Rats treated with somatotropin select diets higher in protein

It was previously shown that somatotropin (STH) increases growth rate, improves food efficiency and stimulates protein accretion. In rats, STH also increases food intake. This study examined the effect of exogenous STH on rats' selection of diets varying in protein content. It was hypothesized that the increase in food intake in response to STH is driven by an increased protein requirement. Rats were allowed to select between two diets varying in casein (5 and 30%) or given a diet of a single casein level (20%). In each diet group, rats were treated with 0 or 4 mg of porcine STH/d. Rats treated with STH showed greater food intake (20%) and protein accretion (125%), regardless of diet. However, the greater food intake in rats allowed to select was due to greater consumption of the high protein diet. Diet selecting, STH-treated rats consumed 75% more of the 30% casein diet than did the saline-treated controls, while consuming a similar amount of 5% casein diet. Total protein intake (g/d) was 22 and 53% greater in rats injected with STH consuming the 20% casein diet and selection diets, respectively. The results indicate that rats injected with somatotropin select a diet greater in protein when compared with those not receiving somatotropin. It is suggested that the STH-induced increase in protein accretion results in a greater demand for essential amino acids. The mechanism whereby animals are able to monitor this greater need is not clear.