Over the last two decades, evidence about the health benefits of the Mediterranean diet has been emerging. While these benefits are not exactly controversial, there has still been a need to provide insights into the effects of specific dietary components on mortality – particularly cardiovascular disease and cancer mortality. The Spanish trial PREDIMED (‘PREvención con DIeta MEDiterránea’) started in 2003 as a long-term dietary intervention study aiming to assess the efficacy of the Mediterranean diet in the primary prevention of cardiovascular diseases.
In the latest analysis published in BMC Medicine, Jordi Salas-Salvadó from the Rovira i Virgili University, Spain, and colleagues evaluated 7,216 men and women aged 55 to 80 years randomized to one of three interventions: Mediterranean diets supplemented with mixed nuts or extra virgin olive oil or advice on a low-fat diet (control diet). Trained dieticians completed validated food frequency questionnaires in face-to-face interviews with participants to record self-reported information on nut intake, prior to participants starting the interventions. The questionnaire included one item regarding the consumption of almonds, peanuts, hazelnuts, pistachios and pine nuts and a separate question specifically inquired about walnuts. At enrolment, all participants were considered to be at high cardiovascular disease risk.
During a median follow-up of 4.8 years, 323 total deaths were recorded of which 81 were due to cardiovascular disease and 130 due to cancer. The analysis showed that compared to those who rarely or never consumed nuts before starting the interventions, a significant decrease in all-cause and cardiovascular disease mortality was observed in those who consumed up to three servings of nuts per week (one serving = 28 g of nuts). In addition, cancer mortality was inversely associated with the consumption of walnuts (but not with other types of nuts).
In a commentary also published in BMC Medicine, Sabine Rohrmann and David Faeh from the University of Zurich, Switzerland, discuss the possible explanation for these findings. It’s already known that nuts are a rich source of polyunsaturated fatty acids, minerals, vitamins, and phytochemicals. Walnuts in particular contain high levels of alpha-linolenic acid, and are usually consumed with the skin, which contains the highest levels of phytochemicals.
Rohrmann and Faeh also highlight that for the first time, this study shows that nuts confer protection against premature mortality, as reduced mortality was only seen in the intervention group in which nut consumption increased during the 4.8 years of follow-up. This was not the case for the groups supplemented with olive oil consumption or in the control group.
There are still many unresolved questions about the mechanisms via which nuts such as walnuts can protect against cancer mortality. Also, the quantity and duration over which certain types of nuts should be eaten to reduce all-cause mortality needs to be explored. Given the current available evidence however, it’s clear that the general population should be encouraged to incorporate whole nuts, particularly walnuts, into their daily diets.
Written by Sabina Alam, Senior Executive Editor for BMC Medicine.
BMC Medicine 2013, 11:164
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