The degree of replenishment of the body's glycogen stores influences the contribution made by glucose and free fatty acids to the fuel mixed oxidized. The expansion of the adipose tissue mass required to promote fat oxidation to rates commensurate on average with fat intake is therefore influenced not only by the diet's fat content, but by glycogen levels as well. It seems possible that recent changes in the food supply and a further decline in physical activity could have led to some increase in the range within which glycogen levels are habitually maintained, and that this could be a cause for the recent increase in the incidence of obesity noted in many countries.