From the Institute of Optimum Nutrition:

Nuts are associated with several health benefits, and may even offer some protection against cardiovascular disease when eaten as part of a healthy diet. However, the widespread belief that nuts may lead to unwanted weight gain can discourage consumption among some individuals.

Epidemiological evidence (i.e. ‘observational’ studies that look at relationships, rather than causation) points to an inverse association between the frequency of nut consumption and body mass index (BMI) and risk of obesity. However, clinical trials that have examined the effects of nuts on weight gain are limited.

A recent review and meta-analysis published in the American Journal of Nutrition* sought to examine the available research on nut consumption and adiposity measures, such as body weight, body mass index (BMI) or waist circumference. The study looked at randomized clinical trials in which nut-rich diets were compared with different control diets, and adiposity measures were reported. Thirty-three clinical trials met the inclusion criteria.

Overall, the findings indicated that nut-enriched diets do not increase adiposity measures, and support the inclusion of nuts in healthy diets for cardiovascular prevention. The authors present several mechanisms that may explain the results of the meta-analysis, including:

Nuts are rich in unsaturated fatty acids, which are more readily oxidized and have a greater thermogenic (heat-producing) effect, which can lead to less fat accumulation. Because of their energy density and abundance of unsaturated fatty acids, fibre and protein, nuts are highly satiating. The physical structure of nuts may also contribute to their satiety effect because they must be thoroughly chewed before swallowing.

*Flores-Mateo G, Rojas-Rueda D, Basora J, Ros E, Salas-Salvado J (2013). Nut intake and adiposity: meta-analysis of clinical trials. Am J Clin Nutr; 97:1346-1355.