To be more specific, the doctors wanted Stefansson to start with an all-lean diet. Stefansson had already lived for much of 10 years on a meat diet and had a better idea of what to eat. Quoting the original medical publication at http://www.jbc.org/content/87/3/651.full.pdf :

> At our request he began eating lean meat only, although he had previously noted, in the North, that very lean meat produced digestive disturbances. On the 3rd day nausea and diarrhea developed. When fat was added to the diet, a full recovery was made in 2 days.

Concerning the meat,

> The meat used included beef, lamb, veal, pork, and chicken. The parts used were muscle, liver, kidney, brain, bone marrow, bacon, and fat. While on lecture trips V. S. occasionally ate a few eggs and a little butter when meat, was not readily available. The carbohydrate content of the diet was very small, consisting solely of the glycogen of the meat. The men, except during short periods of special observation, ate as much as they wanted and proportioned the lean meat to the fat as they desired. T. S., in 31 days of special diet in the ward in which he was free from digestive disturbances, took an average of 0.81 kilos of meat per day while K. A. for 110 days averaged 0.70 kilos per day.