Barcelona, Spain - Another study has suggested that eating walnuts can reverse the impairment of endothelial function associated with eating a fatty meal [1]. But olive oil did not have the same beneficial effect.

Senior author of the study, Dr Emilio Ros (Hospital Clínico, Barcelona, Spain), explained to heartwire: "When we eat a fatty meal, inflammatory molecules are increased that prevent the endothelium from producing nitric oxide, which thus leads to endothelial dysfunction. In our study, eating a handful of walnuts prevented the increase in the inflammatory substances and the endothelial dysfunction, whereas olive oil prevented the increase in inflammatory molecules but did not prevent the endothelial dysfunction associated with eating fatty food. Olive oil does have some beneficial effects—it is not bad, but walnuts are better."

Ros et al have previously reported a study showing eating walnuts over a four-week period can improve endothelial dysfunction. The current study, published in the October 17, 2006 issue of the Journal of the American College of Cardiology, adds to this by showing that the effect is seen after just one serving.

But Ros said that people should not be told that they can continue eating unhealthy fats provided they add walnuts to their meals. Instead, they should consider making walnuts part of a healthy diet that limits saturated fats.

He noted that walnuts have several components that could be contributing to their benefits. These include polyunsaturated fats, including alpha-linolenic acid; arginine, which is a precursor of nitric oxide; and many antioxidants. "It could be any one of these or maybe all three together that protect the vessels." Olive oil also contains antioxidants but has more mono- rather than polyunsaturated fats, and it does not contain arginine or omega-3 oils, he added.

He said that while walnut oil would probably also be somewhat beneficial, eating the nuts themselves was a better option, as the oil does not contain all the beneficial components. "The oil contains the fats and the fat-soluble antioxidants, but it does not contain arginine, which is not fat soluble," he explained to heartwire. He also pointed out that it was better to eat the raw nuts, rather than cooking them, as heating could inactivate some of the beneficial components.

"I would recommend that people eat a handful of walnuts every day—about six to eight nuts. They could eat these before or after a meal or as part of the meal—for example, in salads and desserts. Or they could replace unhealthier options that are usually used for snacks," he advised.

The current study had a crossover design and involved 24 nonsmoking adults with normal body weights and blood pressures, half of whom had normal cholesterol levels and half had moderately high levels. Each was asked to follow a cholesterol-lowering Mediterranean diet for two weeks before the study and throughout its duration. They were 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 g of walnuts, or about eight shelled nuts. Venipunctures and ultrasound measurements of brachial artery endothelial function were performed after fasting and four hours after test meals.

Results showed that in both study groups, flow-mediated dilation (a measure of endothelial dysfunction) was worse after the olive oil meal than after the walnut meal. Levels of oxidized LDL decreased after both meals, as did concentrations of soluble inflammatory cytokines and adhesion molecules. But the adhesion molecule E-selectin was reduced more after the walnut meal.

Commenting on the study in a journal press release, Dr Robert Vogel (University of Maryland, College Park), 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." Vogel added that 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."