Laurie Barclay, MD
The frequency of nut consumption was inversely associated with total and cause-specific mortality independent of other predictors of death, according to a study from 2 large prospective US cohorts, published in the November 21 issue of the New England Journal of Medicine.
"Nuts are rich in nutrients, such as unsaturated fatty acids, fiber, vitamins, and minerals, which may confer cardioprotective, anticarcinogenic, antiinflammatory, and antioxidant properties," lead author Ying Bao, MD, ScD, associate epidemiologist and instructor in medicine, Brigham and Women"s Hospital and Harvard Medical School, Channing Division of Network Medicine, Boston, Massachusetts, told Medscape Medical News. "Previous studies have shown that nut consumption has beneficial effects on various mediators of chronic diseases (eg, blood cholesterol, inflammation, and insulin resistance), and increased nut intake is associated with reduced risks of many chronic diseases (eg, coronary heart disease and type 2 diabetes mellitus)."
The investigators studied the association between nut intake and mortality among 76,464 women in the Nurses’Health Study (1980 - 2010) and 42,498 men in the Health Professionals Follow-up Study (1986 - 2010). The authors excluded participants with a history of cancer, heart disease, or stroke. They evaluated nut consumption at baseline and every 2 to 4 years thereafter.
"[F]ew studies have investigated nut consumption in relation to total mortality, and these investigations have often been limited by small samples, single assessment of diet and other covariates, or inadequate adjustment for important confounding factors," the authors write.
"To our knowledge, this study is the largest study on nut intake and total mortality, and the first to examine the associations between nut intake and a variety of causes of death," Dr. Bao said.
During 3,038,853 person-years of follow-up,16,200 women and 11,229 men died.
Compared with participants who did not eat nuts at all, those who ate nuts less than once per week had a 7% decrease in mortality risk (pooled multivariate hazard ratio [HR], 0.93; 95% confidence interval [CI], 0.90 - 0.96), after adjustment for other known or suspected risk factors, including total sodium intake, adherence to a Mediterranean diet, and olive oil intake. Pooled multivariate HRs were 0.89 (95% CI, 0.86 - 0.93) for eating nuts once per week, 0.87 (95% CI, 0.83 - 0.90) for 2 to 4 times per week, 0.85 (95% CI, 0.79 - 0.91) for 5 to 6 times per week, and 0.80 (95% CI, 0.73 - 0.86) for 7 or more times per week (P < .001 for trend).
There were also significant inverse associations between nut consumption and deaths resulting from cancer, heart disease, and respiratory disease. Results were similar for peanuts and tree nuts, and the inverse association persisted across all subgroups.
"In two large prospective U.S. cohorts, we found a significant, dose-dependent inverse association between nut consumption and total mortality, after adjusting for potential confounders," the authors write. "[C]ompared with participants who did not eat nuts, those who consumed nuts seven or more times per week had a 20% lower death rate."
Study Strengths and Limitations
Study strengths include prospective design, large sample size (more than 27,000 deaths), 30 years of follow-up with a follow-up rate exceeding 90%, repeated assessment of diet and lifestyle variables (including separate data on peanuts and tree nuts), and extensive data on known or suspected confounding variables.
"Given the observational nature of our study, it is not possible to conclude that the observed inverse association between nut consumption and mortality reflects cause and effect," the authors write. "However, our data are consistent with a wealth of existing observational and clinical-trial data in supporting the health benefits of nut consumption for many chronic diseases. In addition, nutrients in nuts, such as unsaturated fatty acids, high-quality protein, fiber, vitamins (e.g., folate, niacin, and vitamin E), minerals (e.g., potassium, calcium, and magnesium), and phytochemicals (e.g., carotenoids, flavonoids, and phytosterols), may confer cardioprotective, anticarcinogenic, antiinflammatory, and antioxidant properties."
Other study limitations include reliance on self-report, possible underestimation of the association, lack of data on method of nut preparation, and restriction of the study sample to health professionals, which could limit generalizability of the results.
Clinical Implications
"[C]linical trials have shown that nut consumption has beneficial effects on some intermediate markers of chronic diseases, such as high cholesterol levels, oxidation, endothelial dysfunction, hyperglycemia, and insulin resistance," the authors write. "Moreover, recent findings from the [Prevencion con Dieta Mediterranea (PREDIMED)] trial [ N Engl J Med. 2013;368:1279-1290] have shown a protective effect of a Mediterranean diet against cardiovascular disease, and one component of the diet was the availability of an average of 30 g of nuts per day."
Dr. Bao added, "A reasonable recommendation would be to follow the [USDA] Dietary Guidelines, and adding nuts to daily diet may also be beneficial."
Because the mechanisms through which nuts may affect health and longevity are still unclear, more research is needed to elucidate the underlying biological mechanisms. Also unknown and meriting additional research are the associations between nut consumption and cancer recurrence and mortality among cancer patients.
"There may be a concern that frequent nut consumption can result in weight gain," the authors conclude. "However, in these two cohorts, increased nut intake was associated with less weight gain. Similarly, in other prospective cohort studies and clinical trials, increased nut consumption was associated with reduced waist circumference, less weight gain, and a decreased risk of obesity."
This study is supported by grants from the National Institutes of Health and the International Tree Nut Council Nutrition Research and Education Foundation. Full conflict-of-interest information is available on the journal’s Web site.
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