In this large prospective study, frequent nut consumption in women was associated with a significantly reduced risk of coronary heart disease. Women who consumed nuts
5 times per week had about a 35% lower risk of coronary heart disease than those women who rarely ate nuts.
We considered the possibility that confounding may explain the observed inverse association because women who frequently consumed nuts had a healthier lifestyle and diet than those who rarely ate nuts (table ). Our detailed analyses, however, took the differences in these factors into account. As expected such adjustments attenuated the associations, but a clear and statistically significant risk reduction persisted. Women who ate nuts tended to eat less meat than those who rarely ate nuts, but the differences were modest, and very few women were vegetarians. Adjustment for intakes of vegetables, fruits, red meat, and fat did not appreciably alter our results. In addition the inverse association was present in all subgroup analyses further suggesting an apparent independent protective effect. Although we cannot rule out the possibility of residual confounding it is unlikely that it can fully explain the observed strong inverse association.
Another potential concern is inaccurate assessment of nut consumption. However, nut consumption was reported on dietary questionnaires with reasonable accuracy.11
Moreover, misclassification of exposure would most likely bias the relative risks toward null. We assessed nut consumption repeatedly during follow up so that our updated analyses took into account dietary changes.
Our results are consistent with other epidemiological studies.2,3
In the Adventist Health Study, subjects who consumed nuts
5 times per week had a 50% reduced risk of coronary heart disease than those who never consumed nuts.2
In the Iowa Women’s Health Study, the relative risk of death from a coronary event in women who consumed nuts 2-4 times per week with those who almost never consumed nuts was 0.43.3
It is biologically plausible that nut consumption can reduce the risk of coronary heart disease. Although nuts are high in fat, the fat is mostly unsaturated,8
which has beneficial effects on blood lipids.4
Sabate randomised 18 men with normal serum cholesterol concentrations to either a control diet or a diet enriched with walnuts and then switched the diets after 4 weeks.5
Both diets conformed to those of the national cholesterol education programme. Walnuts provided 20% of calories in the walnut diet, replacing animal and visible fat. The walnut diet lowered the ratio of serum concentrations of low density lipoprotein: high density lipoprotein by 12%. Diets supplemented with almonds have also shown a cholesterol lowering effect.6,7
As well as having potential beneficial effects on blood lipids, nuts may protect against coronary heart disease through other mechanisms.16,17
Most nuts are rich in arginine, the precursor of nitric oxide, an endothelium derived relaxing factor.18
Nitric oxide is a potent vasodilator and can inhibit platelet adhesion and aggregation. In addition, walnuts are high in α linolenic acid (about 10% of weight). High intakes of α linolenic acid have been associated with reduced risk of coronary heart disease in several prospective studies, possibly due to the antithrombotic and antiarrhythmic effects of α linolenic acid.19,20
Other postulated explanations for benefits of nuts include their high content of magnesium, copper, folic acid, protein, potassium, fibre, and vitamin E.8,17
Frequent consumption of peanuts was associated with a low risk of coronary heart disease. Although one study showed that peanut oil was atherogenic when fed to rats and rabbits,21
in another study it was antiatherogenic when fed to cynomolgus monkeys; the blend of fats approximated the fatty acid composition of the average diet in the United States.22
In addition, a low fat diet supplemented with peanuts appreciably improved serum lipoprotein profiles when compared with a regular low fat diet in postmenopausal women with raised serum cholesterol concentrations.23
As different types of nuts comprise similar nutrients, reflected by their similar lipid profile,8
we do not expect the inverse association to differ between peanuts and other nuts. Peanut butter was only weakly associated with a risk of coronary heart disease, but this may be due to the addition of hydrogenated fat to major American brands.24
In conclusion, frequent nut consumption was associated with a reduced risk of both fatal coronary heart disease and non-fatal myocardial infarction in our large prospective study. These data, and those of other epidemiological and clinical studies, support a role for nuts in reducing coronary heart disease risk.