Menstrual cycle phase and oral contraceptive effects on triglyceride mobilization during exercise

We examined the effects of menstrual cycle phase and oral contraceptive (OC) use on triglyceride mobilization during 90 min of rest and 60 min of leg ergometry exercise at 45 and 65% peak O₂ uptake (V̇O _{2 peak}) in eight moderately physically active, eumenorrheic women (24.8 ± 1.2 yr). Subjects were tested during the follicular phase (FP) and the luteal phase (LP) before OC use and during the inactive phase (IP) and high-dose phase (HP) after 4 complete mo of OC use. Glycerol rate of appearance (R_a), a measure of triglyceride mobilization, was determined in a 3-h postabsorptive state using a primed constant infusion of [1,1,2,3,3- ²H]glycerol. Before OC use (BOC), there were no significant differences between FP and LP in any of the variables studied. Dietary composition, exercise patterns, plasma glycerol concentrations, growth hormone concentrations, and exercise respiratory exchange ratio did not change with OC use. However, 4 mo of OC use significantly (P < 0.05) increased glycerol R_a in HP during exercise at 45% V̇O_{2 peak} (6.2 ± 0.2, 6.5 ± 0.4, and 7.7 ± 1.1 μmol·kg ^-1·min^-1 for BOC, IP, and HP, respectively) and in IP and HP at 65% V̇O_{2 peak} (6.6 ± 0.1, 8.2 ± 0.6, and 8.1 ± 0.7 μmol·kg^-1·min^-1 for BOC, IP, and HP, respectively). Plasma cortisol concentrations were significantly higher with OC use at rest and during exercise at 45 and 65% V̇O_{2 peak}. In summary, although fluctuations of endogenous ovarian steroids have little effect on triglyceride mobilization, the synthetic ovarian steroids found in OCs increase triglyceride mobilization and plasma cortisol concentrations in exercising women. We conclude that the hierarchy of effects of ovarian steroids and their analogs on triglyceride mobilization in exercising women is as follows: energy flux > OC use > recent carbohydrate nutrition, menstrual cycle effects.