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 type (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, or very low-carb (Atkins) - for 4 weeks at a time. After 4 weeks on one diet, each subject would crossover
to another diet until they had been on all 3 diets for 4 weeks. These diets were designed to maintain weight loss meaning calories-in were equal to expected calories burned.
The metabolism of each subject while on each diet was measured using indirect calorimetry and doubly-labeled water (techniques that measure carbon dioxide production as an indirect measure of metabolism).
- The study found the following differences in average metabolic rates with each diet:
- Very low-carb (Atkins) - Resting energy expenditure - 1643 cal/d, Total energy expenditure - 3137 cal/d
- Low-glycemic index - Resting energy expenditure - 1614 cal/d, Total energy expenditure - 2937 cal/d
- Low-fat diet - Resting energy expenditure - 1576 cal/d, Total energy expenditure - 2812 cal/d
- cal/d = calories per day
- While this study is interesting, it has a number of weaknesses:
- The sample size was small - 21 patients
- The length of dietary intervention was short - 4 weeks
- Measurements of metabolism are indirect
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
for a guide to losing weight and counting calories.