The Impact of Natural Menstrual Cycle and Oral Contraceptive Pill Phase on Substrate Oxidation During Rest and Acute Submaximal Aerobic Exercise.

Background. Previous research has identified sex differences in substrate oxidation during submaximal aerobic exercise including a lower respiratory exchange ratio (RER) in females compared to males. These differences may be related to differences in sex hormones. Our purpose was to examine the impact of the natural menstrual cycle (NAT) and 2(nd) and 3(rd) generation oral contraceptive pill (OCP2, OCP3) cycle phases on substrate oxidation during rest and submaximal aerobic exercise. Methods. Fifty female participants (18 NAT, 17 OCP2, 15 OCP3) performed two experimental trials that coincided with the low (i.e., non-active pill/early follicular) and the high hormone (i.e., active pill/mid-luteal) phase of their cycle. RER and carbohydrate and lipid oxidation rates were determined from gas exchange measurements performed during 10min of supine rest, 5min of seated rest, and two 8min bouts of submaximal cycling exercise at similar to 40% and similar to 65% of peak oxygen uptake ((V) over dot O-2peak). Results. For all groups, there were no differences in RER between the low and high hormone phases during supine rest (0.73 +/- 0.05 vs 0.74 +/- 0.05), seated rest (0.72 +/- 0.04 vs 0.72 +/- 0.04), exercise at 40% (0.77 +/- 0.04 vs 0.78 +/- 0.04) and 65% (V) over dot O-2peak (0.85 +/- 0.04 vs 0.86 +/- 0.03; p>0.19 for all). Similarly, carbohydrate and lipid oxidation rates remained largely unchanged across phases during both rest and exercise, apart from higher carbohydrate oxidation in NAT versus OCP2 at 40% (V) over dot O-2peak (p=0.019) and 65% VO2peak (p=0.001). Conclusion. NAT and OCPs do not appear to largely influence substrate oxidation at rest and during acute submaximal aerobic exercise.