Published reports show that low carbohydrate weight loss diets provide a metabolic advantage, a greater weight loss per calorie consumed compared to isocaloric high carbohydrate diets. These reports have not been refuted but rather largely ignored, presumably because of the apparent violation of the laws of thermodynamics ("a calorie is a calorie"). In this review, we show that there is no such violation of thermodynamic laws. Energy utilization of different diets depends on the chemical pathway taken and a metabolic analysis of the efficiency of different pathways reveals large differences. Likewise, thermogenesis produced by diets of different macronutrient composition varies widely. We present a plausible mechanism that depends on the inefficiency of metabolic cycles and, in particular, protein turnover. A low carbohydrate diet makes demands on protein turnover for gluconeogenesis. From a theoretical point of view,energy balance between two diets is to be expected only if the subjects have the same final physiologic state, and only if all of the changes contributing to the energy, heat, work and chemical effects are known. Most diet experiments do not conform to this ideal. There is no theoretical contradiction in metabolic advantage and no theoretical barrier to accepting reports describing this effect.