In 1920, while Vilhjalmur Stefansson was just beginning his campaign to convince nutritionists that an all-meat diet was a uniquely healthy diet, it was already making the transition into a reducing diet courtesy of a New York internist named Blake Donaldson. Donaldson, as he wrote in his 1962 memoirs, began treating obese patients in 1919, when he worked with the cardiologist Robert Halsey, one of four founding officers of the American Heart Association. After a year of futility in trying to reduce these patients ("fat cardiacs," he called them) with semi-starvation diets, he spoke with the resident anthropologists at the American Museum of Natural History, who told him that prehistoric humans lived almost exclusively on "the fattest meat they could kill," perhaps supplemented by roots and berries. This led Donaldson to conclude that fatty meat should be "the essential part of any reducing routine," and this is what he began prescribing to his obese patients. Through the 1920s, Donaldson honed his diet by trial and error, eventually settling on a half-pound of fatty meat-three parts fat to one part lean by calories, the same proportion used in Stefansson's Bellevue experiment-for each of three meals a day. After cooking, this works out to six ounces of lean meat with two ounces of attached fat at each meal. Donaldson's diet prohibited all sugar, flour, alcohol, and starches, with the exception of a "hotel portion" once a day of raw fruit or a potato, which substituted for the roots and berries that primitive man might have been eating as well. Donaldson also prescribed a half-hour walk before breakfast.

Over the course of four decades, as Donaldson told it, he treated seventeen thousand patients for their weight problems. Most of them lost two to three pounds a week on his diet, without experiencing hunger. Donaldson claimed that the only patients who didn't lose weight on the diet were those who cheated, a common assumption that physicians also make about calorie-restricted diets. These patients had a "bread addiction," Donaldson wrote, in that they could no more tolerate living without their starches, flour, and sugar than could a smoker without cigarettes. As a result, he spent considerable effort trying to persuade his patients to break their habit. "Remember that grapefruit and all other raw fruit is starch. You can't have any," he would tell them. "No breadstuff means any kind of bread…. They must go out of your life, now and forever." (His advice to diabetics was equally frank: "You are out of your mind when you take insulin in order to eat Danish pastry.")

Had Donaldson published details of his diet and its efficacy through the 1920s and 1930s, as Frank Evans did about his very low-calorie diet, he might have convinced mainstream investigators at least to consider the possibility that it is the quality of the nutrients in a diet and not the quantity of calories that causes obesity. As it is, he discussed his approach only at in-house conferences at New York Hospital. Among those who heard of his treatment, however, was Alfred Pennington, a local internist who tried the diet himself in 1944-and then began prescribing it to his patients.