The following paper reviews the current data regarding the physiological origins of variations in feed efficiency, and the effects of genetic selection on energy balance parameters. Effects of diet composition on response to genetic selection are considered by focusing on three feed factors: protein concentration, energy concentration and utilisation of specific ingredients. Analyses of effects of genetic selection on responses to diet composition highlight the problems associated with the increased dietary protein concentrations needed by the latest commercial broilers as a result of genetic selection. The advantages and disadvantages in producing lean birds are discussed from the point of view of feed efficiency and adaptation to feed variation. Lastly, the problems of variation in digestibility resulting from broiler selection are discussed.