DIETARY-COMPOSITION, SUBSTRATE BALANCES AND BODY-FAT IN SUBJECTS WITHA PREDISPOSITION TO OBESITY

Ecological, cross-sectional and prospective longitudinal studies show that obesity is positively associated with dietary fat energy percenta ge and negatively with carbohydrate energy percentage. The relationshi ps are concordant with the concept of separately regulated macronutrie nt balances and the higher satiating effect of carbohydrate than of fa t. Dietary records have suggested that obese subjects tend to consume a diet with a higher fat content than normal weight controls. Due to a carry-over effect of the habitual diet on the next day's substrate us e, we were able to show that an obese group of women oxidized 40.2% fa t energy while a normal weight group oxidized only 36.0% fat energy, a lthough they all consumed a diet with 30% fat energy. Percentage fat o xidation (24 h) was positively correlated with fat mass, which support s the theory that the expansion of fat stores is a prerequisite to an increase in fat oxidation to match a high dietary fat energy percentag e. Post-obese subjects did not differ in 24 h macronutrient balances f rom a control group when consuming diets with 20 and 30% fat energy. I n contrast, they failed to increase the ratio of fat to carbohydrate o xidation appropriately when exposed to a high-fat diet (50% fat energy ), which resulted in a positive fat balance and a negative carbohydrat e balance. The post-obese subjects seem to have normal insulin sensiti vity, and preliminary results suggest that exercise-induced stimulatio n of lipolysis is normal, while fat oxidation is reduced in spite of h igher circulating levels of free fatty acids. In conclusion, the studi es suggest that a positive fat balance is elicited by a high-fat diet in subjects with a genetic predisposition to fail to increase fat oxid ation appropriately.