“Particularly overconsumption of the wrong things,” Cooper replied. “Very often in the poor we see people who are plump who might be called obese, and people would then conclude that they do not have a deficiency because they look rotund, healthy in one sense of the word. But it is true that the consumption of high carbohydrate sources with the induction of obesity constitutes a very serious public health problem in the underprivileged and economically disadvantaged. I would agree with that.”

This response seems clear enough: the overconsumption of “high carbohydrate sources”—a phrase used to describe carbohydrate-dense starches and refined carbohydrates rather than leafy green vegetables and fruits—was associated with obesity in the poor, and perhaps even the cause. McGovern then asked Cooper to provide a “general rule of thumb” about eating habits that would help prevent disease and lengthen our lives, and Cooper reluctantly agreed to do so.

“What kinds of foods in general should we be consuming less of and what should we be eating more of?” McGovern asked.

“I think what we need to consider doing is to reduce our total fat intake,” Cooper replied. “Fat adds a caloric substance—almost twice as much—nine calories per gram—as compared to sugar. I think in order to have an effective reduction in weight and realignment of our composition we have to focus on reducing fat intake.”

With that answer, Cooper had contradicted himself, and the conventional wisdom on diet and health in America had shifted. The problem was no longer overconsumption of high-carbohydrate sources, but the overconsumption of fatty foods. And if Cooper realized that reducing our total fat intake meant increasing our consumption of carbohydrates, he neglected to say so.

Between 1973 and the mid-1980s, the notion of the fattening carbohydrate, which had persisted in clinical and popular literature for well over a century, was replaced with the belief that it is dietary fat, with its particularly dense calories, that is responsible for overweight and obesity. The prescription of reducing diets that restricted starches and sugars, and perhaps oils and butter as well, was replaced with diets that targeted fat alone—restricting not just butter and oils but meat, eggs, and dairy products—thereby increasing carbohydrate consumption. Obesity was conceptually transformed from a condition commonly associated with the excessive consumption of carbohydrates and carbohydrate craving to one that would be described by prominent nutritionists as a “carbohydrate-deficiency syndrome,” which in turn explained why “an increase in dietary carbohydrate content at the expense of fat is the appropriate dietary part of a therapeutical strategy.”

What makes this shift all the more perplexing is that it occurred immediately after the science of fat metabolism evolved to explain why carbohydrates were uniquely fattening, and it followed a six-year period in which carbohydrate-restricted diets achieved unprecedented credibility among clinicians. The latter coincided precisely with the genesis of obesity research as what would be considered a legitimate field of scientific study, a transformation marked by the increasingly frequent appearance of conferences and symposiums dedicated to reporting the latest findings in obesity research, all of which, through 1973, had been dominated by discussions of the peculiar efficacy of carbohydrate-restricted diets.