Glycine is the simplest amino acid; it is synthesized in the body and is one of three amino acids required to synthesize glutathione, the “super antioxidant” of vision. It is not essential in that the body can produce it from the amino acid serine, but your body may not produce enough glycine from serine.
It is an inhibitory neurotransmitter (non-stimulating) in the brainstem and spinal cord. In the retina, it plays a mediation role in neurotransmission.1 Half of certain retinal neurons, called amacrine cells, release glycine at their synapses with a variety of other neurons. Some of these cells relay light from rods, and some modulate different types of ganglion cell responses.2
It is also a protein precursor in the body, where the greatest use is in collagen, which is about 35% glycine.3
Glycine and the brain. Glycine and serine, along with amino acid alanine and enzymes are essential parts of blood sugar processing, and are lower in Alzheimer’s cases compared to normal brain tissue samples.6 Glycine modulates certain neurobiochemicals in the brain and improves sleep quality.4 The details of why it does so are poorly understood. Researchers found that oral glycine induced non-rapid eye movement sleep, along with lowered core temperature, and they conclude that the effect may be due to dilation of peripheral blood vessels.5
Source. Egg whites are a good source of glycine. Beans, spinach, kale, cauliflower, cabbage and pumpkin are vegetable source of glycine. It is found in fruits, including banana and kiwi, and in meat, dairy products, poultry, fish, and eggs (especially egg whites).
1. Glycine. Wikipedia. Retrieved Oct 31 2017 from https://en.wikipedia.org/wiki/Glycine.
2. Havercamp, S. (2012). Glycine Receptor Diversity in the Mammalian Retina. Webvision, Mar 27..
3. Ibid. Glycine. Wikipedia..
4. Bannai, M., Kawai, N., Ono, K., Nakahara, K., Murakami, N. (2012). The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers. Font Neurol, Apr 18;3:61.
5. Kawai, N., Sakai, N., Okuro, M., Karakawa, S., Tsuneyoshi, Y., et al. (2015). The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus. Neuropsychopharmacology, May;40(6):1405-16.
6. An Y, Varma VR, Varma S, Casanova R, Dammer E, et al. (2018). Evidence for brain glucose dysregulation in Alzheimer's disease. Alzheimers Dement. Mar;14(3):318-329.