To lower our risk of heart disease, doctors have been telling us for years to cut back on the fat and cholesterol in our diets. But according to one recent theory, most heart attacks are actually caused by a deficiency of three B vitamins: B9 (better known as folic acid), B6, and B12.

This trio of B's performs many tasks in the body. One of their jobs is to convert a naturally occurring amino acid called homocysteine into a harmless form that the body can excrete. Having high levels of homocysteine in your blood is more dangerous than having high cholesterol, according to pathologist Dr. Kilmer McCully, co-author of The Heart Revolution (Harperperennial, 2000). Homocysteine (pronounced hoe-mow-SISS-teen) damages the walls of arteries by making them tough and stiff. A high cholesterol level is still unhealthy, says Dr. McCully, but only because LDL cholesterol (the "bad" cholesterol) carries homocysteine molecules to the arteries, where they do their damage.

To support his theory, Dr. McCully points to several large population studies, which show that people who have heart attacks tend to have more homocysteine in their blood than people with healthy hearts. A study published in 1998 suggested that folic acid and B6, in particular, might be the key to preventing cardiovascular trouble. The Harvard School of Public Health studied over 80,000 women and found that people who consumed the greatest amounts of these two vitamins had about a 50 percent lower risk of heart attack than women who ate the least.

Until there's stronger proof that homocysteine causes heart disease, the American Health Association says that it's not necessary for most people to have their blood levels measured. But if you have heart disease already, or come from a family with a strong history of cardiovascular problems, talk with your doctor about having a homocysteine test or taking nutritional supplements.

The AHA recommends getting healthy doses of folic acid, B6, and B12 from food, and Dr. McCully heartily agrees. "These three vitamins are crucial in determining homocysteine levels," he says. To get your share, Dr. McCully offers the following tips:

• Eat six to 10 servings of fruits and vegetables each day. Avoid canned and frozen produce; processing strips away many nutrients. For example, asparagus is a good source of folic acid, but the canned variety has 35 percent less of the vitamin than you get from fresh leaves. Some other good choices for heart health include broccoli, cabbage, cantaloupe, and kale.

• Make beans a bigger part of your diet. They're a terrific source of the three B's, too, as are whole-grain wheat products. (White bread and other refined-grain foods lose vitamins, as well as fiber, minerals, and other nutrients when they're milled.)

• Limit your consumption of sugary snacks and desserts -- they crowd out healthier sweets, namely fruit. When you get a hankering for cookies or cake, just think: An apple a day might keep the cardiologist away.