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

Ecological, cross-sectional and prospective longitudinal studies show that obesity is positively associated with dietary fat energy percentage and negatively with carbohydrate energy percentage. The relationships are concordant with the concept of separately regulated macronutrient balances and the higher satiating effect of carbohydrate than of fat. 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 use, we were able to show that an obese group of women oxidized 40.2% fat energy while a normal weight group oxidized only 36.0% fat energy, although they all consumed a diet with 30% fat energy. Percentage fat oxidation (24 h) was positively correlated with fat mass, which supports the theory that the expansion of fat stores is a prerequisite to an increase in fat oxidation to match a high dietary fat energy percentage. Post-obese subjects did not differ in 24 h macronutrient balances from a control group when consuming diets with 20 and 30% fat energy. In contrast, they failed to increase the ratio of fat to carbohydrate oxidation appropriately when exposed to a high-fat diet (50% fat energy), which resulted in a positive fat balance and a negative carbohydrate balance. The post-obese subjects seem to have normal insulin sensitivity, and preliminary results suggest that exercise-induced stimulation of lipolysis is normal, while fat oxidation is reduced in spite of higher circulating levels of free fatty acids. In conclusion, the studies suggest that a positive fat balance is elicited by a high-fat diet in subjects with a genetic predisposition to fail to increase fat oxidation appropriately.