The traditional paradigm of weight loss suggests that the amount of weight gained or lost is a result of the energy input and energy expenditure of an individual. In this respect, it would be expected that increased physical activity and reduced energy intake would result in weight loss, while reduced physical activity and increased energy intake would result in weight gain. However, the nutritional literature is quite clear in that the weight loss achieved by the individual is only weakly associated with the exercise they perform or the energy intake they obtain. Therefore it is clear that other factors must be involved that control weight change. Nuts are a great example of how the traditional energy in energy out equation of weight change breaks down in the face of empirical evidence. Nuts are high in energy, and yet consuming nuts, to a level that the energy intake of the individual increases significantly, can cause weight loss and improvements in body composition.
The reasons for the weight loss effects of nuts are not fully understood. This May relate to the fact that nuts benefit body composition in multiple ways. However, studies investigating the digestion of nuts have shown that although they are a significant source of energy, the calories within nuts may not be fully absorbed. For example, a recent study in the American Journal of Clinical Nutrition1 investigated the digestion and absorption of the lipids contained within almonds. Healthy subjects were fed natural whole almonds or whole roasted almonds. The researchers then measured a number of aspects of the digestive and absorptive processes associated with the almonds. The results of the study showed that only a small proportion of the lipid content of the almonds was released on chewing (8.5 and 11.3 % of total lipids for natural and roasted almonds, respectively). In addition, there were a large number of particles in the chewed almonds that were of large size (>500 μM).
Therefore most of the lipid content of the almonds remained inside cells upon swallowing. The cell walls of almonds, as with all plant cell walls, are non-digestible to human enzymes, and so the fat contained within these undigested particles could not be absorbed through the normal lipid absorption pathway. Bacteria that inhibit the colons of humans do possess enzymes capable of breaking down the cellulose cell walls of plants. Some of these non-digested almond particles may therefore be digested by gut bacteria in the colon, where the lipid content would be released, metabolised and absorbed. However, the colon does not possess a highly absorptive surface like the small intestine, and some of the energy released from the nuts would likely be used by the bacteria for their own energy needs. Therefore it is not clear how much of this released energy is available for absorption by the host individual. The intactness of the cell walls following chewing is likely one of the main ways that nuts provide weight loss benefits.
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