Substrate oxidation and availability during acute exercise in non-obese, obese, and post-obese sedentary females

OBJECTIVE: This study compared fat oxidation rates during an acute bout of cycle ergometry exercise (E) typical of progressive fat oxidation in health y, but sedentary, women of different obesity histories. DESIGN: Five never-obese (NO) (mean age = 25 +/- 3 (s.e.) y, mean body fat = 25.0 +/- 2.8 (s.e.)%), five obese (O) (26 +/- 3 y, 44.4 +/- 1.7%), and fi ve post-obese (PO) (22 +/- 1 y, 32.2 +/- 3.0%) women cycled for 60 min at 6 0 - 65% peak VO2. To identify the specific effects of E, a control trial co nsisting of 60 min of seated rest (R) was also performed. E and R trials we re counterbalanced one month apart in the follicular phase and conducted fo llowing a 3 d normalized, eucaloric diet. MEASUREMENTS: Dual energy X-ray absorptiometry (DEXA) was used to determine body composition, and all were weight stable for at least eight weeks prio r to experimentation. During both trials breath by breath measurements of V O2 and RER were used to determine substrate oxidation and energy expenditur e. Blood samples were collected for hormone and metabolite analysis before, and every 15 min during exercise or rest. RESULTS: All three groups showed a similar and progressive shift toward fat oxidation as exercise progressed. No group differences were observed for E energy expenditure or fat oxidation. Glycerol (P<0.0001) and free fatty ac ids (P<0.0001) increased similarly in all three groups, but PO maintained t he highest free fatty acid level during exercise (group effect; P<0.01). E and R decreased (P<0.001 for both) insulin levels across groups, with lowes t levels noted in PO and highest in O. Plasma epinephrine (<0.0001) and nor epinephrine (P<0.001) increased similarly during E in all three groups. Pla sma growth hormone (GH) levels rose (P<0.05) during E, with a pronounced in crease observed in PO. CONCLUSION: We conclude that exercise of equal relative intensity elicited similar fat oxidation rates among NO, O, and PO women, despite group differ ences in free fatty acid availability. The PO women's persistently lower in sulin and higher plasma GH levels may have enhanced free fatty acid availab ility.