Good Calories, Bad Calories

But this, too, fails as a viable explanation. The liver increases ketone-body synthesis only when carbohydrates are unavailable and the body is relying predominantly on stored fat for its fuel. Ketone bodies could be responsible for appetite suppression, as Spark and Brody suggested, but so could the absence of carbohydrates or the burning of fat, or something else entirely. All of these are associated with the absence of hunger. In fact, the existing research argues against the claim that ketone bodies suppress appetite. Individuals with uncontrolled diabetes, for example, will suffer from ketoacidosis, during which ketone-body levels can be tenfold or even forty-fold higher than the mild ketosis of carbohydrate restriction, and yet these people are ravenous. “It is not clear why the sensation of hunger subsides [in starvation studies], but the disappearance is apparently not related to ketosis,” wrote Ernst Drenick in 1964 about his fasting studies at UCLA. Hunger sensations often disappeared in his subjects before ketone bodies could be detected in their blood or urine, “and it did not reappear” in those periods when ketone body levels were low. The same dissociation between ketone bodies and hunger was reported in 1975 by Duke University pediatrician James Sidbury, Jr., in the treatment of obese children.

Another common explanation for the absence of hunger on carbohydrate-restricted diets is that fat and protein are particularly satiating—“these foods digest slowly, making you feel satisfied longer,” as Brody has explained in the Times. (Even those investigators who published studies supporting Yudkin’s idea that carbohydrate-restricted diets work by restricting calories would invariably comment that high-protein, high-fat diets still induced the least hunger and the greatest feeling of satiation. “There is a good reason to believe that the satiety value of such diets is superior to diets high in carbohydrate and low in fat, and hence, may be associated with better dietary adherence,” the metabolism researcher Laurance Kinsell wrote in an influential 1964 article entitled “Calories Do Count.”) But this is also unsatisfying as an explanation. The statement that fat and protein satisfy us longer is equivalent to the statement that carbohydrates are less satisfying—they either make us experience hunger sooner than fat and protein or perhaps induce hunger, whereas fat and protein suppress it. This leads us back to the now familiar question: what is it about carbohydrates, or about the speed with which we digest them, that accelerates or exacerbates our sensation of hunger and our desire to eat?

Even Yudkin had struggled with the question of why people would willingly semi-starve themselves on a carbohydrate-restricted diet. “For reasons I do not clearly understand,” he wrote, there must be something unique about carbohydrates that either stimulates our appetites or fails to satiate us. “It would seem from this that carbohydrate does not satisfy the appetite,” he noted; “it may even increase it….”

This conclusion is simply hard to avoid, considering the half century of experimental observations on these diets. It leaves us with two seemingly paradoxical observations. The first is that weight loss can be largely independent of calories. The second is that hunger can also be. Even if we could establish that weight loss on these diets is universally attended by a decrease in calories consumed—no bread, no butter—we then have to explain why the subjects of these diets don’t manifest the symptoms of semi-starvation. If they eat less on the diets, why aren’t they hungry? And if they don’t eat less, why do they lose weight?

“It is better to know nothing,” wrote Claude Bernard in An Introduction to the Study of Experimental Medicine, “than to keep in mind fixed ideas based on theories whose confirmation we constantly seek, neglecting meanwhile everything that fails to agree with them.” In the study of human obesity, that fixed idea has been what Yudkin called “the inevitability of calories,” which in turn is based on the ubiquitous misconception of the law of energy conservation. If we believe that conservation of energy—calories in equal calories out—implies cause and effect, then we will refuse to believe that obese patients can lose significant weight without restricting their energy intake beneath some minimal expenditure. Any reports to the contrary will be rejected on the basis that they cannot possibly be true. “Claims that weight loss occurs even with high-caloric intake, but no carbohydrate, are absurd,” as the American Medical Association insisted in 1974. “Although authors of popular diet books frequently say that loss of body fat can occur regardless of high-calorie intake, this is not supported by evidence and, in fact, is refuted by the laws of thermodynamics.”