Glycine

What is glycine?

Glycine is the simplest protein-forming amino acid. The body can produce it, but it is also supplied through food.

Glycine is found in many proteins and is especially abundant in collagen. It also serves as a precursor for several important compounds.

What functions does glycine have?

Glycine is involved in:

• Protein and collagen synthesis
• Formation of glutathione
• Production of creatine
• Synthesis of heme
• Bile-acid conjugation
• Neurotransmission
• One-carbon metabolism

In the nervous system, glycine acts mainly as an inhibitory neurotransmitter in the spinal cord and brainstem. It also participates in NMDA-receptor signaling.

Food sources

Glycine occurs in:

• Meat and fish
• Gelatin
• Collagen-rich connective tissue
• Bone broth
• Dairy products
• Legumes

Modern diets often contain less collagen-rich tissue than traditional diets, which may reduce glycine intake.

Glycine and collagen

Approximately one third of the amino acids in collagen are glycine. It is therefore essential for the structure of skin, tendons, cartilage, bone and blood vessels.

Whether additional glycine alone improves connective tissue depends on overall protein intake, vitamin C status and other factors involved in collagen synthesis.

Glycine and glutathione

Glutathione is an important intracellular antioxidant composed of glycine, cysteine and glutamate. In some older adults, the availability of glycine and cysteine may limit glutathione synthesis.

This is one reason combinations of glycine and N-acetylcysteine, often called GlyNAC, are being investigated in healthy-aging research.

Glycine and sleep

Small clinical studies have examined glycine taken before bedtime. Some reported improvements in perceived sleep quality, daytime alertness or time required to fall asleep.

Possible mechanisms include effects on body temperature and neurotransmission. Larger studies are needed before glycine can be considered an established sleep treatment.

Glycine and metabolic health

Low blood glycine concentrations have been associated with insulin resistance and metabolic disease in observational studies. It is not yet clear whether low glycine is a cause, a consequence or both.

Supplementation has been studied in relation to glucose metabolism, inflammation and oxidative stress, but clinical evidence remains preliminary.

Dosage and safety

Studies commonly use several grams of glycine per day. It is generally well tolerated. Possible adverse effects include nausea, soft stools or abdominal discomfort.

People with kidney or liver disease, pregnancy or regular medication use should consult a healthcare professional before taking high doses.

Conclusion

Glycine is a versatile amino acid involved in collagen, glutathione, creatine and nervous-system function. It is scientifically interesting in relation to sleep, metabolic health and healthy aging, but many proposed benefits still require stronger clinical evidence.