That nuts promote weight loss is well reported in the nutritional literature. Nuts are known to be good sources of many nutrients and their consumption is generally associated with health benefits. For example they have been shown in numerous studies to be beneficial to lipid profiles and decrease the risk of cardiovascular disease. But nuts are also associated with a high fat content, which deters many people from eating them regularly. Fat contains nine calories per gram and of the macro nutrients it is the most energy dense, with over twice the energy content of protein or carbohydrate. Those of individuals trying to lose weight might assume that this high density of calories precludes nuts from the menu. However, there are a number of reasons why they should be included on a weight loss diet.
A number of studies have investigated the association between nut consumption and body weight or weight loss. Studies seem to confirm that those who regularly consume nuts have lower body weights than those who do not incorporate nuts into their diets. This seems to be the case for both peanuts and tree nuts1. This is interesting as peanuts are not nuts but legumes, but both are associated with high levels of fat. Just why nuts are associated with reduce body weight despite their high energy content is not known. One possible explanation is that the high fat content of nuts is associated with satiety. However, there is little evidence that calorie consumption is reduced in regular nut eaters, or that dietary fat is able to reduce food intake long term. More likely, the polyunsaturated fats in nuts alter the metabolism in the body.
For example, studies have shown that long term consumption of nuts is associated with an increase in the energy expended during rest. A study published in the British Journal of Nutrition in 20072 demonstrated that consumption of almonds resulted in a modest rise in resting energy expenditure of 209 kilojoules per day. This may be due to the high unsaturated fat content of the nuts. Omega-3 fatty acids, a form of fatty acid found in high concentration is various nuts and seeds has been shown to be oxidised as an energy source more readily that saturated fat. Walnuts and flax seeds have higher levels of omega-3 fatty acids than almonds and so it might be expected that they are able to raise resting energy expenditure even higher.
Further to this metabolic effect, studies have demonstrated that the nutrients from nuts are not efficiently absorbed from the gut. Studies with peanuts, almonds, pecans have all detected increase faecal fat content with increased consumption. Estimates have indicated that 10 to 20 % of the energy associated with the nuts is accounted for by increase fat excretion in this way. The likely reason for this is that the parenchymal cell walls of the nuts and seeds are not able to be penetrated by bacterial or endogenous enzymes, and so many of the cells not ruptured during chewing are simply passed through the body undigested. This evidence is backed up by a study published in the Journal of nutrition in 20053, that showed that the bioavailability of flax seeds is enhanced by milling and crushing the seeds.
Unless you have an allergy to nuts, it seems sensible to incorporate them into your diet, especially if you are attempting to lose weight. Many studies are now reporting positive effects of fish oils on weight loss due to their omega-3 content, and many nuts and seeds also contain fatty acids from the same family. This supports the hypothesis that nuts promote weight loss. If you have read my previous article on vitamin E (here) you will already know that vitamin E shows beneficial health effects, and nuts and seeds contain high levels of this vitamin. Nuts are also a good source of vegetable protein and research has shown that nut consumers also have higher intakes of folate, carotenoids, vitamin K, phosphorus, copper, selenium, potassium, and zinc. Nuts and seeds are also a good source of antioxidants, which has been estimated to be of comparable levels to those in broccoli or tomatoes.