A review was conducted that evaluated studies comparing weight loss and energy expenditure in adults consuming high protein and/or low carbohydrate with low fat diets. I believe this is where all the support for low carb diets got its momentum.
The review analyzed nine studies where diets composed of high protein and/or low carbs resulted in a 5.5 pound greater weight loss after twelve weeks as compared to diets of high carbs and/or low fat. However, neither macronutrient-specific differences in the availability of food energy nor changes in energy output could explain these differences in weight loss.If a calorie is a calorie, then what other factors could account for the differences in weight loss between the two diets?
Low carb diets facilitate the loss of glycogen stores and associated water, which can be as great as 4.4 pounds.
Because the Rubner and Atwater factors used to calculate metabolizable energy are not exact, the standard macronutrient values are not perfect, and small errors can occur. The substitution of one macronutrient for another has been shown in some studies to be statistically significant regarding the effect on the expenditure half of the energy balance equation, especially in high-protein diets. Because the energy expenditure is minimal, however, it may account for less than one-third of the differences in the weight losses reported between high-protein or low-carb diets and high-carb or low-fat diets.