The amount of calories matters when it comes to weight loss, but whether the type of calories has any impact is more controversial. The results of several recent studies provide a convincing argument that the type of calories do matter. A study published in the American Journal of Clinical Nutrition examined the effects of increasing protein in place of fat while keeping carbs constant. In this study, protein was increased from 15% to 30% and fat reduced from 35% to 20% (carb was constant at 50%). The increase in protein was clearly associated with reduced appetite and caloric intake, which ultimately led to a significant weight loss.
Exactly how protein curbs appetite is unknown, but this study suggests it may affect leptin, one of the most potent appetite hormones. Regardless of how it works, the data seem clear - increasing protein helps reduce appetite and caloric intake. This means it is a lot easier to lose weight when you bump up the protein intake.