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

This study compared fat oxidation rates during an acute bout of cycle ergometry exercise (E) typical of progressive fat oxidation in healthy, but sedentary, women of different obesity histories.

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 five post-obese (PO) (22+/-1 y, 32.2+/-3.0%) women cycled for 60 min at 60-65% peak VO2. To identify the specific effects of E, a control trial consisting of 60 min of seated rest (R) was also performed. E and R trials were counterbalanced one month apart in the follicular phase and conducted following a 3 d normalized, eucaloric diet.

Dual energy X-ray absorptiometry (DEXA) was used to determine body composition, and all were weight stable for at least eight weeks prior to experimentation. During both trials breath by breath measurements of VO2 and RER were used to determine substrate oxidation and energy expenditure. Blood samples were collected for hormone and metabolite analysis before, and every 15 min during exercise or rest.

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 acids (P<0.0001) increased similarly in all three groups, but PO maintained the 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 lowest levels noted in PO and highest in O. Plasma epinephrine (P<0.0001) and norepinephrine (P<0.001) increased similarly during E in all three groups. Plasma growth hormone (GH) levels rose (P<0.05) during E, with a pronounced increase observed in PO.

We conclude that exercise of equal relative intensity elicited similar fat oxidation rates among NO, O, and PO women, despite group differences in free fatty acid availability. The PO women's persistently lower insulin and higher plasma GH levels may have enhanced free fatty acid availability.