Kriketos AD et al (2000) investigated the contributions of physical fitness and body composition to 24-hour fat oxidation in adults under sedentary conditions in a whole-room calorimeter.
The results show that
In male subjects under sedentary conditions, 24-hour fat oxidation is positively related to body fat mass and negatively related to VO2max (the marker used here for level of physical fitness). This supports our hypothesis that regularly active males maintain lower body fat stores as the low contribution to daily fat oxidation from a lower body fat mass is counterbalanced by the high contribution to fat oxidation from daily physical activity.
The lack of a relationship between VO2max and 24-hour energy expenditure under the sedentary conditions of this study suggests that the major effects of physical activity on total daily energy expenditure and fat oxidation may occur during and relatively quickly after an exercise bout. Further, cessation of regular exercise will likely be associated with a high risk of positive fat balance and weight gain.