The global prevalence of overweight and obesity is growing. Correction of the problem through dietary intervention has been disappointing and has prompted research into variations of diet composition or pattern to promote negative energy balance. Limited research suggests that selected approaches, such as low-carbohydrate diets, augment weight loss and offer an advantage over conventional low-fat diets. This has led to the hypothesis that although in theoretical terms a calorie is a calorie, in practice this is not the case. Support for such views and plausible mechanisms exist. Although any advantage may augment weight management, reported discrepancies between theoretical and observed effects are small. Thus, energy-restricted diets should be based primarily on energy intake and expenditure.