as schizophrenia, using glycine transport antagonists have shown great promise.
Oral glycine can boost tissue levels of glutathione, especially with concurrent NAC and/or lipoic acid. Because glutathione levels decline during the aging process, supplementing with glycine can impact elderly patients with low protein intake.
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Low glycine may be due to decreased intake, or GI malabsorption and 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.
Elevated glycine may be due to dietary intake (i.e. meat, fish, legumes, and gelatin) or supplementation. Enzymatic SNPs or cofactor deficiencies in glycine production and metabolism (vitamin B6, B12, and folate) may result in abnormal levels of glycine.
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