1. The present study was done to clarify the relationship between the amount of dietary protein given to, and the gut microflora of, the host. Day-old chicks were given diets containing three concentrations of dietary protein (50, 200 and 400 g/kg) for 14 d. Body-weight gain, food consumption, body consumption, and protein and energy utilization were measured.
2. There was no difference in body-weight gain and food consumption between germ-free (GF) and conventional (CV) chicks, but food conversion efficiency (g body-weight gained/g food consumed) was significantly higher in GF than in CV chicks.
3. Little difference was found in protein retention (g protein retained/14 d), but protein retention rate (g protein retained/g protein consumed) tended to be higher in GF chicks, particularly those given the diet with the lowest protein.
4. The presence of micro-organisms improved metabolizable energy (ME) values of the diets, but not all of the digested energy in CV chicks was utilized for growth. Therefore there was little difference in energy retention (kJ energy retained/14 d) between environments, although energy retention rate (kJ energy retained/kJ ME consumed) was significantly lower in CV chicks. The amount of body fat in GF chicks was higher than that in CV chicks, especially in those fed on the low-protein diet.
5. It is suggested that although the gut microflora may have beneficial effects on the digestion of dietary energy components, they may have detrimental effects on utilization of ME by their hosts, because chicks harbouring a gut microflora seem to have higher energy requirements for maintenance.