Background: Nuts are known to have numerous health benefits but most such data have come from small studies only. In addition, the effect of nut consumption on the hard end-point of all-cause or disease-specific mortality has also not been studied adequately.
Material and methods: Data from two large prospective studies – the Nurses' Health Study (NHS, 76,464 women) and the Health Professionals Follow-up Study (HPFS, 42,498 men) were analyzed to determine the association between nut consumption and subsequent total and cause-specific mortality. Average duration of follow-up in NHS was 30 years and in HPFS 24 years. Nut consumption (1 serving = 1 oz) was assessed at baseline and updated every 2–4 years. Participants with a history of cancer, heart disease, or stroke were excluded. Extensive statistical analyses and sub-group analyses were performed to eliminate the effects of various known and unknown confounding factors.
Results: A total of 27,429 deaths (16,200 women and 11,229 men) occurred over 3,038,853 person-years of follow-up. There was a significant, dose-dependent inverse relationship between nut consumption and total mortality in both the studies. The pooled multivariate hazard ratios for death among participants who ate nuts were 0.93, 0.89, 0.87, 0.85 and 0.80 for those who consumed nuts less than once per week, once per week, two to four times per week, five or six times per week, and seven or more times per week, respectively (p < 0.001 for trend). This relationship persisted across all subgroups based on age, physical activity, body-mass index, smoking status, aspirin use, consumption of fruits/vegetables, etc. A similar beneficial effect of nut consumption was seen on cause-specific mortality also, with significant inverse associations observed for deaths due to cancer, heart disease, and respiratory disease. Finally, comparison between peanuts and tree-nuts revealed similar associations with total and cause-specific mortality for both the types of nuts.
Conclusions: The two large, independent cohort studies involving nurses and other health professionals demonstrated that nut consumption was inversely associated, in a dose-dependent manner, with total and cause-specific mortality. This relationship was independent of other predictors of death.
1. Perspective
Nuts are nutrient-dense foods which are rich in high-quality protein, fiber, unsaturated fatty acids, vitamins (e.g., folate, niacin, and vitamin E), minerals (e.g., potassium, calcium, and magnesium), and phytochemicals (e.g., carotenoids, flavonoids, and phytosterols). Previous studies have shown that nuts possess cholesterol-lowering, anti-inflammatory and antioxidant properties and have favorable effects on endothelial function, glucose homeostasis, and visceral adiposity. Together, these properties result in reduced risk of type 2 diabetes mellitus, hypertension, metabolic syndrome, cardiovascular disease, various forms of cancers and inflammatory diseases. However, most such data have come from studies that were either small or had methodological limitations, thereby compromising the strength of the findings. In addition, only a few of these studies had evaluated the effect of nut consumption on disease-specific or total mortality. In this context, the information provided by the present study assumes great clinical significance. As discussed above, among apparently healthy nurses and other healthcare professionals, regular consumption of nuts was found to be associated with significantly lower risk of all-cause and disease-specific mortality. The magnitude of the observed benefit was directly related to the frequency of nut consumption with almost 20% relative risk reduction observed in total mortality among individuals who consumed seven or more servings of nuts every week. This beneficial effect was seen across all subgroups and with both peanuts and tree-nuts.
There are several strengths of this analysis. First, the data is derived from nearly 120,000 individuals who were followed up for almost three decades resulting in more than 3.0 million patient-years of follow-up. This is thus the most robust evidence-base till date to address this issue. Second, the dietary pattern of the study participants was assessed in a prospective manner, using well-defined questionnaires, thus minimizing chances of reporting errors. A validation study of the food-frequency questionnaire confirmed that nut intake was indeed reported reasonably accurately. Third, dietary assessment was repeated every 2–4 years which ensured that any alterations in dietary patterns were accounted for while determining the relationship between nut consumption and the observed clinical outcomes. Fourth, extensive statistical analyses were performed to minimize the effects of confounding factors and other potential biases, permitting a more accurate assessment of the independent impact of nut consumption on reduction in mortality from various causes. However, despite these strengths, a cause and effect relationship between consumption of nuts and the mortality benefit could not be determined because of the observational nature of the present study. Nevertheless, the fact that the observed benefits had a dose–response relationship and were seen across all subgroups supports a direct pathophysiological role of nuts in these health benefits. Although a randomized, placebo-controlled study is required to definitely establish the cause and effect relationship, considering the findings of the present study, it is now hard to envisage a large-scale randomized study, ever in future, to address this issue.
Thus, in summary, the present report, for the first time, provides strong evidence to demonstrate that regular nut consumption is associated with significantly lower risk of all-cause mortality as well as deaths due to cancers, heart disease and respiratory disease. Although most healthcare authorities already recommend routine consumption of nuts, the findings of the present study should allay any apprehensions that the physicians or the patients may harbor regarding the beneficial effects of nuts.