Changing diet composition with no energy intake change
Altering diet composition without a change in total energy intake should have relatively modest effects on body weight and body fat content. There are at least two ways that such a change in diet composition could affect body weight. First, the thermic effect of carbohydrate is greater than the thermic effect of fat. Changing to a lower fat diet (assuming total energy and protein intake remain constant) means changing to a higher carbohydrate diet, which will increase total energy expenditure. The magnitude of increase in energy expenditure depends on the magnitude of change of the carbohydrate/fat ratio, but is probably relatively small and of questionable importance in body weight regulation for reducing dietary fat from 35-40% to 20-25% of total energy intake. Second, altering the carbohydrate/fat ratio of the diet requires that substrate oxidation rates be readjusted to the new macronutrient intakes. If total energy expenditure is not changed, these changes occur relatively rapidly, with carbohydrate and protein balance being reachieved more quickly than fat balance (108,109). Negative fat balance and some loss of body fat will occur until fat balance is reachieved. It is difficult to predict the rapidity with which fat balance will be reachieved following a reduction in fat (and an accompanying increase in carbohydrate intake).