In this prospective cohort, we observed a lower incidence of hypertension with nut consumption among US male physicians. In addition, this association was mainly observed among lean subjects (BMI < 25 kg/m2
) and not in overweight or obese individuals. These findings are consistent with preliminary reports from the PREDIMED trial, which showed that compared with a low-fat diet, Mediterranean diets supplemented with nuts and olive oil were associated with 7.1 and 5.9 mm Hg reduction in systolic blood pressure after 3 months of intervention, respectively, among ~257 adult men and women per intervention group15
. In that study, corresponding effects for diastolic blood pressure were 2.6 and 1.6 mm Hg for Mediterranean diet with nuts and olive oil15
, respectively. Because the Mediterranean diet with nuts was compared to either Mediterranean diet with olive oil or low-fat diet, it is difficult to estimate the individual contribution of nuts on blood pressure reduction in that trial.
In addition to direct effects of nuts on blood pressure, previous studies have reported beneficial effects of nut consumption on other major risk factors for hypertension. Data from the SUN Study23
found that subjects who consumed nuts 2+ times per week had a 31% (OR: 0.69; 95% CI: 0.53-0.90) lower risk of weight gain (defined as adding 5+ kg of body weight during the follow up) than those who never ate nuts. Similarly, individuals who never ate nuts gained on average 424 g (95% CI: 102-746) more than frequent nut eaters after a median follow up of 28 months23
. In addition, nut consumption has been associated with a lower risk of type 2 diabetes among female nurses in a dose-response fashion24
Though nut consumption has been shown to reduce the risk of cardiovascular disease and improve lipid profiles, concerns that intake of nuts (rich in fat) may lead to obesity remain in the general population. However, such concerns have not been substantiated by epidemiological data. For example, in the SUN study23
, the relative risk of becoming overweight/obese among subjects consuming nuts at least 2 times per week was 0.73 (0.48-1.11) compared with people who never ate nuts after adjustment for age, sex, baseline BMI, exercise, smoking, television viewing, and energy intake. Furthermore, there was no evidence for a statistically significant weight gain with Mediterranean diet supplemented with nuts compared with baseline weight15
. What physiologic mechanisms could help establish a causal relation between nuts and hypertension?
Nuts are low in sodium and also contain a variety of nutrients including mono- and polyunsaturated fatty acids, minerals such as magnesium and potassium, fiber, antioxidants, and vitamins with beneficial influence on blood pressure13,14
. Sodium reduction has been shown to result in lower blood pressure8
. Dietary fiber influences satiety and energy intake and could improve blood pressure through weigh reduction. A meta-analysis of 25 studies reported blood pressure lowering effects of dietary fiber, especially among hypertensive subjects25
. Magnesium intake has been shown to lower blood pressure26
by acting as a calcium channel blocker and causing vasodilation27
. Magnesium stimulates prostacyclin production and nitric oxide synthesis and modulates endothelium-dependent and independent vasodilation28
. However, results from clinical trials have been heterogeneous and inconsistent29
. Other minerals contained in nuts could favorably influence blood pressure14
. In the Multi-Ethnic Study of Atherosclerosis30
, nut and seed consumption was associated with lower levels of C-reactive protein, interleukin-6, and fibrinogen. This suggests that nuts may inhibit inflammation and prevent progression of atherosclerosis, an important factor in the development of hypertension. It is however unlikely that these possible physiologic mechanisms could have been differential in lean vs. overweight/obese subjects to explain the observed interaction between adiposity and nuts on the risk of hypertension in the present study. A plausible explanation is that overweight/obese subjects avoided nuts as energy-dense foods in order to manage their weight. Alternatively, overweight/obese subjects may have been more likely to underreport their nut consumption, thereby diluting any beneficial effect of nut consumption on blood pressure.
Our study has some limitations. Participants were male physicians who may have different behaviors or lifestyle habits than the general population, thereby limiting the generalizability of our findings. Nut consumption was assessed only once (12 months post-randomization) and since subjects may have changed their dietary habits, we were not able to account for such changes in our analyses. Since we used a simple questionnaire to assess nut consumption, we were unable to adjust for total energy intake and other nutrients consumed by study subjects. In addition, we did not have data on types of nuts consumed; their preparation including salted, spiced, roasted, or raw nuts to examine the influence of types of nuts or preparation method on the risk of hypertension. It is possible that some of the subjects were misclassified due to inaccurate recall on nut intake. However, since information on nuts was collected before the occurrence of hypertension, it is more likely that such misclassification was non-differential and led to an underestimation of the true relation between nuts and hypertension. Since study participants were physicians, it is less likely that consumption of salted nuts was important in this population given the positive association between sodium intake and hypertension. Lastly, in the absence of random allocation of nuts, we can not exclude unmeasured or residual confounding (i.e. over- or underreporting of nut consumption) as a possible explanation for observed findings. Nevertheless, the large sample size, the longer duration of follow up, and the complete ascertainment of outcome via standardized follow-up questionnaires are strengths of the present study.
In conclusion, our data suggest that frequent consumption of nuts is associated with a lower risk of incident hypertension in US male physicians, and such relation appears to be limited to lean individuals. Further examination of the relation between nuts and incident hypertension in the general population and differential effects of types of nuts consumed is warranted.