In the nineteenth century, Carl von Voit, Max Rubner, and their contemporaries demonstrated that this was indeed what happened, at least in animals. Francis Benedict, Ancel Keys, George Bray, Jules Hirsch, and others have demonstrated this in humans, showing that neither eating less nor exercising more will lead to long-term weight loss, as the body naturally compensates. We get hungry, and if we can’t satisfy that hunger, we’ll get lethargic and our metabolism will slow down to balance our intake. This happens whether we’re lean or obese, and it confounds those authorities who recommend exercise and calorie restriction for weight loss. They operate on the assumption that the only adjustment to the caloric deficit created by either dieting or exercise will be a unilateral reduction in fat tissue. This would be convenient, but the evidence argues against it.

Among researchers who study malnutrition, as opposed to those whose specialty is obesity, these compensatory effects to caloric deprivation are taken for granted, as is the fact that hormones regulate this process. “Changes in…hormones such as insulin and glucagon*87 play an important role in this metabolic response to energy restriction,” explains Prakash Shetty, director of the Nutrition Planning, Assessment and Evaluation Service of the United Nations’ Food and Agriculture Organization. “These physiological changes may be considered as metabolic adaptations which occur in a previously well-nourished individual and are aimed at increasing the ‘metabolic efficiency’ and fuel supply of the tissues at a time of energy deficit.” We should not be surprised that “dieting is difficult,” as Keith Frayn of Oxford University says in his 1996 textbook, Metabolic Regulation. “It is a fight against mechanisms which have evolved over many millions of years precisely to minimize its effects…. As food in take drops, the level of thyroid hormone falls and metabolic rate is lowered. Food intake has to be reduced yet further to drop below the level of energy expenditure. Hunger mechanisms, including the feeling of an empty stomach, lead us to search for food….”

Though the traditional response to the failure of semi-starvation diets to produce long-term weight loss has been to blame the fat person for a lack of willpower, Bruch, Rony, and others have argued that this failure is precisely the evidence that tells us positive caloric balance or overeating is not the underlying disorder in obesity. No matter what technique is used to achieve a caloric deficit, whether eating less or exercising more, it will only serve to induce hunger and/or a compensatory decrease in energy expenditure. These are the “usual symptoms resulting from reduced food intake,” as Ancel Keys and his collaborators described them, and anyone will experience them, regardless of weight.