Low-carbohydrate diets are based on an alternative theory of obesity where dietary carbohydrate, particularly unprocessed sugars, causes hyperinsulinemia, leading to insulin resistance, obesity, and cardiovascular disease. In this model, carbohydrate is viewed as a "metabolic poison" and therefore is limited in the diet. This article systematically reviews and refutes the 6 major physiologic claims made by proponents of low-carbohydrate diets. Any benefits or advantages resulting from these diets must therefore be derived from factors other than those stated by the alternative theory.