Eating more nuts — especially tree nuts such as walnuts and almonds — was associated with lower cardiovascular risk in people with diabetes, according to a large prospective cohort study.
Compared with study participants who ate less than one serving of nuts per month, those who consumed five or more servings per week had a 17% reduction in total cardiovascular disease incidence (HR 0.83, 95% CI 0.71-0.98, P=0.01), reported Gang Liu, PhD, of Harvard T.H. Chan School of Public Health in Boston, and colleagues in Circulation Research.
They also found a 20% reduction in coronary heart disease incidence (HR 0.80, 95% CI 0.67- 0.96, P=0.005), a 34% decrease in cardiovascular mortality (HR 0.66, 95% CI 0.52-0.84, P<0.001), and a 31% drop in all-cause mortality (HR 0.69, 95% CI 0.61-0.77, P <0.001) for the group consuming five or more servings.
The trend persisted in subgroup analyses stratified by gender, body weight, smoking status, and other factors, and was also evident in people who increased their nut consumption after being diagnosed with diabetes.
Compared with participants whose consumption of nuts remained the same, the risk was lower in those who ate more nuts after their diagnosis by 11% for cardiovascular disease, 15% for coronary heart disease, 25% for cardiovascular mortality, and 27% for all-cause mortality (95% CIs and P values not given).
The study also found a dose-response relationship: Each additional one-ounce serving of nuts per week was associated with a 3% reduction in cardiovascular disease incidence (95% CI 1%-6%) and a 6% decrease in cardiovascular mortality (95% CI 3%-10%, P<0.001 for trend). In addition, the association was stronger for tree nuts -- walnuts, almonds, Brazil nuts, cashews, pistachios, pecans, macadamias, hazelnuts, and pine nuts -- compared with peanuts (which are actually legumes because they grow underground), the researchers said.
“Higher consumption of nuts, especially tree nuts, is associated with lower cardiovascular disease incidence and mortality among participants with diabetes. These data provide novel evidence that supports the recommendation of incorporating nuts into healthy dietary patterns for the prevention of cardiovascular disease complications and premature deaths among individuals with diabetes,” Liu and colleagues wrote.
Asked for her perspective, Lauri Wright, PhD, a nutrition scientist at the University of North Florida in Jacksonville, who was not involved with the study, agreed that people should eat more nuts. They have been linked with reduced inflammation, improvements in blood pressure, and other benefits, she told MedPage Today via email.
In addition, the study was unique in its finding of a dose-response relationship associated with as little as a one-ounce serving per week, Wright said. “Other studies have found health benefits with four to seven servings per week.”
According to a CDC report on nut consumption among U.S. adults, only 38% of Americans eat nuts on any given day. The FDA suggests that a 1.5-ounce daily serving of nuts may reduce the risk of heart disease, but the report showed that only 14% of men and 11% of women ate this amount.
Europeans eat more nuts than Americans, averaging about an ounce per day, Wright said. “Some of the best ways to add nuts to your diet: snack on nuts between meals, sprinkle nuts in your salad, add nuts to muffins or bread, do a nut-crusted chicken or tilapia, or mix nuts into cream cheese for a yummy spread,” she said.
The study included 16,217 men and women with diabetes at baseline or diagnosed during follow-up from the Nurses’ Health Study, established in 1976, and the Health Professionals Follow-Up Study, established in 1986. Participants were followed until 2014, and their nut consumption was assessed with a validated food frequency questionnaire and updated every 2-4 years.
There were 3,336 incident cases of cardiovascular disease and 5,682 deaths during the study period. The researchers used Cox proportional hazards models to investigate the association between nut consumption and cardiovascular outcomes.
“Although the exact mechanism underlying the beneficial effects of nut consumption for individuals with diabetes remains to be further elucidated, accumulating evidence from clinical trials among participants with diabetes suggests that nut consumption may improve glycemic control, blood pressure, lipid metabolism, inflammation, and endothelial dysfunction,” the study authors said.
“These beneficial effects can be at least partially explained by the unique nutritional composition of nuts, including unsaturated fatty acids, fiber, vitamins (such as vitamin E and folate), minerals (such as calcium, potassium, and magnesium), and phytochemicals (such as flavonoids and phytosterols),” the team added.
For example, evidence suggests that the polyunsaturated fats in nuts could regulate gene expression in lipid metabolism through various mechanisms, including changing membrane composition, eicosanoid production, and intracellular calcium levels, the researchers explained. The phytosterols in nuts have also been shown to reduce cholesterol absorption.
“In addition, several clinical trials demonstrated that nut supplementation could positively modulate the expression of certain microRNAs (such as miR-192 and miR-375) related with glucose metabolism and insulin sensitivity,” Liu’s group wrote.
The investigators cautioned that the study was observational and therefore not able to prove causality. Another limitation was that the analysis did not take into account how nuts were prepared (for example, raw, roasted, or salted) and how that may have influenced outcomes. “In addition, no information on nut allergy was collected in our population, which should be taken into account in future studies. Lastly, the role of confounding by genetic susceptibility or psychosocial stress, residual confounding due to measurement errors of covariates, including other lifestyle factors, or chance in the present study could not be excluded,” the team noted.
The study was supported by the National Institutes of Health.
Liu reported having no conflicts of interest; two co-authors reported grants from the California Walnut Commission.