For a long time I’ve been an advocate of tree nuts (e.g. cashews, hazelnuts, walnuts, pecans, brazil nuts, almonds) and peanuts in the diet. One reason for this is that they are a highly nutritious food, and their consumption has been consistently linked in the scientific literature with a reduced risk of heart disease and even death from heart disease.
However, even know this may not persuade someone to incorporate nuts into their diet, and a major reason for this is that fact that nuts are intensely calorific, and therefore are generally believed to be a highly fattening food. However, studies show that individuals eating the most nuts tend to have the lowest body weight. Of course the explanation here might be that individuals of lower body weight feel more relaxed about eating ‘fattening’ nuts than heavier individuals. However, studies show that including nuts in the diet of individuals generally leads to little or no weight gain. In fact, there is at least some evidence which finds that inclusion of nuts in the diet may promote weight loss. All this strongly supports the notion that nuts, despite being one of the most calorific foods on the planet, are not fattening. The question is, why?
There are a number of potential mechanisms that explain why nuts may not be fattening which were explored in a recent review that appeared in the Journal of Nutrition recently [1]. I’ve summarised them (along with one mechanism that is not discussed but I think is important) below:
Nuts are satisfying
Studies show that nuts tend to be effective at satisfying the appetite, which can mean that individuals just end up eating less of other foods. The percentage of calories that come from nuts that are compensated for by eating less of other foods varies from study to study, but comes in at around 70 per cent.
Two things in the scientific literature have been identified as factors which may influence a food’s ability to sate the appetite: its glycaemic index (the lower, the better) and its protein content. Nuts have a low GI value and are also relatively rich in protein, and these properties may help to explain why they tend to be quite effective at sating the appetite.
Because nuts have a low GI, they may tend not to promote fat storage
This mechanism is actually not dealt with in the review, but I’ve added it in for good measure. Theoretically, a food will tend to cause more fatty accumulation in the body if it releases sugar relatively quickly into the bloodstream. Such foods tend to cause surges in the hormone insulin which is the chief fat storage hormone in the body. The low GI nature of nuts may therefore help to account for why it is that this food does not appear to be especially fattening.
Nuts can stimulate the metabolism
Imagine you have a fire going in a hearth. If you want to whatever fuel you put on the fire to burn, then it makes sense to use fuels that tend to burn well, right? Dry logs would be an example. And when you put appropriate fuel on the fire, it tends to cause the fire to burn brighter too. Which means that subsequent fuel may burn more quickly. Well, studies show that eating nuts in the long term has positive effects on the metabolism: some studies show that nuts can raise the metabolic rate by more than 10 per cent.
Not all of the fat in nuts is absorbed from the gut
Studies typically show that about 10-15 per cent of the calorific value of nuts is not absorbed by the gut, and passes straight out of the body in the stool.
What these mechanisms remind me of is how it’s simply not accurate to claim that whether someone loses, gains or maintains their weight is simply down to ‘calories in and calories out’. This, clearly, is a gross oversimplification of reality. Also, getting people focused on calories can often cause them to construct their diet that are:
1. low in calories, but not particularly healthy (many ‘diet’ foods fall into this category)
2. don’t really satisfy the appetite very well, causing people to be hungry and therefore more likely to default back to their original diet
Nuts are a prime example of why, when considering whether a food is likely to be fattening or not, we need to go way beyond the calories it contains. The authors of the review comment: There are claims that energy-dense foods are especially problematic for weight loss and maintenance. Nuts are among the most energy-dense foods consumed, yet the literature consistently documents little impact of their ingestion on body weight. These data suggest that each food must be evaluated objectively for its impact on body weight and total diet quality to ensure recommendations about its use is sound and empirically based.
References:
1. Mattes RD, et al. Impact of peanuts and tree nuts on body weight and healthy weight loss in adults. J Nutr 2008;138:1741S-1745S
