Author(s): LÖFBERG, I., KARPPINEN, J.1,2, LAATIKAINEN-RAUSSI, V.1, IHALAINEN, J.1,3, LEHTI, M.1, KYRÖLÄINEN, H.1, HACKNEY, A.4, MIKKONEN, R.1, Institution: UNIVERSITY OF JYVÄSKYLÄ, Country: FINLAND, Abstract-ID: 2030

INTRODUCTION:
Estradiol (E2) and progesterone (P4) are suggested to influence substrate use during exercise [1]. Previous literature indicates higher fat oxidation during submaximal continuous exercise in the luteal phase (LP) of the menstrual cycle (MC) compared to the follicular phase (FP) [2]. Evidence regarding fat oxidation during exercise in women using hormonal contraceptives (HC) is sparse but indicates no clear differences between active (AP) and inactive hormonal phases (IP) [3]. This study aimed to examine differences in peak fat oxidation (PFO) between hormonally confirmed MC phases in naturally menstruating women (NoOC) and HC phases in women using combined monophasic oral contraceptives (COC).
METHODS:
Healthy untrained women (Tier 1) [4] aged 18–35 years with a body mass index between 19.5 and 35 kg/m² were recruited to this study and divided into NoOC- (n=35) and COC-group (n=19). Participants were measured after overnight fast in the FP/LP or the AP/IP of the MC or HC cycle, respectively. E2 and P4 were measured using immunoassays and body composition via bioimpedance. Fat oxidation was measured using indirect calorimetry during a graded treadmill PFO test consisting of 4-min stages. The initial speed and gradient were 4.5 km/h and 1.2°, respectively. The speed increased by 1 km/h in the first two stages and the gradient increased by 1.2° from the third stage onwards until participants reached a respiratory exchange ratio of 0.95 [5]. Fat oxidation was calculated for the last minute of each stage and the highest rate achieved was selected as the PFO. MC or HC phases were compared using paired t-test. Associations between E2/P4 and PFO were analyzed with multiple linear regression using fat free mass (FFM) as a covariate.
RESULTS:
All participants in the NoOC-group had P4 level ≥16 mmol/l in the LP and higher E2 and P4 concentrations in the LP than in the FP (p<0.001). E2 and P4 did not change from AP to IP in the COC-group. In the NoOC-group, PFO was 0.40 g/min (SD 0.09) in the FP and 0.43 g/min (SD 0.12) in the LP (mean difference 0.03, 95% CI -0.02–0.07, p=0.253). In the COC-group, PFO was 0.44 g/min (SD 0.11) in the AP and 0.48 g/min (SD 0.12) in the IP (mean difference 0.04, 95% CI -0.01–0.09, p=0.099). E2/P4 were not significantly (p>0.05) associated with PFO in any of the MC/HC phases.
CONCLUSION:
E2 and P4 fluctuations during the MC cycle or the use of COC do not notably affect PFO. This discrepancy with previous work may result from the use of different testing protocols, as most studies reporting higher fat oxidation during the LP have used prolonged single-stage exercise, which is known to be crucial for maximal lipolysis [6].
REFERENCES:
1. Oosthuyse & Bosch. 2010. Sports Med 2010, 40, 207–227
2. Hackney et al. 1994. Int J Sport Nutr 4(3):299–308
3. Williams et al. 2023. J Appl Physiol 135(3), 642–654
4. McKay et al. 2021. Int J Sports Physiol Perform 17(2):317–31
5. Amaro-Gahete et al. 2019. 7(23):10:909
6. Wolfe. 1998. Adv Exp Med Biol 441:147-56