Nuts show a number of beneficial health effects in humans. In particular, consumption of nuts is associated with lower body weight and improved blood lipid profiles. These effects are shared by a large number of different nut varieties, suggesting that the beneficial component or components in nuts is common throughout the nut food group. Nuts contain a number of constituents that have been suggested to be responsible for the beneficial health effects including monounsaturated fatty acids, antioxidant phytochemicals and fibre. That nuts produce weight loss effect in healthy normal weight humans, even in the absence of energy restriction, suggests that they may have insulin sensitising effects. In fact, evidence supports the contention that the weight loss effects seen by nuts may be through improvements in postprandial glycaemia in combination with reduced oxidative stress, which together can produce significant improvements in insulin sensitivity, particularly in those with metabolic syndrome.
Epidemiological evidence suggests that nut consumption is associated with leanness. In countries where nut consumption is high, particularly Mediterranean countries, rates of obesity are low. Comparisons between nut eaters and non-nut eaters shows that the former have significantly lower body weights than the former. These are associations and cause and effect are difficult to prove in nutrition. However, intervention studies show that consumption of nuts including pecans, almonds and walnuts cause healthy individuals to lose weight and at the same time to report feeling hungry. In addition, studies that supplement regular diets with nuts do not cause weight gain, despite the extra energy added to the diets. The body weight loss associated with nut consumption may explain the improved lipoprotein profiles seen with nut consumption, as body weight, particularly when in the form of visceral fat, is associated with detrimental effects on the liver that negatively modify lipoprotein profiles.
The weight loss effects of nuts are complex and not fully understood. It has been suggested that the association between nuts and body weight is confounded because nuts may be a marker for a healthier diet. However, this does not explain the weight loss effects of nuts in intervention studies. Nuts may improve satiety because of their protein and fibre content, and this may relate to the beneficial glycaemic effects they possess. By slowing the absorption of nutrients, the blood glucose levels of the individual become more stable and this reduces the nutrient overload on the cells. As a result energy oxidation is more controlled and free radical production is kept within the capacity of the cells antioxidant defences. As free radicals may inhibit the insulin signal cascade and cause insulin resistance, reduced postprandial glycaemia may have insulin sensitising effects. The antioxidant phytochemical in nut may also bolster cellular antioxidant defences and contribute to the insulin sensitising effects of nuts.
Further, the fatty acid content of the nuts may also provide beneficial effects. Polyunsaturated fatty acids are present in some nuts such as walnuts, and these include essential fatty acids of the omega-3 variety, particularly alpha linolenic acid (ALA, C18:3 (n-3)). Consumption of ALA results in the production of short lived signal molecule and these may be involved in regulating the sensitivity of cells to the hormone insulin. In addition, most nuts are high in monounsaturated fatty acids, which may enhance β-cell function in the pancreas, thus improving the efficiency of the insulin system and having a pronounced blood glucose lowering effects. Polyunsaturated fatty acids may also increase the oxidation of fatty acids in the liver and muscle tissues, decreasing the quantity of accumulated fats that have been suggested to interfere with insulin signalling in these tissues. The fatty acid components of nuts may therefore contribute significantly to the beneficial weight loss effects of nuts, through multiple mechanisms.
RdB
