The Calories-In / Calories-Out model is often criticized by Low Carb diet advocates and there is evidence that the criticism has some validity. A well designed and executed study compared Low Carb to Low Fat diets (Bonnie J. Brehm, Randy J. Seeley,Stephen R. Daniels, David A. D’Alessio. A Randomized Trial Comparing a Very Low Carbohydrate Diet and a Calorie-Restricted Low Fat Diet on Body Weight and Cardiovascular Risk Factors in Healthy Women. The Journal of Clinical Endocrinology & Metabolism, Volume 88, Issue 4, 1 April 2003, Pages 1617–1623).
The results were very interesting. The Very Low Carb group did quite well on the diet and of course much better than the Low Fat group. But most significantly to this subject was that the Low Fat group weight loss was explained well by their caloric restriction, but the weight loss of the Low Carb test subjects was not accounted for in their caloric intake.
As the study noted:
The mechanism of the enhanced weight loss in the very low carbohydrate diet group relative to the low fat diet group is not clear. Based on dietary records, the reduction in daily caloric intake was similar in the two groups. For the greater weight loss in the very low carbohydrate group to be strictly a result of decreased caloric consumption, they would have had to consume approximately 300 fewer calories/d over the first 3 months relative to the low fat diet group. Although the inaccuracy of dietary records for obese individuals is well documented, it seems unlikely that a systematic discrepancy of this magnitude occurred between groups of subjects who were comparably overweight. Therefore, it is difficult to explain the differences in weight loss between the two groups primarily as a function of differing caloric intake. Despite instructions to maintain baseline levels of activity, it is possible that the women in the very low carbohydrate diet group exercised more than those in the low fat diet group. Additionally, it is possible that consuming a very low carbohydrate diet increases resting or postprandial energy expenditure. The possibility that differences in the macronutrient composition of the diet alter energy expenditure is an interesting question that bears further investigation.
To take it a step further, in this diet they put the Low Fat people on a restricted Calorie diet but the Low Carb test subjects were allowed to eat as much as they wanted. The study noted:
Another unexplained, but important, observation was the spontaneous restriction of food intake in the very low carbohydrate diet group to a level equal to that of the control subjects who were following a prescribed restriction of calories. This raises the possibility that the very low carbohydrate diet may have been more satiating. Previous studies have suggested that, calorie for calorie, protein is more satiating than either carbohydrate or fat, and it may be that the higher consumption of protein in the very low carbohydrate diet group played a role in limiting food intake. Another explanation for restricted food intake in the very low carbohydrate group is that food choices were probably greatly limited by the requirements of minimizing carbohydrate intake, and that dietary adherence per se may have forced caloric restriction due to practical factors. Although it has been proposed that ketosis developing from severe carbohydrate intake contributes to a decrease in appetite, this does not seem likely based on our data. Although the women following the very low carbohydrate diet developed significant ketonemia, the elevation of circulating β-hydroxybutyrate was mild, well below what is seen in other clinical states of ketosis, such as starvation and diabetic ketoacidosis, and was noted only at 3 months. In addition, there was no correlation between the level of plasma β-hydroxybutyrate and weight loss (r = 0.29; P = 0.43).
Keto for the win. Calories-In / Calories-Out loses once again.