However, Dr. Aletta E. Schutte of North-West University, Potchefstroom and colleagues found that eating a diet rich in walnuts actually impaired baroreflex sensitivity. They call for more research into the effects of eating large quantities of nuts on heart health.
The baroreflex becomes less sensitive as heart disease develops. People with metabolic syndrome, a constellation of symptoms including obesity, high cholesterol and high blood sugar, have an increased risk of heart disease and diabetes, and also have impaired baroreflex sensitivity.
Cashews are high in monounsaturated fatty acids (MUFA), while walnuts contain polyunsaturated fatty acids (PUFA). To investigate the effect of the nuts in people with the metabolic syndrome, Schutte and her colleagues divided 62 people with the syndrome into three groups. One followed a normal diet, the second ate a diet with 20% of calories from walnuts, and the third got 20% of their calories from unsalted cashews.
After eight weeks, there was no change in baroreflex sensitivity among people on the control diet. Those on the walnut-rich diet showed significant impairment in baroreflex sensitivity, while baroreflex sensitivity improved among those eating the cashew-heavy diet. No other significant changes were seen in other components of the metabolic syndrome, and neither diet produced changes in levels of cholesterol and other blood fats. However, the people on the cashew diet showed higher blood sugar levels.
The most obvious interpretation of the findings is to attribute them to differing effects of MUFA and PUFA on baroreflex sensitivity, Schutte and her team note. However, the nuts also contain a wide variety of other ingredients that can benefit blood vessel function such as fiber and folic acid, they add.
"An overwhelming body of evidence has demonstrated the beneficial effects of nuts, but this study suggests that intake of large quantities of walnuts and cashews may have detrimental effects on the baroreflex sensitivity and glycemia, respectively, at least in individuals with metabolic syndrome," the researchers write.
"Further research defining the potential benefits and harmful effects of nuts, especially when consumed in large quantities, is therefore needed," they conclude.