The PREDIMED nutrition trial based in Spain looked at the effect on the primary prevention of cardiovascular disease of over 7000 older people (aged 55 to 90) randomized to a Mediterranean Diet supplemented with extra virgin olive oil or nuts, compared to a control group following a low fat diet. In Mediterranean regions, nut consumption is relatively high compared to other countries. People who ate nuts tended to have a lower BMI and smaller waist. They were also less likely to smoke and were more physically active than those who rarely or never ate nuts. Nut eating was associated with a better diet in general as these people ate more vegetables, fruit and fish.
There were fewer people with type 2 diabetes or people taking medicine for hypertension in the group of people who ate the most nuts. Overall, nut eaters had a 39% lower mortality risk and walnut eaters 45% lower — meaning that they were less likely to die than the non-nut eaters.
People eating more than 3 servings (1 serving — 28 g) a week of nuts reduced risk of death due to cardiovascular disease by 55% and cancer by 40%. A similar effect was demonstrated for walnuts.
Prof Jordi Salas-Salvadó, from the Universitat Rovira i Virgili who led this study explained, "Quite how nuts are able prevent premature mortality is not entirely clear, nor why walnut should be better for you than other nuts. Walnuts have particularly high content of alpha-linoleic acid and phytochemicals, especially in their ‘skin’ both of which, along with fibre and minerals such as calcium, magnesium and potassium, may contribute to their healthy effect."
source : http://www.sciencedaily.com/releases/2013/07/130715202458.htm