The disturbing trends in increasing numbers of overweight and obese Americans tend not to reflect a lack of effort on the part of the American public to control their weight, but rather a failure to make the attempt in a scientifically sound and effective manner. It may be the frustration associated with this trend that accounts for the popularity of high-fat, low-carbohydrate diets, pushed in books and in the popular media. This paper reviews the physiological basis for these diets, as compared to the low-fat, high-carbohydrate diet recommended by conventional scientific wisdom, with particular attention paid to the effects of these diets on fat-free mass. Fat-free mass, or total body mass minus fat mass, is an important indicator of resting metabolic rate. It is therefore important to the future success of the diet to conserve relative to fat mass when losing weight. In a survey of reliable, peer-reviewed sources, referenced specifically below, it was found that initial weight loss and changes in body composition are independent of the macronutrient sources consumed, but that a negative energy balance is the fundamental key to weight and fat loss. In addition, associated with low-carbohydrate diets are serious health concerns, such as the long-term effects of these diets on cholesterol, blood pressure, bones and kidneys.