Glycine

Glycine is the simplest amino acid, consisting of little more than an amino group and a carboxyl group. It is classed as non-essential because the body can synthesise it, though several researchers argue that synthesis falls short of what optimal collagen turnover requires, making it conditionally essential in practice.

It accounts for roughly a third of the amino acid content of collagen, appearing at every third position in the triple helix, a role no other amino acid can fill because none other is small enough.

Dietary sources are the connective-tissue-rich parts of animals: skin, bone broth, gelatin and tougher cuts. Modern diets favouring muscle meat over these supply considerably less glycine than traditional eating patterns, which is the argument for supplementing it. It also tastes distinctly sweet, which makes it unusually pleasant to take.

In the central nervous system glycine acts as an inhibitory neurotransmitter, binding its own receptors in the brainstem and spinal cord. It also acts as a co-agonist at NMDA receptors, which is a more complex modulatory role.

The sleep effect appears to work through body temperature. Glycine promotes peripheral vasodilation, increasing heat loss through the extremities and lowering core body temperature, which is one of the physiological triggers for sleep onset.

Glycine is also one of three amino acids required for glutathione synthesis, alongside cysteine and glutamate. Research into combining glycine with N-acetylcysteine, known as GlyNAC, targets glutathione restoration on the basis that both precursors can be limiting in older adults.

Sleep is the best-supported application. Trials using 3 g before bed in people with poor sleep quality have reported improved subjective sleep quality, faster sleep onset and reduced daytime fatigue, with objective measures supporting shorter time to sleep onset.

The GlyNAC research in older adults is an interesting emerging area, with small trials reporting improvements in glutathione levels, oxidative stress markers and several measures of physical function. Sample sizes are small and the work concentrated in a limited number of groups.

Evidence for collagen synthesis, metabolic markers and joint outcomes is more mechanistic than demonstrated. The reasonable summary is a cheap, safe amino acid with genuine sleep evidence and plausible but less established benefits elsewhere.

Glycine is exceptionally well tolerated. It is a normal dietary component and trials using several grams daily report very few adverse effects, with mild digestive upset or nausea the occasional complaint at higher doses.

Because it has inhibitory activity in the nervous system, some people find it mildly sedating, which is the point when taken at night but worth knowing if taken during the day. Combining it with anything else that makes you drowsy will compound that.

Glycine acts at receptors that some prescription medicines also target, so it is not a neutral addition for everyone. If you take regular medication, particularly anything affecting the brain or nervous system, check with your doctor before starting it.