Nuts are interesting because they have particular nutritional reasons why they won’t make you fat. For a start nuts are high in fibre, and this fibre is found mainly in the cell walls of the plant material. Because the fibre is indigestible, and because it surrounds the nutrients within the cell, it acts as a physical barrier to digestion. Cells that are not crushed open during chewing are therefore unable to be digested efficiently by digestive enzymes and some of their content is never absorbed. In fact studies show that the energy content of the faeces increases considerably if nuts are eaten. Another consideration is the type of fat that nuts contain. Most nuts are high in mono- or polyunsaturated fatty acids, and evidence suggests that polyunsaturated fatty acids in particular may increase fat oxidation in the liver, and this may improve liver function. The essential fatty acid alpha linolenic acid in some nuts is preferentially oxidised rather than stored as fat. Therefore biologically, nuts possess a number of properties that prevent them causing weight gain.
However, there is another good reason that nuts may not cause weight gain. This related to their plant biology, and in particular their location within a hard outer shell. While consuming de-shelled nuts is relatively easy, anyone who has struggles to open a walnut with even a specially designed nutcracker will attest to the fact that the shell can considerable reduce the rate of consumption. This is an important concept nutritionally, because the rate at which energy is consumed is a primary determinant of the amount of energy that can be absorbed. This is because the appetite regulatory systems required time to allow feedback signals in the gut to be generated and then arrive at the brain in order to increase satiety. Processed foods that have a high energy content on account of their high fat and high sugar content as well as their lack of fibre, not only cause a long-term dysfunction in the appetite regulatory mechanisms, but also overwhelm the nutritional needs of the individual before energy intake has been adequately assessed and regulated.
Having to overcome the hard shell of a tree nut in order to obtain the energy of the seed contained within allows adequate time for the energy content of the first nuts consumed to be assessed before overconsumption of energy becomes a problem. In the same way whole fruits must often be peeled and prepared and this greatly reduces their rate of consumption compared to consumption of the juice of that fruit in a drink. This slower rate of consumption is pivotal in the correct function of the digestive process, as generally concentrated and liquid forms of energy have not been available to humans until relatively recently. Fibre also plays a role in this regulatory function because high fibre foods often require more chewing, and are therefore much more slowly consumed that low fibre foods. The tradition of eating over prolonged periods that is common in Mediterranean areas may also benefits the correct function of the appetite. The Mediterranean diet for example not only provides high quality nutrition, but is often consumed socially over extended periods.
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