Reverse causality is another possible explanation for our findings, because people with chronic disease and poor health might abstain from nut consumption. However, we excluded participants with a history of cancer, heart disease, or stroke at baseline, and we suspended further updating of all dietary variables when participants reported a diagnosis of stroke, heart disease, angina, or cancer. Moreover, the results remained significant when we excluded the first 2 years of follow-up and added a 2-year lag period between nut-intake assessment and each follow-up period.
Indeed, clinical trials have shown that nut consumption has beneficial effects on some intermediate markers of chronic diseases, such as high cholesterol levels,3 oxidation,6,7 endothelial dysfunction,13 hyperglycemia,6,10 and insulin resistance.11,12
The above quotes from the NEJM article imply more than simple correlation, but the conclusion of that article stops short of stating that, specifically, "Nonetheless, epidemiologic observations establish associations, not causality, and not all findings from observational studies have been confirmed in controlled, randomized clinical trials."
Source: http://www.nejm.org/doi/full/10.1056/NEJMoa1307352#t=article