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Homocysteine
Homocysteine
(pronounced homo-SIS-teen) is an amino acid and is found normally
in the body. Its metabolism is linked to that of several vitamins,
especially folic acid, B6, and B12. Deficiencies of those vitamins
may cause elevated levels of homocysteine. In recent years, studies
have accumulated suggesting that a high level of homocysteine increases
a person's chance of developing heart disease, stroke, and peripheral
vascular disease (a reduced blood flow to the hands and feet). In
September 1995, the National Heart, Lung, and Blood Institute (NHLBI)
convened a special panel to review the scientific evidence about
homocysteine's possible link to heart disease. The information that
follows is based on the panel's conclusions. Briefly, the panel
said that an elevated homocysteine level appears to increase the
risk of heart disease, stroke, and peripheral vascular disease.
However, no studies have been done to show that lowering the homocysteine
level reduces the risk of heart disease. The panel stressed that
more research, especially a clinical trial, must be done to understand
the possible association between the level of homocysteine and heart
and related diseases.
HOMOCYSTEINE
AND HEART DISEASE
Various
studies have found that persons with elevated levels of homocysteine
in their blood are at an increased risk of heart and vessel disease.
These studies include the Physicians' Health Study, the Tromso Study
from Norway, the Framingham Heart Study, and a meta-analysis of
nearly 40 studies. Some studies indicate that persons with elevated
homocysteine levels tend to also have other risk factors for heart
disease, especially smoking, high blood pressure, and high blood
cholesterol. So far, no clinical trial has been done to show that
lowering homocysteine levels alters the progression of heart disease,
or prevents heart attacks or strokes.
WHY
HOMOCYSTEINE?
Much
more basic research must be done before scientists understand how
an elevated homocysteine level affects the development and progression
of heart disease. However, scientists have several theories: First,
a high level of homocysteine may be involved with the process called
atherosclerosis, the gradual buildup of fatty substances in arteries.
Homocysteine also may make blood more likely to clot by increasing
the stickiness of blood platelets. Clots can block blood flow, causing
a heart attack or stroke. Increased homocysteine may affect other
substances involved in clotting too. Finally, higher homocysteine
levels may make blood vessels less flexible--and so less able to
widen to increase blood flow. However, none of theories has so far
been proven.
WHAT
DETERMINES HOMOCYSTEINE LEVELS?
Individuals
differ in their levels of homocysteine. Two key factors affect a
person's homocysteine level--genetics and environment. *Genetics.
Genetic factors help regulate the level of homocysteine in the blood.
For instance, genetic flaws (mutations) can affect homocysteine's
metabolism. The NHLBI Family Heart Study found families with genetic
mutations in the enzymes involved in homocysteine metabolism. The
NHLBI Framingham Heart Study and other investigations have found
a relationship between elevated homocysteine levels and families
with early heart disease. *Environment. The level of homocysteine
in the blood also is affected by the consumption of vitamins, especially
folic acid, B6, and B12. Data from the Framingham Heart Study show
that only 30-40 percent of the population was getting 200 or more
micrograms of folic acid in their diet. The data indicated that
for many persons an intake of at least 400 micrograms was needed
to keep homocysteine levels from becoming elevated. Data also indicate
that homocysteine levels are higher in older persons than younger
ones, and in women after menopause than in those before. But more
research is needed to confirm and explain these differences.
SOURCES
OF FOLIC ACID, B6, AND B12
Americans
who follow a well-balanced diet should get enough vitamins, including
folic acid, B6, and B12. There are no data to support the benefit
of taking a folic acid supplement for heart and vessel diseases.
Some food sources of folic acid, B6, and B12 are given below. The
list includes sample percentages of the recommended daily value
(RDV) for each vitamin. These RDVs are: 400 micrograms for folic
acid, 2 milligrams for B6, and 6 micrograms for B12.
*Folic
acid, also called folate: More than a third of the folic acid
in most Americans' diet comes from citrus fruits, tomatoes, and
vegetables. Grain products also are an important source of folic
acid. For example, 1/2 cup of spinach gives at least a third of
the RDV for folic acid. A 1/2 cup of asparagus, broccoli, or green
peas gives 10-24 percent of the RDV. An 8-ounce glass of orange
juice and 1/2 cup of asparagus each has about a quarter of RDV.
Americans also can use beans and lentils as sources of folic acid.
A 1/2 cup of black-eyed peas, lentils, lima beans, pinto beans,
or navy beans gives at least a third of the RDV for folic acid.
*Vitamin
B6: Americans' major sources of vitamin B6 are meat, poultry,
fish, fruits, vegetables, and grain products. For example, 1 banana
has up to 40 percent of the RDV for B6. One baked potato, a 1 3/4-cup
serving of watermelon, or a 3-ounce serving of salmon or turkey
gives up to a quarter of the RDV for B6.
*Vitamin
B12: Americans' major sources of vitamin B12 are meat, poultry,
fish, and milk and milk products. B12 is not found in fruits, vegetables,
beans, grains, nuts, or seeds. For example, a 3-ounce serving of
mackerel or trout has more than 40 percent of the RDV for B12. A
3-ounce serving of tuna has up to 40 percent of the RDV for B12.
One cup of nonfat plain yogurt has about a quarter of the RDV for
B12. Some foods, such as breakfast cereals, have folic acid and
other nutrients added to them. Check the food label for the RDV
for folic acid. Beginning in January 1998, certain foods have been
required by the U.S. Food and Drug Administration to add folic acid
in order to help prevent birth defects, such as spina bifida. These
foods include enriched breads and rolls, all enriched flours, corn
meals, all enriched macaroni and noodle products, and breakfast
cereals. Food labels may say the product has been fortified with
folic acid.
WHAT
LIES AHEAD?
It
is not yet definitely known if elevated homocysteine is a risk factor
for heart disease--that is, if it really increases a person's chance
of developing heart disease. Known risk factors for heart disease
are age (being 45 or older for men; 55 or older for women), a family
history of early heart disease, high blood pressure, high blood
cholesterol, smoking, obesity, physical inactivity, and diabetes.
Until more research is done, Americans can protect their health
by following a heart-healthy food plan. Those concerned about homocysteine
should talk to their doctor. The NHLBI panel has called for more
research to help answer the many questions about homocysteine's
possible role in the development and progression of heart disease
and stroke.
Reference:
from the National Heart, Lung and Blood Institutue
FOR
MORE INFORMATION:
A healthy eating plan supplies enough folic acid, B6, and B12.
For more information about healthy eating, write to the NHLBI Information
Center, P.O. Box 30105, Bethesda, MD 20824-0105.
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