Homocysteine: An amino acid that is produced by the human body, usually as a byproduct of consuming meat. Homocysteine is normally converted into other amino acids. An abnormal accumulation of homocysteine, which can be measured in the blood, can be a marker for the development of heart disease. Elevated levels of homocysteine in the blood appear to increase the risk of heart attack, stroke, peripheral vascular disease, and venous thrombo'embolism (blood clots in the veins). Homocysteine is believed to damage blood vessels in several ways. It injures the cells that line arteries and stimulates the growth of smooth muscle cells. Homocysteine can also disrupt normal blood clotting mechanisms. Elevated levels of homocysteine also appear to increase the risk of Alzheimer's disease. The ways to lower homocysteine are to eat less meat and take supplements of the B vitamins folic acid (folate), B6, and B12 that are needed by the enzymes that process homocysteine.