Homocysteine is a common amino acid, that is naturally present in our bodies. It is produced from the metabolism of methionine - an amino acid found primarily in animal protein that is involved in lipid metabolism and immune system function. Homocysteine can either be recycled back into methionine, or converted into cysteine, which is a precursor to the antioxidant glutathione. Both of these pathways require B vitamins.
Studies have shown that high levels of homocysteine in plasma are associated with advanced age, as well as the rate of brain atrophy in healthy older individuals.