But the message going to consumers should caution that eating walnuts does not give them carte blanche for the rest of their diet. "Consumers would get the wrong message from our findings if they think they can continue eating unhealthy fats provided they add walnuts to their meals," said corresponding author Emilio Ros from the Universitat Autonoma de Barcelona.
The study, funded by the California Walnut Commission and the Spanish Ministry of Health, investigated the effects of the addition of walnuts or olive oil to a fatty meal on a series of markers for cardiovascular health.
Twelve healthy people and 12 patients with high cholesterol levels were randomly assigned to a high-fat meal (80 g fat, 35 per cent saturated fat) supplemented with 40 grams of walnuts or 25 grams of olive oil. One week later, the participants were crossed over to eat the other supplemented high-fat meal.
The researchers analysed the activity of blood vessels after the meal, lipoprotein levels, markers of oxidative stress, and plasma levels of asymmetric dimethylarginine (ADMA), a by-product of protein metabolism that is said to interfere with the amino acid L-arginine for nitric oxide (NO) production. NO has been shown to act upon smooth muscle in blood vessels and increase blood flow (vasodilation).
The Barcelona researchers report that blood flow in the arm, so-called flow-mediated dilation (FMD) in the brachial artery, was improved in the people with high cholesterol after consumption of the walnut-supplemented meal (24 per cent increase) while the olive oil-supplemented meal actually resulted in a decrease in FMD (36 per cent decrease).
However, lipoprotein levels decreased in similar quantities after both meals, while plasma ADMA concentrations were unaffected.
"The fact that a single walnut meal positively affects postprandial vasoactivity further supports the beneficial effects of walnuts on cardiovascular risk," wrote lead author Berenice Cortés in the Journal of the American College of Cardiology (published on-line ahead of print, doi:10.1016/j.jacc.2006.06.057).
Inflammation, measured as a function of soluble inflammatory cytokine levels, were found to decrease independently of the meal, except for E-selectin, a molecule that plays a role in cell adhesion, which fell after the walnut meal.
"Adding walnuts to a high-fat meal acutely improves FMD independently of changes in oxidation, inflammation, or ADMA. Both walnuts and olive oil preserve the protective phenotype of endothelial cells," concluded Cortés.
Dr. Ros said: "Many people forget that walnuts are an important part of the Mediterranean diet, providing numerous health benefits."
The Med diet, rich in cereals, fruits, nuts, legumes and whole grains, fish and olive oil, has been linked to longer life, less heart disease, and protection against some cancers. The diet's main nutritional components include beta-carotene, vitamin C, tocopherols, polyphenols, and essential minerals.
In fact, he said: "Walnuts, unlike olive oil and other nuts, contain significant amounts of polyunsaturated fatty acids, specifically alpha-linolenic acid (ALA), an essential plant based omega-3. They also provide antioxidants and L-arginine, components identified in past studies as potential nutrients that improve artery function."
Indeed, the researchers wrote: "Recently, [other researchers] reported increased FMD in diabetic patients after meals enriched with either marine n-3 PUFA or ALA, thus supporting the beneficial role of ALA-rich walnuts on endothelial."
Robert Vogel, a researcher from the University of Maryland, who did not participate in the study, said: "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.
"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," said Vogel in a statement.