Homocysteine

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Medical Author: Siamak Nabili, MD, MPH
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

• What is homocysteine?
• Why is it important to monitor homocysteine levels?
• What are the possible symptoms or features of elevated homocysteine levels?
• What is considered a high level for homocysteine?
• What causes elevated homocysteine levels?
• Can elevated homocysteine levels be genetic?
• Can nutritional problems cause elevated homocysteine levels?
• How common is hyperhomocysteinemia?
• How can homocysteine levels be lowered?
• How many vitamins should I take to lower my homocysteine level?
• Does lowering homocysteine levels prevent heart attacks and strokes?
• What should I do to prevent heart attacks and strokes?
• Who should undergo testing for homocysteine blood levels?

What is homocysteine?

Homocysteine is an amino acid that is produced by the body, usually as a byproduct of consuming meat. Amino acids are naturally made products, which are the building blocks of all the proteins in the body.

Why is it important to monitor homocysteine levels?

Elevated levels of homocysteine (>10 micromoles/liter) in the blood may be associated with atherosclerosis (hardening and narrowing of the arteries) as well as an increased risk of heart attacks, strokes, blood clot formation, and possibly Alzheimer's disease.

In 1969, Dr. Kilmer S. McCully reported that children born with a genetic disorder called homocystinuria, which causes the homocysteine levels to be very high, sometimes died at a very young age with advanced atherosclerosis in their arteries. However, it was not until the 1990's that the importance of homocysteine in heart disease and stroke was appreciated.

What are the possible symptoms or features of elevated homocysteine levels?

Theoretically, an elevated level of homocysteine in the blood (hyperhomocysteinemia) is believed to cause narrowing and hardening of the arteries (atherosclerosis). This narrowing and hardening of the vessels is thought to occur through a variety of ways involving elevated homocysteine. The blood vessel narrowing in turn leads to diminished blood flow through the affected arteries.

Elevated levels of homocysteine in the blood may also increase the tendency to excessive blood clotting. Blood clots inside the arteries can further diminish the flow of blood. The resultant lack of blood supply to the heart muscles may cause heart attacks, and the lack of blood supply to the brain causes strokes.

Elevated homocysteine levels also have been shown to be associated with formation of blood clots in veins (deep vein thrombosis and pulmonary embolism). The mechanism is complex, but it is similar to the way that they contribute to atherosclerosis. In some studies, even moderate levels of homocysteine level showed higher rates of repeated incidence of blood clot formation. ^(1,2)