Glycine甘氨酸 Glycine

Place · Level 3 · Supplement

最小的氨基酸 · 条件必需每日缺口 ~10 g · 一分子 5 命运 (胶原/GSH/肌酸/血红素/神经) · 3 g 改善睡眠

Conditionally essential

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身份 · 条件必需

Glycine (Gly, G) is the smallest of the 20 proteinogenic amino acids — its side chain is just a hydrogen atom. This makes it structurally unique, able to fit anywhere a protein needs 'very little space + high flexibility', most famously at the inner turns of the collagen triple helix.

Classic textbooks list glycine as a 'non-essential amino acid': the body can synthesize it from serine via SHMT enzyme + 5,10-methylene-THF, feeding into one-carbon metabolism.

Meléndez-Hevia 2009 J Biosci re-counted daily glycine demand:

Collagen synthesis: ~10-20 g new collagen/day in adults, containing ~3-7 g glycineGSH synthesis: ~5-10 g GSH turnover/day, containing ~1-2 g glycineCreatine synthesis: ~1.5-2 g creatine/day, containing ~0.5 g glycineHeme synthesis: red cells turn over daily; each porphyrin ring contains 4 glycines, ~0.2 g/dayPurine bases + bile acids + neurotransmitters: ~1 gTotal demand: ~10-15 g/day
Endogenous capacity: the SHMT pathway from serine produces ~2.5-3 g/day. The gap is ~10 g/day, which has to come from diet. A typical Western diet supplies 1.5-3 g/day (eggs / meat / legumes / gelatin), still well below the requirement.

Meléndez-Hevia's conclusion: glycine isn't 'non-essential' — it's conditionally essential. The body makes some, but not nearly enough for daily use, so dietary intake is required.

This is the Atlas's first systematic treatment of the 'conditionally essential' concept in nutrition. It also applies to glutamine, arginine, cysteine, and tyrosine — a third category beyond the textbook 'essential vs non-essential' split.

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