I spent last week in Portugal, and while I was there my girlfriend and I had a lot of food given to us by local farmers. We were lucky enough to be given quite a stash of fresh produce including citrus fruit, onions, broad beans, fresh peas, lettuce and cabbages. We were also given a carrier bag full of walnuts in their shells. The shelling kept us busy, but the end result was worth it: for a few days we had fresh, tasty walnuts that we used as a snack food and pre-dinner nibble for family and friends.
I like to eat nuts, not just because I like their taste and texture, but also because for me (like for a lot of people) they do a decent job of quelling my appetite. That’s a useful property to have in a snack or pre-meal nibble, if part of the reason for you eating it is to stop your appetite running out of control at mealtime. The sating effect of nuts is at least in part related to their relatively high-protein nature.
However, nuts have other things going for them in that they have relatively low glycaemic index, and therefore tend not to stimulate much in the way of secretion of insulin (the chief fat storage hormone), and help to stimulate the metabolism. These and other factors may help to explain why studies have found that, generally speaking, nuts are not a fattening food despite being intensely calorific.
Further support for this notion came from a meta-analysis (analysis of several similar studies) on the effect of walnut eating on measures of health [1]. 13 studies where included in the review, and each study involved individuals consuming 10-24 per cent of their calories in the form of walnuts over 4-24 weeks. Taking the evidence of all studies together, there was not evidence that walnut eating led to a significant change in weight.
This study also looked at the effect of walnut eating on specific blood fat levels including supposedly unhealthy low density lipoprotein (LDL) cholesterol. Walnut eating led to a reduction in the levels of this fat. It did not, however, lead to significant changes in the levels of supposedly healthy high density lipoprotein (HDL) cholesterol or triglyceride levels.
If we were to interpret these results from a traditional perspective, we would say that overall, the evidence suggests that walnut eating leads to an improvement in cardiovascular risk factors. As the authors point out, longer-term studies would be useful for helping to verify this finding.
However, I think it is worth bearing in mind that nuts are generally rich in a range of nutrients that might have benefits for cardiovascular health, including monounsaturated fat, magnesium, potassium, copper and vitamin E. Walnuts are also quite rich in a form of omega-3 fat known as alpha linolenic acid which might also have benefits for cardiovascular health. There is indeed some evidence which links nut eating with benefits in this respect: One study found that women consuming at least five ounces (about 125 g) of nuts each week had one-third fewer heart attacks compared to women who rarely or never ate nuts [2]. In another study, men eating nuts twice a week, compared to those who rarely or never ate nuts, were found to be at about half the risk of ‘sudden cardiac death’ [3].
References:
1. Banel DK, et al. Effects of walnut consumption on blood lipids and other cardiovascular risk factors: a meta-analysis and systematic review. Am J Clin Nutr (May 20, 2009). doi:10.3945/ajcn.2009.27457
2. Hu FB, et al. Frequent nut consumption and risk of coronary heart disease in women: prospective cohort study. BMJ 1998;317(7169):1341-5
3. Albert CM, et al. Nut consumption and decreased risk of sudden cardiac death in the Physicians’ Health Study. Archives of Internal Medicine 2002;162(12):1382-7
