Friday, October 2, 2009
The reduction in caloric intake was also not sustained. Weight loss averaged 6 kg at 6 months,
which fits reasonably with the planned daily deficit of 750 kcal. However, after 12 months, subjects started to regain weight, which suggests that they were eating more than planned. Final weight losses averaged 3 to 4 kg after 2 years. This weight loss is similar to the weight loss that can be achieved with pharmacotherapy, and it is a clinically relevant effect that will slow the onset of type 2 diabetes. 4,5 To that extent, all the diets were successful. But the weight regain during the second year, although slow, suggests that in the end many participants might have regained their original weight even if treatment had continued.
Within each diet group, some participants achieved much better weight loss than others. Participants who lost more weight attended more counseling sessions and adhered more closely to the prescribed dietary composition. These observations led Sacks et al. to conclude that behavioral factors rather than macronutrient composition are the main influences on weight loss. That is a plausible hypothesis, and it has been observed before, but the present data do not allow a firm conclusion to be reached, because differences in macronutrient intake were too small.
Even if the planned differences in macronutrient intake had been achieved, the absence of
blinding would have made it difficult to ascribe the effect of a particular diet to protein, fat, or
carbohydrate molecules. Weight-loss studies are behavioral studies; they require participants to eat less. Cognition and feelings have a huge impact on such behavior. Participants may eat less not because of the protein or carbohydrate content of a diet but because of the diet’s reputation or novelty or because of the taste of particular foods in the diet.
Specific effects of fat, protein, and carbohydrates on food intake and body weight can be determined only when all diets look and taste the same. Studies that have accomplished this goal with the use of porridges (similar to oatmeal) and standardized snacks or with covertly manipulated foods have been carried out for short periods, but few subjects would be willing to eat those foods for the several years that would be needed to examine long-term effects. Therefore, this issue is unlikely to be settled soon. If behavior rather than diet composition is the key to weight loss, macronutrient composition may be of secondary importance anyway.
