STUDY FINDS THAT CALORIE TYPE MAY AFFECT METABOLISM (JUNE 2012) - It has long been assumed that calories consumed in a diet are equivalent regardless of the the macronutrients they come from (fats, protein, carbohydrates). A study in the JAMA measured the effect of 3 different diets on the metabolism of 21 people. The subjects were first placed on a calorie-restricted diet for 16 weeks where they lost an average of 13.6% of their body weight. Subjects were then randomized to 1 of 3 diets (low-fat, low-glycemic index, very low-carb) for 4 weeks at a time. After 4 weeks on one diet, each subject crossed over to another diet until they had spent 4 weeks on all 3 diets. The diets were designed to maintain weight loss meaning calories-in were equal to expected calories burned. The metabolism of each subject was measured on each diet using indirect calorimetry and doubly-labeled water (techniques that measure carbon dioxide production as an indirect measure of metabolism). [PubMed abstract]


StraightHealthcare analysis:

In their discussion of the study, the authors note that the difference in total energy expenditure between the low-fat diet and the very low-carb diet was 300 cal/day in favor of the very low-carb diet. If this were the case, then subjects would have been expected to lose an average of 2-3 additional pounds while on the very low-carb diet (extra energy expenditure of 8400 calories over the course of 4 weeks) when compared to the low-fat diet. There were no significant differences in patient weights while on the 3 diets.

This study received a lot of media attention when it was released, and low-carb/low-glycemic index diet proponents were quick to promote its findings. But the reality is, these types of diets have already been tested against each other in long-term trials and there is no conclusive evidence that one diet is superior to another in terms of weight loss (see popular diets for a review of some of these trials).

The primary focus of people who are trying to lose weight should continue to be overall calorie intake versus calorie expenditure. See our discussion on weight loss for a guide to losing weight and counting calories.