Whether the association with gut microflora modifies the energy metabolism of chickens was investigated by varying the metabolizable energy consumption level from zero to above the maintenance requirement in the germ-free and conventional states. Single comb White Leghorn chicks were either fasted for 3 d (Expt 1), or fed for 6 d at a fixed daily meal intake of 2, 5 or 8 g/d (Expt 2), or 5, 10 or 15 g/d (Expt 3). Changes in carcass energy deposition and heat production indicated that when no dietary energy was available the presence of the gut microflora could benefit the birds by reducing energy losses, whereas when dietary energy was supplied the efficiency of energy utilization was reduced by the presence of the gut microflora. It was concluded, therefore, that the heavy burden of the gut microflora modifies energy metabolism by exerting a buffering or a counter-productive action on the energy utilization of the chicken.
