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1 Center for Human Nutrition and Department of Internal Medicine, Washington University School of Medicine, St. Louis, MO, USA
* To whom correspondence should be addressed. E-mail: sklein{at}im.wustl.edu.
The effect of relative body fat mass on exercise-induced stimulation of lipolysis and fatty acid oxidation was evaluated in 15 untrained men (5 lean, 5 overweight, and 5 obese with body mass indeces of 21±1, 27±1, 34±1 kg/m2, respectively, and % body-fat ranging from 12% to 32%). Palmitate and glycerol kinetics and substrate oxidation were assessed during 90 min of cycling at 50% VO2 peak by using stable-isotope labeled tracer infusion and indirect calorimetry. An inverse relationship was found between % body fat and exercise-induced increase in glycerol Ra relative to fat mass (r2=0.74; p<0.01). The increase in total fatty acid uptake during exercise ([µmol/kg fat-free mass] x 90 min) was ~50% smaller in obese (181±70; p<0.05) and ~35% smaller in overweight (230±71; p<0.05) than in lean (354±34) men. The percentage of total fatty acid oxidation derived from systemic plasma fatty acids decreased with increasing body fat, from 49±3 % in lean to 39±4% in obese men (p<0.05); conversely, the percentage of non-systemic fatty acids, presumably derived from intramuscular and possibly plasma triglycerides, increased with increasing body fat (p<0.05). We conclude that the lipolytic response to exercise decreases with increasing adiposity. The blunted increase in lipolytic rate in overweight and obese men compared with lean men limits the availability of plasma fatty acids as a fuel during exercise. However, the rate of total fat oxidation was similar all groups, because of a compensatory increase in the oxidation of non-systemic fatty acids.
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