There has been much controversy regarding the formation of fat from protein in the animal body. A number of observations by Voit which were believed to demonstrate such a production of fat were subjected to vigorous criticism by Pfluger and apparently shown to be capable of other interpretations. Later experiments by Cremer in Voit's laboratory appear, however, to establish the formation of body fat from protein food beyond reasonable doubt.

Thus in one of these experiments a cat after a preliminary period of fasting was placed in a respiration apparatus and fed liberally with lean meat for eight days. The amount of protein broken down in the body was estimated from the nitrogen eliminated. The carbon eliminated was also measured, and it was found that 58.4 grams of carbon had been retained in the body. This would correspond to 130 grams of glycogen, but the total amount of glycogen in the body at the end of the experiment was only 35 grams, hence about three fourths of the carbon retained by the cat from the protein food must have been stored as body fat.

The evidence of formation of milk fat in part from protein, while perhaps not amounting to a mathematical demonstration, is still very strong.

Since there is already abundant experimental evidence of the production of carbohydrate from protein and of the transformation of carbohydrate into fat, it is evident that protein food can indirectly, if not directly, contribute to the formation of fat in the body.