Walnuts may help you lose weight and make better food choices.  

Gaining weight is complicated, but it is not less complicated than losing it.  Weight gain is a consequence of total energy intake.  It is an imbalance between what we eat and burn during our lives.  If we burn more, we lose weight, and if we burn too little, we gain weight.  It is simple in concept yet complex in reality.  At the population level, consuming foods, such as nuts, would appear to be inversely associated with increasing weight because they are very dense in calories.  The reasons for the difference are unclear, but they could be examined in the context of dietary advice for weight loss.