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Homocysteine is a sulfur-containing amino acid that occurs naturally in all humans. It is broken down in the body through two metabolic pathways. The chemical changes that must occur to break down homocysteine require the presence of folic acid (also called folate) and vitamins B6 and B12. The level of homocysteine in the blood is influenced by the presence of these substances.

Homocystinuria is a rare genetic disorder that occurs in about one in every 200,000 individuals. This congenital metabolic disorder causes large amounts of homocysteine to be excreted in the urine. Homocystinuria is associated mental retardation and the development of heart disease before age 30.

In the late 1960s, doctors documented that individuals with homocystinuria developed narrowing of the arteries at a very early age, sometimes even in childhood. Although homocystinuria is rare, this finding stimulated research on whether people who did not have homocystinuria but who did have unusually high levels of homocysteine in their blood were at greater risk of developing heart disease or stroke.

Many risk factors, including family history of heart disease, smoking, obesity, lack of exercise, diabetes, high levels of low-density lipoprotein cholesterol (LDL or "bad" cholesterol), low levels of high-density lipoprotein cholesterol (HDL or "good" cholesterol), and high blood pressure have been documented to increase the risk of stroke and heart disease. With so many other risk factors, it has been difficult to determine whether high levels of homocysteine are an independent risk factor for the development these diseases. However, a substantial number of controlled, well-designed, and well-documented studies have shown that individuals who have high levels of homocysteine in the blood are at increased risk of developing blocked blood vessels, a condition known as occlusive arterial disease or at risk to worsen atherosclerosis ("hardening of the arteries").

In the 2000s, studies also suggested that high levels of homocysteine were associated with poorer mental functioning, leading to ongoing investigations into the role of homocysteine in Alzheimer's disease. Additional studies have also suggested that high levels of homocysteine can lead to osteoporosis and an increased risk of broken bones in the elderly. As of 2005, homocysteine was being tested in half a dozen clinical trials to determine its role in these and several other conditions. Information on clinical trials that are enrolling patients can be found on-line at 〈〉.

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