Eating walnuts with high-fat meals helps to protect arteries against short-term damage

If you've been going a little nuts trying to eat a healthier diet, you just might be on the right track.

New research shows that consuming a handful of raw walnuts along with meals high in saturated fat appears to limit the ability of the harmful fat to damage arteries.

Adding olive oil, however, does not provide the same type of vascular protection. The research, conducted at the University of Barcelona's Hospital Clínico, appears in the Oct. 17, 2006 edition of the Journal of the American College of Cardiology.

“People would get the wrong message if they think that they can continue eating unhealthy fats provided they add walnuts to their meals,” said Emilio Ros, MD, PhD, director of the Lipid Clinic at Hospital Clínico in Barcelona, Spain, the principal site where the research was conducted. “Instead, they should consider making walnuts part of a healthy diet that limits saturated fats.

“Each time we eat a high-fat meal, the fat molecules trigger an inflammatory reaction that, among other ill effects, reduces the elasticity of the arteries,” Dr. Ros said. “Over time, this repeated damage is thought to contribute to hardening of the arteries and, in turn, to heart disease. Our latest research shows that eating walnuts helps to maintain the elasticity of the arteries.”

For the study, Dr. Ros and his colleagues recruited 24 nonsmoking adults with normal body weights and blood pressures. Half of the participants had normal cholesterol levels and half had moderately high levels. Each was asked to follow a cholesterol-lowering Mediterranean diet for two weeks prior to the study and throughout its duration. A Mediterranean diet includes foods low in saturated fat but high in fiber or monounsaturated fat, such as fruits and vegetables, whole grains and olive oil.

Study participants were randomly assigned to one of two groups. Each was provided with two high-fat meals, eaten one week apart. The meals were identical, consisting of a salami-and-cheese sandwich on white bread and a small serving of full-fat yogurt. For one meal, the researchers added about 5 teaspoons (25 ml) of olive oil. For the other, they added 40 grams of walnuts, or about eight shelled nuts.

According to their findings, both the olive oil and the walnuts helped to decrease the sudden onset of inflammation and oxidation in the arteries. These harmful processes, which typically follow consumption of high-fat meals, can lead to hardening of the arteries, or atherosclerosis, a precursor to heart disease.

But unlike olive oil, adding walnuts also helped to preserve the elasticity and flexibility of the arteries, regardless of people's cholesterol levels. This elasticity allows the arteries to expand when needed to increase blood flow to the body.

“The inner lining of the arteries produces a substance called nitric oxide that is needed to keep the arteries flexible,” Dr. Ros said. “When we eat high-fat meals, the fat molecules temporarily disrupt the production of nitric oxide, preventing the arteries from increasing blood flow in response to physical activity.”

One of the nutrients found in walnuts, he said, is arginine, an amino acid used by the body to produce nitric oxide. Walnuts also contain antioxidants and alpha-linolenic acid (ALA), a plant-based omega-3 fatty acid. Olive oil does not contain ALA, a specific type of healthy, polyunsaturated fat.

Currently, Dr. Ros is beginning a new trial to determine whether the ALA in walnuts will help people with abnormal heart rhythms. The ALA, although from plants, is similar to the omega-3 fatty acids found in fish. These fatty acids already have been shown to help prevent atrial fibrillation in some people.

For people worried about gaining weight from adding walnuts or olive oil to their diets, Dr. Ros said no evidence exists that links healthy fats to increases in body weight. Eating more saturated fats, such as butter or lard, however, often leads to weight gain because these fats usually are consumed with simple sugars, he said.

Robert A. Vogel, MD, a professor of medicine at the University of Maryland, did not participate in the study, but said it provides further insight into how eating the right foods and the right combinations of foods can help to preserve people's health.

“This demonstrates that the protective fat from walnuts actually undoes some of the detrimental effects of a high-saturated-fat diet, whereas a neutral fat, such as olive oil, does not have as much protective ability,” Dr. Vogel said. “This raises a very interesting issue because many people who eat a Mediterranean diet believe the olive oil is providing the benefits. But this research and other data indicate that's not true. There are probably other factors in the diet, including that it is a relatively rich source of nuts. This is not to say that olive oil is bad, but it's not the key protective factor in the Mediterranean diet.”

In fact, Dr. Vogel said, research continues to indicate that all monounsaturated-rich foods, including olive oil, likely are relatively neutral in terms of their ability to protect vascular health. On the other hand, he said, omega-3 rich oils and fats – including walnuts, canola oil and flaxseed oil – “are probably quite protective.”

Dr. Vogel said more research is needed to determine how heating and cooking walnuts affects their protective abilities and whether they need to be eaten raw in order to provide the greatest health benefit.

Research for this study was conducted at Hospital Clínico's Lipid Clinic and Ultrasound Laboratory in collaboration with the hospital's Biological Diagnosis Department and with Autonomous University, all in Barcelona.

This study was supported with grants from the California Walnut Commission, the Spanish Ministry of Health and Fundación Carolina. Dr. Ros serves on the scientific advisory board of the California Walnut Commission.

Also in this issue of JACC

Researchers in Milan and Monza, Italy, studied the effects of adding polyunsaturated fatty acid supplements to the diets of heart-attack survivors with systolic heart failure, a condition in which the heart is unable to pump enough blood to the body.

The supplements provided the dietary equivalent of eating 200 grams (7.1 ounces) of fish rich in healthy omega-3 fatty acids every day. The researchers speculated the supplements would enhance heart rate variability as well as improve the heart's ability to detect and respond to blood pressure changes.

According to their findings, the study “provides the first demonstration that a 4-month dietary supplementation of polyunsaturated fatty acids is accompanied by a partial restoration of several indices of cardiovascular homeostatic control that are typically depressed in the heart failure patient.”