Effects of time-restricted access to protein and to carbohydrate in adult mice and rats.

Specific appetites for protein and carbohydrate were investigated by allowing mice and rats time-restricted access to either protein or carbohydrate. Animals were maintained over a period of weeks on a self-selection regimen consisting of three separate macronutrient sources (carbohydrate, fat, and protein). After a baseline phase during which all three diets were freely available, protein-restricted animals were allowed access to the protein diet only during a one-hour period each day, but other diets were always available; access of carbohydrate-restricted animals to the carbohydrate diet was restricted in a similar fashion. Control animals had continuous free access to all three diets throughout the experiment. During the restriction phase, protein-restricted animals showed substantial selective increases in protein intake during the one-hour periods of protein availability, consuming 40-45% of their normal daily protein intake during this phase of the experiment. In contrast, carbohydrate-restricted rats demonstrated no evidence for the development of a carbohydrate appetite, and the carbohydrate appetite of mice was weak and transient. These results are related to current notions concerning specific appetites.