For some time, researchers have been finding that people who exercise don’t necessarily lose weight. A study published online in September in The British Journal of Sports Medicine was the latest to report apparently disappointing slimming results. In the study, 58 obese people completed 12 weeks of supervised aerobic training without changing their diets. The group lost an average of a little more than seven pounds, and many lost barely half that.

How can that be? Exercise, it seems, should make you thin. Activity burns calories. No one doubts that.

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THE STUDY
J Appl Physiol (October 15, 2009)
When energy balance is maintained, exercise does not induce negative fat balance in lean sedentary, obese sedentary, or lean endurance-trained individuals

Fat oxidation during exercise is increased by endurance training, and evidence suggests that fat oxidation during exercise is impaired in obesity. Thus, the primary aim of this study was to compare the acute effects of exercise on 24 h fat oxidation and fat balance in lean sedentary (LS, N=10, body mass index (BMI) = 22.5±6.5 kg/m2), lean endurance-trained (LT, N=10, BMI = 21.2±1.2 kg/m2) and obese sedentary (OS, N=7, BMI = 35.5±4.4 kg/m2) men and women. 24 h energy expenditure and substrate oxidation were measured under sedentary (CON) and exercise (EX) conditions while maintaining energy balance. During EX, subjects performed 1 hr of stationary cycling at 55% of aerobic capacity. 24 h fat oxidation did not differ on the CON or EX day in LS (43±9 vs. 29±7 g/day, respectively), LT (53±8 vs. 42±5 g/day), or OS (58±7 vs. 80±9 g/day). However, 24 h fat balance was significantly more positive on EX compared to CON (P<0.01). 24 h glucose, insulin, and free fatty acid (FFA) profiles were similar on the EX and CON days, but after consumption of the first meal, FFA concentrations remained below fasting levels for the remainder of the day. These data suggest that when exercise is performed with energy replacement (i.e. energy balance is maintained), 24 h fat oxidation does not increase and in fact, may be slightly decreased. It appears that the state of energy balance is an underappreciated factor determining the impact of exercise on fat oxidation.
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