Journal of the American Medical Association 2012; 307(24):2627-34.
After weight loss, the human body fights hard to regain weight. One of the ways it does this is to decrease the amount of calories it burns on a day to day basis. This is known as a decrease in resting energy expenditure. The objective of this study was to examine the effects of 3 different diets varying in macronutrient content and glycemic load on resting energy expenditure following weight loss.
21 overweight young men and women completed a 12 week weight loss program that resulted in an average weight loss of 13.6% of body weight. Once the weight was lost, they each went on one of 3 diets for a period of 4 weeks. Each of these diets contained the same exact number of calories.
Diet #1: Low Fat: 60% carbohydrate, 20% protein, 20% fat
Diet #2: Low Glycemic Index: 40% carbohydrate, 20% protein, 40% fat
Diet #3: Low Carbohydrate: 10% carbohydrate, 30% protein, 60% fat
After each 4 week diet intervention, a variety of measures were taken, including resting energy expenditure, hormone levels, and components of the metabolic syndrome.
The decrease in resting energy expenditure was greatest with the low fat diet (-205 calories per day). Next was the low glycemic index diet (-166 calories per day). The least was the low carbohydrate diet (-138 calories per day).
This is a ground breaking study that challenges the assertion that a calorie is a calorie. The 3 diets that were tested were isocaloric, in other words they had the same amount of calories. The fact that each had a different impact on resting energy expenditure means that something is going on hormonally that will impact your body’s reaction to weight loss. It would be easy to look at the results of this study and say that the low carb diet is the way to go.
Not so fast! The researchers measured a bunch of other things besides resting energy expenditure. During the low carb phase, subjects had a significantly higher cortisol level. This is a hormonal measure of stress and has been associated with a variety of negative health outcomes, such as insulin resistance and cardiovascular disease. Subjects in the low carb phase also had a higher level of C-reactive protein. This is a measure of body wide inflammation and has also been associated with a variety of chronic diseases.
The low fat diet showed the greatest decrease in resting energy expenditure and also showed some negative metabolic effects, including an increase in insulin resistance, a decrease in HDL cholesterol, and an increase in triglycerides.
The low glycemic index diet seemed to be the best of both worlds. You get a reduced decrease in resting energy expenditure without the metabolic changes that increase risk of chronic disease.
Take Home Message
The study provides more evidence that a low fat diet is not the way to go if you want to lose weight and keep it off. While the low carb diet had the most favorable impact on resting energy expenditure, it also resulted in some very unfavorable metabolic changes. The low glycemic approach seems to offer the best combination of blunting the reduction in resting energy expenditure that comes with weight loss while protecting you from chronic disease.