Good Calories, Bad Calories

Yudkin was the most prominent advocate of carbohydrate-restricted diets among nutritionists through the 1970s. He also had unconditional faith, however, in the popular misinterpretation of the law of conservation of energy. “The irrefutable, unarguable fact is that overweight comes from taking in more calories than you need,” Yudkin explained in a 1958 diet book entitled This Slimming Business. He reconciled this belief with his advocacy of carbohydrate-restricted diets because he also believed that “much of the extra fat today” in the diet “comes together with carbohydrate in cakes, biscuits, ice cream, and sweetmeats of various sorts.” If we remove the carbohydrates, Yudkin proposed, the fat calories will come down, too.

In 1960, Yudkin provided experimental evidence to support this statement in a Lancet article entitled “The Treatment of Obesity by the ‘High-Fat’ Diet.” He had asked four women and two men to consume a carbohydrate-restricted diet for two weeks. They all lost weight, he reported, by consuming significantly less carbohydrates and no more fat than they typically ate on a balanced diet. The two men ate roughly twenty-nine hundred and thirty-five hundred calories normally, but reported consuming only fifteen to sixteen hundred when they abstained from carbohydrates. Their fat consumption dropped by two hundred calories a day as well. This led Yudkin to the “unequivocal” conclusion that “the high-fat diet leads to weight-loss because, in spite of its unrestricted allowance of fat and protein, it is in fact a low-calorie diet….” Weight is lost by restricting calories, even if calorie-restriction is not required by the diet.

Here again, however, Yudkin was confusing an association with cause and effect. Even if Yudkin’s subjects had reduced their calorie consumption on the carbohydrate-restricted diet, which is a common finding in these studies, it does not mean that the reduction in calories caused the weight loss, only that the diet was associated with a reduction in calories as well as a reduction in weight. The diet could have worked by some other mechanism entirely, but both weight loss and decreased appetite were consequences. The fact that a reduction of appetite associates with weight loss does not mean that it is the fundamental cause.

And, of course, what may have been true, on average, for Yudkin’s seventeen subjects—six in his 1960 study and eleven a decade later—is not necessarily the case for everyone who loses weight on such diets.^*105 Even before Yudkin published This Slimming Business, Weldon Walker and the Columbia University physician Sidney Werner had both reported that their subjects lost significant weight while consuming at least twenty-seven hundred and twenty-eight hundred calories a day respectively. In 1954, when the Swiss clinician B. Rilliet discussed his experiences using Pennington’s diet to treat obese patients at the County Hospital of Geneva, he reported that his successes were “numerous and encouraging” with both a twenty-two-hundred-calorie version of the diet and a three-thousand-calorie version. It’s hard to avoid the observation that at least some individuals lose weight on carbohydrate-restricted diets while eating considerably more calories than would normally be consumed in a semi-starvation diet. This is why Werner speculated that his obese subjects must have typically been eating four to five thousand calories a day before he set about experimentally reducing them. But if that is true, why don’t obese patients regularly lose weight on twenty-seven or twenty-eight-hundred calorie balanced diets, and why have clinicians always believed it necessary to semi-starve them with twelve to fifteen hundred calories, or even feed them very low-calorie diets of eight hundred calories or less, to achieve any significant weight loss? Something else is going on here, and it has nothing to do with calories.