According to mainstream dogma, weight gain is a result of an excessive energy intake combined with low levels of physical activity. This dogmatic consensus is so ingrained in medical thinking, nutritional research and even amongst the layman, that proposing an alternative hypothesis more often than not leads to derision and incredulity. However, the hypothesis that it is not how much you eat but what you eat that determines your bodyweight, is gaining acceptance in the nutritional literature. Increasingly it is being shown that high quality diets containing plant foods, organic grass fed meats, fatty fish, whole grain cereals and high intakes of fruits and vegetables, particularly legumes, cause weight loss even if eaten ad libitum with no effort to restrict calories. Nuts are one high quality food that are often avoided by those who wish to lose weight but this strategy may be counterproductive, because evidence suggests that they do not cause weight gain, but are an important source of essential fatty acids and micronutrients.
Many observational studies have shown that frequent nut consumers have lower body weights and less risk of obesity compared to those those who infrequently consume nuts. However, cause and effect are not able to be proven from observational studies and so it is not clear from this evidence if nuts cause weight loss. The clinical trials examining the effects of nut consumption are more inconsistent, but this will always be the case with human subjects who complicate the data with their behavioural nuances. That being said, while data from clinical trials might not show weight loss effects for nuts consistently, there is strong evidence that they do not cause weight gain when eaten as part of a high quality diet. This is important, because fat makes up a large proportion of the content of nuts and the energy contained within nuts might be considered a cause of weight gain based on the ‘eat-too-much, do-too-little’ theory of weight gain. That nuts do not cause weight gain further supports the contention that the energy balance equation of weight gain is erroneous.
The effects of nuts on body weight have been investigated in a recent meta-analysis and review of the feeding trials that have been performed since december 20121. Studies included in the analysis were those that provided data on nut intake and had measurements of body weight, body mass index or waist circumference in peer reviewed literature. Of the thirty clinical trials included in the analysis, pooled data showed no significant effect for nuts on body mass index, body weight or waist circumference for diets containing nuts, compared to control diets. The authors noted that the data was surprisingly robust in a sensitivity analysis, a measure of the uncertainty often used in mathematical models. Therefore the avoidance of nuts based on the premise that they contain fat and that this energy may increase body weight is not supported by the evidence in the literature. The inclusion of nuts to a high quality diet may therefore beneficial even during periods of fat loss because of their high micronutrient and essential fatty acid content.
A number of mechanisms may explain the ability of nuts to contribute to correct body weight and prevent increases in the measures of adiposity. Nuts are a rich source of unsaturated fatty acids, including polyunsaturated and monounsaturated varieties. Unsaturated fatty acids may contribute to increased fatty acid oxidation within the liver, thus reducing energy available for storage in adipose tissue. It has been shown for example, that the essential fatty acid alpha linolenic acid (ALA, C18:3 (n-3)) is preferentially oxidised rather than being used as a structural component in the body. Some nuts such as walnuts contain ALA, and this may suggest that the fat absorbed from such nuts is destined to be largely oxidised rather than stored as adipose tissue. In addition, the increased hepatic oxidation of unsaturated fatty acids lowers the plasma triglyceride concentrations and this may decrease the deposition of fatty acids to muscle tissue, a phenomenon thought to contribute to the development of insulin resistance, a driver of weight gain.
In addition, studies have shown that much of the energy within nuts is not absorbed (here). This related to the tough fibrous parenchymal cell walls that surround the nutrients and which are not degradable by human enzymes. This mechanisms has also been shown to occur in legumes and may explain some of their weight loss effects. Measurements of faecal fat contents shows that much of the fat energy within nuts is not absorbed and so the amount of fat absorbed from nuts does not reflect their nutritional profiles. Nuts are not therefore as calorie dense as is suggested from direct colorimetry experiments. The high protein and fibre content of nuts may also improve satiety and this may have beneficial long term effects on maintaining a neutral calorie balance, as well as decreasing the risk of overeating low quality foods which may cause weight gain. The chewing required by nut consumption may also increase the cephalic phase of digestion and thus initiate feedback mechanisms to prevent overeating.
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