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FRIDAY, March 26 (HealthDayNews) -- Walnuts protect against heart disease, a small Spanish study contends.

"This is the first time a whole food, an intact food, has shown this effect," says Dr. Emilio Ros, lead author of research appearing in the March 23 online edition of Circulation. "This adds another piece of scientific evidence."

But others note the study, which was partially funded by the California Walnut Commission, was a small one and leaves several questions unanswered.

"This is promising preliminary evidence that walnuts as a foodstuff may have some interesting properties that help coronary disease, but it's too short-term and too limited in the number of patients," says Dr. Marc Siegel, a clinical associate professor of medicine at New York University School of Medicine in New York City.

Nuts have been shown to have heart-beneficial effects and, last summer, the U.S. Food and Drug Administration allowed the California Walnut Commission to use a qualified claim on the nut's health properties. It reads, "Supportive but not conclusive research shows that eating 1.5 ounces per day of walnuts as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease." The FDA had previously decided that "there is not significant scientific agreement that walnuts may reduce the risk of CHD [coronary heart disease]."

Coronary heart disease affects an estimated 12.6 million Americans, making it the most common form of heart disease. CHD usually results from a condition known as atherosclerosis, which occurs when plaques or fatty substances form inside the arteries that supply blood to the heart.

Epidemiological studies, in particular, have shown that eating nuts on a frequent basis reduces the risk of coronary artery disease.

"These studies have shown that when you eat nuts more than five times a week, which means almost daily, you get a relative risk reduction of coronary heart disease close to 50 percent," says Ros, head of the Lipid Clinic at Hospital Clinic of Barcelona. "That's really an important reaction and not explained by cholesterol lowering."

To find out what other processes might be at play, Ros and his colleagues randomly assigned 21 men and women with high cholesterol levels to follow a Mediterranean diet or a similar diet in which walnuts replaced about one-third of the calories from olive oil, olives, avocadoes and other monounsaturated fats. No nuts other than walnuts were allowed in the Mediterranean diet.

Measurements revealed that the walnuts actually had a dual effect on heart health.

First, the walnut-laced diet increased the elasticity of arteries by 64 percent. Second, it reduced the level of molecules that can gum up blood vessels by 20 percent. It also decreased total and LDL ("bad") cholesterol levels.

It's not entirely clear why walnuts may have this beneficial effect. But, unlike other nuts, they do have high levels of omega-3 fatty acids that have healthful properties. Walnuts also contain the amino acid L-arginine and a form of vitamin E that may prevent vessel blockage, Ros says.

Is all this cause to up your walnut intake? While Ros sees no reason not to, Siegel is more cautious and cites some methodological limitations to the new study.

For one thing, Siegel says, the researchers looked specifically at the brachial artery (in the arm), which is not the same as the coronary artery (supplying blood to the heart). "Blood flow through one artery is not the same," Siegel says. "It might be predictive, but it's not identical."

Also, the researchers removed other monounsaturated fats from the diet. "What is the significance of removing that from the diet?" Siegel asks.

Finally, the duration of the trial could be a problem. "They're only looking at results over a four-week period of time, which is way too short to be drawing conclusions," Siegel says. "Walnuts are not well enough studied to say they're not bad for you."

More information

The American Heart Association has more on eating for a healthy heart and more on different types of fats.

(SOURCES: Emilio Ros, M.D., head, Lipid Clinic, Hospital Clinic of Barcelona, Spain; Marc Siegel, M.D., clinical associate professor, medicine, New York University School of Medicine, New York City; March 23, 2004, Circulation online)

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