With the carbohydrates and added fats minimized in these diets, meat was inevitably the primary constituent. This would provide the protein necessary to ensure that weight loss came mostly from the patient’s fat and not the muscle. The idea was to keep the body in what is called nitrogen equilibrium, with the nitrogen consumed from the protein in the diet balancing out the nitrogen being excreted in the urine from the breakdown of muscle protein.

When these clinicians discussed what plant foods they would allow in their diets, they typically did so on the basis of the carbohydrate content: potatoes are nearly 20 percent carbohydrate by weight (the rest is mostly water), so they were known as 20-percent vegetables. Green peas and artichokes are 15-percent vegetables. Onions, carrots, beets, and okra are 10-percent vegetables. Most of the green vegetables—including lettuce, cucumbers, spinach, asparagus, broccoli, and kale—are 5-percent, which means carbohydrates constitute at most 5 percent of their weight. These weight-loss diets allowed only 5-percent vegetables, which ruled out all starchy vegetables, like potatoes. Because a one-cup serving of a 5-percent vegetable will yield only twenty to thirty calories, as the University of Toronto physician Walter Campbell wrote in 1936, “the inclusion of an extra portion or omission of an undesired portion is of little moment in the [dietary] scheme as a whole.” Some of these diets did allow an ounce or two of bread—usually whole-grain, because white bread had too few vitamins to make it worth including. But most did not. “All forms of bread contain a large proportion of carbohydrate, varying from 45–65 percent,” noted H. Gardiner-Hill of London’s St Thomas’s Hospital Medical School in 1925, “and the percentage in toast may be as high as 60. It should thus be condemned.”

When these physicians talked about lean meat as the basis of a weight-reducing diet, they did not mean a chicken breast without the skin, as has been the iconic example for the past twenty years. They meant any meat, fish, or poultry (bacon, salt pork, sausage, and duck occasionally excepted) in which the visible fat had been trimmed away.

Once weight was satisfactorily lost, weight-maintenance diets were also restricted in carbohydrates, although not so drastically. For maintaining a reduced weight, as described by the Pittsburgh physician Frank Evans in the 1947 edition of the textbook Diseases of Metabolism, the daily diet should include at least one egg, a glass of skimmed milk, a portion of raw fruit, “a generous portion of any cut of lean fresh lamb, beef, poultry or fish,” and a portion of each of three 5-percent vegetables. Individuals trying to maintain their weight loss could then eat anything else they wanted, Evans wrote, but they could do so only as long as they maintained a stable weight and were sufficiently “sparing with” alcohol, added fats and oils, “concentrated carbohydrate foods,” “starches,” “mealy vegetables, which are potatoes, beans, peas,” and “cereals, used as vegetables, which are: macaroni, spaghetti, rice, corn.”