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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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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)