The use of oral contraceptives in women alters the differences in substrate oxidation between phases of the menstrual cycle

We previously reported that women in the luteal (LUT) phase oxidize less leucine, lipid and lipid relative to CHO as a fuel source, whereas they have a more advantageous protein balance, at rest and during exercise as compared with the follicular (FOL) phase ( Hamadeh et al, Physiologist 2004 ; 47 : 306 – 7 ). Moreover, FOL and LUT women oxidize less leucine and have a less negative protein balance at rest and during exercise as compared with men. To investigate whether this is maintained in women taking oral contraceptives (OC), we measured whole body leucine kinetics using the stable isotope L‐[1‐ 13 C]leucine in 6 women taking triphasic OC and compared the results with those for 12 men at rest and during moderate intensity endurance exercise. The women were tested in the FOL (day 9 ± 2; mean ± SEM) and LUT phases (day 21 ± 1) of the menstrual cycle. Following a primed continuous infusion of L‐[1‐ 13 C]leucine in the postabsorptive state, VCO 2 and steady state breath 13 CO 2 and plasma [ 13 C]α‐KIC enrichments were measured at rest and 60, 75 and 90 min during cycling at an intensity of 65% VO 2 max. Menstrual phase had no effect on any outcome measure. Men had higher leucine Ra (P < 0.004), leucine oxidation (P < 0.005) and protein breakdown (P < 0.004), but lower estimated protein balance (P < 0.005), than women. Men had higher protein synthesis (P = 0.046) as compared with FOL women only. We conclude that OC use in women abolishes the differences in substrate oxidation observed in non‐OC users. Studies measuring substrate oxidation should account for the use of OC in women. (Funded by the Hamilton Health Sciences Foundation and NSERC Canada).