GLYCINE

Glycine is a conditionally essential amino acid whose plasma levels are reduced in obesity, diabetes, and malnutrition. Human trials show that 3g oral glycine before bedtime improves subjective sleep quality, likely mediated by a reduction in core body temperature and increased cutaneous blood flow. Supplemental glycine (100 mg/kg/day) corrected obesity-associated glycine deficiency, enhanced glycine conjugation detoxification, and improved markers of metabolic dysfunction-associated liver disease in a small exploratory study. In skeletal muscle, glycine protects against wasting in models of cancer cachexia, sepsis, and caloric restriction, though underlying mechanisms remain unclear. Small-scale clinical trials suggest glycine-site NMDA modulators may ameliorate negative and cognitive symptoms of schizophrenia, but evidence is preliminary. High-dose supplementation may raise homocysteine and increase ammonia via the glycine cleavage system, and well-controlled studies in non-deficient populations are still needed.