Walnuts are a good source of a number of nutrients including the omega-3 fatty acid alpha linolenic acid (ALA, C18:3 (n-3)), minerals, vitamins and antioxidants such as ellagic acid. Evidence suggests that consuming walnuts is associated with a decreased of cardiovascular disease and mortality, making walnuts a healthy food. However many people avoid walnuts based on the fact that they contain high amounts of polyunsaturated fat and as a result have a high total energy content. Avoiding high fat foods that contain a lot of energy is based on the traditional paradigm that such foods cause weight gain because they increase the energy input of the body and as a result tip the energy balance equation in favour of weight gain. However, nuts in general, have been shown to have weight loss effects and this suggests that they are not a source of weight gain and that the traditional energy balance model of weight gain is flawed. The reason for the weight loss effects of nuts are numerous, and have been investigated.
For example, in one study1, researchers fed healthy subjects 42 gram of nuts for 3 weeks. During this time the basal diet was reduced in energy to counterbalance the additional energy of the nuts. This period of feeding was then compared with another 3 week period were no walnuts were consumed, but an isocaloric energy intake was maintained. The researchers then analysed the faecal matter of the subjects to analyse the amount of energy from the diets that was not absorbed. The results showed that a 28 gram serving of walnuts contained a useable 146 kcal (5.22 kcal per gram), 39 kcal less than the calculated value of 185 kcal (6.61 kcal per gram). The walnuts therefore contained 21 % less metabolisable energy that predicted by the Atwater’s factors, an estimate of the energy contained within particular foods. These results are consistent with other studies that show that the energy in nuts in not fully absorbed and so their energy content is actually lower that listed on the nutritional labels they contain.
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