Very low-carbohydrate diets have experienced a resurgence in popularity, perhaps due to their greater weight reducing potential. There were several studies performed 30-40 years ago showing greater weight loss with a very low-carbohydrate compared to an equal energy low-fat diet, suggesting a metabolic advantage and "a calorie is NOT a calorie". Metabolic advantage is defined as a greater weight loss with a very low-carbohydrate compared to a low-fat diet of equal energy content and indeed occurs with a very low-carbohydrate diet (1). Several recent clinical trials have again showed greater weight loss with very low-carbohydrate diets ranging from 3 to 12 mo in duration. Weight loss was on average two-fold greater in subjects following the very low-carbohydrate diet. Metabolic advantage is proclaimed to be a violation of the first law of thermodynamics (conservation of energy), and mistakenly used to support the notion "a calorie is a calorie". However, diets very low in carbohydrate utilize different chemical pathways that vary in efficiency, and thus weight loss can indeed vary compared to a low-fat diet equal in energy without violating the first law of thermodynamics. Very low-carbohydrate diets result in powerful metabolic adaptations to enhance mobilization and utilization of fat while sparing carbohydrate fuel sources. Low-carbohydrate diets appear to decrease efficiency, which is a good thing because it means more energy that you eat is lost as heat.

1. Feinman RD, Fine EJ. Thermodynamics and Metabolic Advantage of Reducing Diets. Metab Syndr Rel Disord. 2003; 1:209-219.