This review from 2019 establishes glycine as a small amino acid that may be conditionally essential. The authors focus on the metabolic implications of glycine deficiency citing preclinical and human clinical research. Diet, gut microbiota, and liver metabolism all determine the extent to which glycine can impact obesity pathogenesis and associated metabolic disorders.
Alves A, Bassot A, Bulteau AL, Pirola L, Morio B. Glycine Metabolism and Its Alterations in Obesity and Metabolic Diseases. Nutrients. 2019 Jun 16;11(6):1356. doi: 10.3390/nu11061356. PMID: 31208147; PMCID: PMC6627940