Good Calories, Bad Calories

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.

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.

This idea that energy expenditure increases to match consumption, and that the ability to do this differs among individuals, also serves to reverse the cause-and-effect relationship between weight and physical activity or inactivity. Lean people are more active than obese people, or they have, pound for pound, a higher expenditure of energy,^*89 because a greater proportion of the energy they consume is made available to their cells and tissues for energy. By this conception, lean people become marathon runners because they have more energy to burn for physical activity; their cells have access to a greater proportion of the calories they consume to use for energy. Less goes to making fat. That’s why they’re lean. Running marathons, however, will not make fat people lean, even if they can get themselves to do it, because their bodies will adjust to the extra expenditure of energy, just as they would adjust to calorie-restricted diets.

Our propensity to alter our behavior in response to physiological needs is what the Johns Hopkins physiologist Curt Richter called, in a heralded 1942 lecture, “total self-regulatory functions.” Behavioral adaptation is one of the fundamental mechanisms by which animals and humans maintain homeostasis. Our responses to hunger and thirst are manifestations of this, replenishing calories or essential nutrients or fluids. Physical activity, as Richter suggested, is another example of this behavioral regulation, in response to an excess or dearth of calories. “We may regard the great physical activity of many normal individuals, the play activity of children, and perhaps even the excessive activity of many manic patients, as efforts to maintain a constant internal balance by expending excessive amounts of energy,” he explained. “On the other hand, the low level of activity seen in some apparently normal people, the almost total inactivity seen in depressed patients, again may be regarded as an effort to conserve enough energy to maintain a constant internal balance.”

In 1936, when Eugene Du Bois published the third edition of his metabolism textbook, Basal Metabolism in Health and Disease, he described the system that accomplished the regulation of a stable body weight as it was then understood. How much we want to eat on any given day, Du Bois explained, is determined by how much we’ve depleted whatever our body considers the necessary reserves of protein, fat, and carbohydrates. If we then consume more calories than we need, the excess will either be burned off as heat or induce physical activity: “When well nourished, the individual tends to become more energetic and it is quite possible that he will soon burn up his stored fat by extra work or exercise which would not have been undertaken had it not been for the overfeeding.” If we consume less food than we might require to replenish our reserves, then the amount of heat generated in response to a meal is minimized, and the stores of carbohydrates (glycogen), fat, and protein are used to make up the difference. Should the caloric deficit continue, the result is “a gradual lowering of metabolism and a tendency toward restriction of activities, due to a lack of energy and initiative.”

However this homeostatic system works to balance energy intake and output and thus maintain a steady supply of fuel to the cells and a stable body weight, it is extraordinarily complex and involves the entire body. Rony discussed this: “The appetite mechanism, which is but a part, although the most important one, of body weight regulation is in itself a highly complex mechanism involving [the central nervous system], endocrine glands, the gastric neuro-muscular apparatus, and the organs of the glycogen, protein, and fat reserves.” This notion was supported by a host of experimental and clinical studies, as we’ll discuss in Chapter 21, which demonstrated that disturbances in body-weight regulation—like obesity—could be caused by “pathological changes in certain parts of the nervous system, endocrine system and depot organs.”

It is also vital to understand that it’s our cells and tissues that require and expend the energy we consume, so this adjustment of intake to expenditure is occurring first and foremost on a cellular level. “Whatever may be the mechanisms controlling food intake,” as the University of California, Berkeley, nutritionist Samuel Lepkovsky wrote in 1948, “the chief site of their action must be the cell.” A fundamental requirement of any living organism is to provide a steady and reliable supply of fuel to its cells, regardless of the circumstances. We apparently evolved an