As the external environment changes, though, our bodies change the manner in which they utilize this heat. Maintaining our bodies at a constant temperature (roughly 98.6°F) requires more energy when it’s cold than when it’s warm. More of the heat from this thermochemical tangle of breakdowns, as Rubner reported, will go to that purpose when it’s cold, as it will when our energy reserves are low—when we’re undernourished—and we need to conserve the biologically useful energy for other purposes. In short, we will put this heat to use when we need to conserve energy, and we will waste it when it might be to our benefit to avoid the accumulation of excess calories as fat.

The primary source of controversy today remains the question that Rubner and Voit disputed a hundred years ago: whether the excess calories consumed have to be dissipated entirely as heat, or whether they can also be used biologically. Rubner argued that the energy requirements of our cells are basically constant. Under some set requirement, determined by temperature among other factors, our cells will adjust by conserving energy. Anything greater, and the energy is wasted as heat. Voit believed that the metabolic rate of our cells responds to the fuel available. The more fuel, the more energy generated. According to Voit, overeating leads to an increase in the available energy for cells, tissues, and muscles, and so perhaps to what the clinical investigators studying obesity in the first half of the century called the “impulse to physical activity” or the “impulse to move.” That feeling of restlessness, they believed, is the manifestation of cells and tissues, literally, having energy to burn.

Both interpretations suggest the same fundamental conclusion about how our bodies work. We have thrifty metabolisms when we are undernourished and so need to use efficiently every calorie we consume, and we have spendthrift metabolisms when we’re overnourished, so as to avoid excessive weight gain and obesity. Our cells may have a certain maximal or ideal capacity for metabolizing nutrients, but the amount that they actually metabolize is ultimately determined by the quantity and perhaps the quality of the nutrients delivered in the circulation. This determination is made on a cellular and hormonal level, not a cognitive or conscious one.