Previous studies have shown that diets enriched in a variety of nuts reduce total and LDL cholesterol by 5% to 15%, although it is speculated that nuts may have antiatherosclerotic effects beyond cholesterol lowering.

In addition to beneficially altering lipid profiles, explain investigators, nuts also contain sizable amounts of antioxidants, as well as L-arginine, the precursor amino acid of the endogenous vasodilator nitric oxide (NO). In addition, walnuts have a high content of -linolenic acid (ALA), a vegetable n-3 fatty acid, which might also confer additional antiatherogenic properties. Previous studies have shown that n-3 fatty acids, antioxidants, and L-arginine improve vascular reactivity, although whole foods have previously not been tested.

To investigate whether walnut intake could improve endothelial function in hypercholesterolemic subjects, the authors conducted a study by substituting a walnut-enriched diet for a healthy Mediterranean-type diet. After a four-week equilibrium period, 21 asymptomatic men and women with hypercholesterolemia were randomized in a crossover design to the different diet sequences for four-week periods. The walnut-enriched diet was similar to the Mediterranean diet, but walnuts partially replaced olive oil and other monounsaturated fatty acid (MUFA) foods such as olives and avocados.

To measure the effects of each diet, investigators obtained ultrasound measurements of brachial artery vasomotor function, as well as circulating markers of endothelial activation.

Compared with the olive-oil-rich diet, a diet enriched with walnuts significantly improved endothelium-dependent vasodilation (EDV). Of the 18 participants who completed the study, nine subjects had EDV improve >2% on the walnut diet. The walnut diet also reduced vascular cell adhesion molecule-1 (VCAM-1). There were no significant differences between diets with regard to endothelium-independent vasodilation, hyperemic flow, or intracellular adhesion molecule-1 (ICAM-1).

Consistent with other studies, the walnut diet also reduced cholesterol levels, although there were no significant differences between the two diets.

"The unique composition of walnuts, a whole food rich in several nutrients and phytochemicals with the capacity to improve vascular reactivity, probably explains why substituting walnuts for part of the MUFA in a Mediterranean diet was associated with improved endothelial function in men and women with hypercholesterolemia," conclude Ros and colleagues. "This finding might explain the cardioprotective effect of regular nut intake beyond cholesterol lowering. It also provides further support for the inclusion of walnuts in healthy diets."