Falck's observations on fasting dogs shows that a lean dogs dies after 25 days, but a fat dog can live for 60 days without food - as it uses less protein when it has fat to metabolize.
Chemistry of Food and Nutrition
Elliott P. Joslin
Protein . The protein burned in the metabolism of a healthy individual from day to day depends chiefly on the protein supplied by the diet . Muscular exercise has little effect upon it , since that is dependent upon carbohydrate or fat, with a preference for the former. Even in the early days of fasting the protein metabolism changes but little from that in health. With a diet rich in carbohydrate and fat and low in protein the protein metabolism is easily brought to less than 50 grams per day, but with an excess of protein in the diet it may rise to 150 or 200 grams. A liver well stored with glycogen protects the body protein of a fasting man for a day just as carbohydrate in the diet , but on a second day fails because the glycogen is nearly exhausted. “The influence of the available supply of body fat upon the protein metabolism of fasting,” as cited by Sherman, "is shown by the following observations of Falck, on the protein metabolism of two fasting dogs -- the one lean, the other fat ." ( See Table 160. ) The fat dog was healthy thirty - five days after the lean dog died .