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A cohort study published in the Journal of Nutrition has evaluated the relationship between nuts and peanut butter consumption and cardiovascular disease (CVD), including coronary heart disease (CHD) and stroke amongst a group of women with type 2 diabetes.
Although nuts are high in fat and calories, previous studies have found that a high consumption has been associated with a reduced risk of CVD. Macronutrients and micronutrients contained in nuts have been found to provide health benefits. Peanuts are proportionally low in saturated fat and high in mono- and polyunsaturated fats which have been shown to improve the blood lipid profile by lowering LDL cholesterol. This study by Li et al from the Harvard Medical School and Harvard School of Public Heath also examined plasma lipids or markers of inflammation to see if nuts influenced the risk of CVD.
The participants of this study were women who were also members of the Nurses’ Health Study. This was carried out from 1980 to 2002 and participants were 6309 women, all with type 2 diabetes. 1171 of these participants also provided blood samples. Participants reported their average frequency of consuming selected foods at commonly used portion sizes including nuts and peanut butter (portion size 28g/1oz and 16g/1 tablespoon respectively) and using a questionnaire starting in 1980 and subsequently every two years. Women were classified into four exposure groups depending on their frequency of nuts and peanut butter consumption, namely almost never, 1-3 serving/month to less than 1 serving a week, 1-4 servings a week and at least five servings a week. Nutrient intake was calculated using consumption frequency and specified portion size. Cardiovascular endpoints, including fatal CHD, non fatal myocardial infarction (MI), stroke and coronary bypass surgery were recorded and where blood samples were provided these were analysed for variety compounds including total cholesterol, triacylglycerols and plasma lipid. Participants also completed a questionnaire every two years assessing a variety of factors including physical activity, smoking, alcohol consumption, menopausal status, duration of diabetes, and hypertension.
Li et al reported 634 cases of CVD endpoints between 1980 and 2002. They discovered that women who, at baseline, consumed more nuts and peanut butter tended to be leaner, more physically active and to smoke less. They reported that these women were less hypertensive and had a slightly longer duration of diabetes. Women in the highest consumption group had higher total energy, polyunsaturated fat, red meat, fruit and vegetable intakes as well as a significantly lower glycaemic load than those in the other groups. The researchers discovered that frequent nut and peanut butter consumption was inversely associated with total CVD risk (after adjustments for age). This association was found to be stronger amongst vegetarians who had higher nut consumption than non vegetarians. Those women in the highest consumption group had a CVD and MI risk lower by 44% than those in the “almost never” group but no significant linear trend across increasing consumption was found. Li et al also found those women in the highest consumption group had significantly lower LDL cholesterol, non-HDL cholesterol and total cholesterol concentrations than those in other groups. No association between HDL cholesterol and nut and peanut butter consumption was found, nor did consumption appear to be associated with those inflammatory markers measured in the blood.
Li et al note that their results agree with and extend previous studies by showing that frequent nut consumption is beneficial for women with diabetes who are at increased risk of CVD. The authors of the study note that as well as improvements in blood lipid levels there are other possible mechanisms by which nut and peanut butter consumption may provide a cardio-protective effect. These include decreased lipoprotein oxidation, inhibition of inflammation, decreasing insulin resistance and improving endothelial function. The authors conclude that although their study has several limitations including possible misreporting of consumption by participants, they say that they would expect this to bias the result toward a null effect. They believe it has many strengths including a large sample size and long duration of follow up. They indicate that their data supports a role for the regular consumption of nuts for patients with diabetes to reduce the risk of CVD.
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