Abstract
Glycine is the proteinogenic amino-acid of lowest molecular weight, harboring a hydrogen atom as a side-chain. In addition to being a building-block for proteins, glycine is also required for multiple metabolic pathways, such as glutathione synthesis and regulation of one-carbon metabolism. Although generally viewed as a non-essential amino-acid, because it can be endogenously synthesized to a certain extent, glycine has also been suggested as a conditionally essential amino acid. In metabolic disorders associated with obesity, type 2 diabetes (T2DM), and non-alcoholic fatty liver disease (NAFLDs), lower circulating glycine levels have been consistently observed, and clinical studies suggest the existence of beneficial effects induced by glycine supplementation. The present review aims at synthesizing the recent advances in glycine metabolism, pinpointing its main metabolic pathways, identifying the causes leading to glycine deficiency—especially in obesity and associated metabolic disorders—and evaluating the potential benefits of increasing glycine availability to curb the progression of obesity and obesity-related metabolic disturbances. This study focuses on the importance of diet, gut microbiota, and liver metabolism in determining glycine availability in obesity and associated metabolic disorders.
Highlights
Glycine is the protein-forming amino acid with the smallest molecular weight (NH2 -CH2 -COOH; 75.067 g/mol)
Using 13 C-glycine infusion, it was observed that the flux through SHMT to 13 C-serine was decreased in the liver of obese rats with nonalcoholic steatohepatitis (NASH) [43], suggesting that hepatic SHMT activity may be altered in obesity and related-metabolic disorders
Considered a promising amino acid for improving metabolic health. It is still unclear whether the decline in glycine levels is causatively involved in the pathogenesis of metabolic disorders, glucose intolerance, insulin resistance, and T2DM
Summary
Glycine is the protein-forming amino acid with the smallest molecular weight (NH2 -CH2 -COOH; 75.067 g/mol). Low plasma glycine concentrations have been consistently reported in association with obesity, T2DM, and NAFLDs [8,9]. The following points will be emphasized: (1) situating glycine metabolism in a context of the physiopathology of metabolic diseases associated with obesity (insulin resistance, T2DM, and NAFLDs);. Note: NGT = Normal glucose tolerance; PGT = pathological glucose tolerance; IS = insulin sensitive; IR = insulin resistant; NA = not available; T2DM = type 2 diabetes; GDR = glucose disposal rate; BMI = body mass index; HOMA-IR = Homeostatic Model Assessment; NAFLD = non-alcoholic fatty liver diseases, PPSDiab: Prediction, Prevention and Sub-classification of type 2 Diabetes
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