THURSDAY, May 6, 2004 (HealthDayNews) -- Eating certain plant-based foods, such as walnuts, opens the arteries and may help lower cholesterol, a small study finds.

People in the study had a 33 percent improvement in vascular function -- blood flow -- after eating a plant-based diet rich in alpha-linolenic acid (ALA), compared to eating a typical American diet. Walnuts and walnut oil were the prime source of ALA in the study.

"It looks like a good study," said Dr. David Jenkins, a professor in the department of nutritional sciences at the University of Toronto. He cited several studies suggesting the type of omega-3 fatty acid found in plants like walnuts may be as beneficial to the heart as eating fatty, cold-water fish, such as mackerel, salmon and herring.

But Paul Coates, director of the National Institutes of Health's Office of Dietary Supplements (ODS), offered a more cautious point of view. "With few detailed studies on the effects of omega-3 fatty acids from plants on cardiovascular disease, this is an area requiring further research," he noted.

The ODS is sponsoring a review of scientific literature on omega-3 fatty acids for a variety of health conditions.

More than 930,000 Americans die of cardiovascular disease each year, according to the U.S. Centers for Disease Control and Prevention.

To help the heart, the American Heart Association recommends that people eat at least two servings of fish a week, particularly fish containing omega-3 fat, which has been shown to boost levels of high-density lipoprotein -- the "good" cholesterol -- and may help lower triglycerides, a blood fat.

The heart association also recommends eating tofu and other plant-based sources of omega-3, such as walnuts, soybeans, flaxseeds, and their oils. But it says more studies are needed to show a cause-and-effect relationship between ALA and heart disease.

Sheila West, an assistant professor of biobehavioral health at Pennsylvania State University and lead author of the new study, was expected to present the findings Thursday at the American Heart Association's annual conference on Arteriosclerosis, Thrombosis, and Vascular Biology in San Francisco.

Her report follows a small Spanish study in the March 23 online edition of the journal Circulation that said walnuts protect against heart disease.

Both studies were funded by California Walnut Board, which petitioned the U.S. Food and Drug Administration more than two years ago for permission to attach a health claim to walnut labels. The board proposed a label advising consumers that walnuts can reduce the risk of heart disease.

After reviewing the scientific evidence, the FDA in March approved a "qualified health claim" for whole and chopped walnuts. It reads, in part: "Supportive but not conclusive research shows that eating 1.5 oz. of walnuts per day, as part of a low saturated fat and low cholesterol diet, and not resulting in increased caloric intake, may reduce the risk of coronary heart disease."

To examine the potential benefits of plant-based ALA, West and her colleagues randomly assigned 13 adults with high cholesterol to eat three different diets. Each participant ate one diet for a six-week period and then took a two-week break before starting the next one.

One regimen in the study replicated the fat and calories of a typical American diet.

The other two diets had the same amount of total fat, but were low in saturated fat and cholesterol and high in polyunsaturated fatty acids. Half of the fat in those diets came from walnuts and walnut oil. Participants ate 1.3 ounces, or about a handful, of walnuts, and 0.53 ounces, or roughly a tablespoon, of walnut oil each day.

The two diets reflect a subtle difference. One contained varying amounts of ALA and linoleic acid. The other, dubbed the "high-dose" ALA diet, contained additional ALA from flax oil.

After each six-week diet, every participant had a noninvasive ultrasound test called flow-mediated dilation. The test showed how their blood vessels responded to changes in blood flow.

Cholesterol levels improved after people ate the two diets that replaced saturated fat with plant-based omega-3 fats. Low-density lipoprotein -- the "bad" cholesterol -- was reduced 10 to 12 percent with either diet, West noted.

But only the high-dose ALA diet improved vascular function. "The dilation responses were bigger when people consumed the high ALA diet," she said.

"I think one of the more important messages is that we didn't add nuts and oils on top of the regular diet," West said. Those healthy fats replaced saturated fats that people typically consume.

What's not entirely clear is whether the improvements noted in the study stemmed from eating plant-based ALA or from cutting out bad fats and replacing them with good ones.

It's a difficult question to test, West conceded. But Jenkins, the Canadian researcher, said diets that simply remove saturated fat have not uniformly succeeded in reducing heart disease to the extent seen with ALA.

And, West added, "It does give people a sort of grocery list idea about how to change a diet."

More information

The U.S. Food and Drug Administration has more on walnuts and heart disease. Or see the American Heart Association's recommendations for eating fish and other omega-3 fatty acids.

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