Investigators studying obesity argued about the same handful of studies on Luxuskonsumption, and then the subject went out of fashion with the general acceptance of Newburgh’s argument that obesity is caused by a perverted appetite. “The idea that people burned off excess energy when overfed was regarded with great disfavor by respectable nutritionists,” as the British clinician John Garrow later noted. “It was a story put about by charlatans to justify magic cures, or by self-indulgent obese people as a justification for their obesity.” It experienced a renaissance in the 1960s, sparked by the British physiologist Derek Miller, who reported that young pigs fed a low-protein diet would consume five times as many calories as those fed a high-protein diet, and yet could burn off the excess so as not to gain weight. This led Miller to speculate that the pigs would eat until they satisfied their protein requirements, and while doing so would stay lean through this process of Luxuskonsumption.*88 It was thought that the ability to burn off excess calories would be of particular survival advantage when confronted with a poor-quality diet, when excessive amounts of food had to be consumed to achieve a requisite amount of protein or essential vitamins or minerals. Miller’s observations prompted the renewed interest in overfeeding experiments of the kind we discussed in the last chapter (Chapter 16). The one consistent finding in these studies has been that individuals vary dramatically in response to prolonged and enforced gluttony. Some will fatten easily, and some will not. The conclusion, seemingly unavoidable, is that a critical variable in the facility with which we gain weight is whether we respond to superfluous calories by storing them away as fat and/or muscle or by converting them to heat and physical activity—i.e., Luxuskonsumption.

At least some of these excess calories are lost in the various chemical reactions required to digest and store the nutrients. Rubner referred to this as the heat generated by the “thermochemical tangle of breakdowns” that occur during the process of digestion. Physicians measure basal or resting metabolism after a twelve-to-eighteen-hour fast because by then this diet-induced thermogenesis has played itself out. The protein in the diet, as Rubner discovered, dominates this effect. The more protein digested over the amount necessary to maintain tissues and organs, the greater the heat generation. It’s what Rubner called the specific dynamic effect of protein that is usually invoked as the rationale to eat high-protein diets for weight loss; excessive calories lost as heat in the process of digesting and utilizing protein can’t then be stored as fat or used for fuel.