Hormonal responses to protein restriction in two strains of chickens with different growth characteristics

The hormonal response to protein restriction was studied in breeds of chickens with different rates of growth and body size. Fast-growing (broiler) chicks and lighter-weight (White Leghorn) birds were fed isoenergetic 5% (low) and 20% (control) protein diets ad libitum starting at 2 wk of age. At 4 wk of age, one-half of each group continued receiving the initial diet, while the other half was fed the alternate diet for two additional weeks. Growth was decreased in birds fed the low protein diet due to restriction of protein, sustained low feed intake and hence possibly insufficient energy for growth. Birds fed low protein diets had higher plasma growth hormone (GH) concentrations (325% in Leghorns and 233% in broilers) compared with controls. Plasma concentrations of somatomedin-C in protein-restricted Leghorn and broiler chicks were only 50 and 34% of those of controls, respectively. Similarly plasma concentrations of thyroxine (T4) and triiodothyronine (T3) were reduced in protein-deprived birds compared with those fed adequate protein. Plasma T3 concentrations of restricted birds were 48 (Leghorns) and 47% (broilers) of those of controls, whereas plasma T4 concentrations were 44 (Leghorns) and 41% (broilers) of those of controls.