A prospective study of male physical activity and fecundability.

study questionTo what extent is male physical activity (PA) associated with fecundability (per-cycle probability of conception)? SUMMARY ANSWER: Preconception levels of vigorous, moderate, or total PA were not consistently associated with fecundability across Danish and North American cohorts, but there was suggestive evidence that bicycling with a 'soft, comfort seat' was associated with reduced fecundability in both cohorts, especially among males with greater BMI. WHAT IS KNOWN ALREADY: Among males, some studies indicate that moderate PA might improve fertility, whereas vigorous PA, especially bicycling, might be detrimental. STUDY DESIGN, SIZE, DURATION: We assessed the association between male PA and fecundability among couples participating in two preconception cohort studies: SnartForaeldre.dk (SF) in Denmark (2011-2023) and Pregnancy Study Online (PRESTO) in North America (2013-2024). We restricted analyses to 4921 males (1088 in SF and 3833 in PRESTO) who had been trying to conceive with their partners for ≤6 cycles at enrollment. PARTICIPANTS/MATERIALS, SETTING,

methodsAt baseline, male partners reported data on medical history, lifestyle, behavioral, anthropometric factors, and their PA levels using different instruments [SF: International Physical Activity Questionnaire (IPAQ); PRESTO: average annual hours/week and type]. Both cohorts included additional questions on bicycling (frequency, bike seat type). After linking couple data at baseline, the female partner completed follow-up questionnaires to update their pregnancy status every 8 weeks for 12 months or until conception, whichever occurred first. We used proportional probabilities regression models to estimate fecundability ratios (FRs) and 95% CIs, controlling for potential confounders. MAIN RESULTS AND THE ROLE OF CHANCE: Average hours/week of vigorous PA, moderate PA, and total metabolic equivalents of task were generally inversely associated with fecundability in SF, but not PRESTO. While there was little association with bicycling overall in either cohort, we observed an inverse association for bicycling using a 'soft, comfort seat' (≥3 vs 0 h/week: SF: FR = 0.75, 95% CI: 0.53-1.05; PRESTO: FR = 0.81, 95% CI: 0.62-1.07) but not a 'hard, racing-style seat' (≥3 vs 0 h/week: SF: FR = 1.16, 95% CI: 0.95-1.41; PRESTO: FR = 1.06, 95% CI: 0.89-1.28). Among males with BMI ≥25 kg/m2, associations with bicycling using a 'soft, comfort seat' were similar or stronger (≥3 vs 0 h/week: SF: FR = 0.75, 95% CI: 0.45-1.24; PRESTO: FR = 0.73, 95% CI: 0.52-1.03). LIMITATIONS, REASONS FOR CAUTION: Misclassification of PA was likely the most important study limitation because we ascertained PA only once at enrollment using different instruments in each cohort. We would expect misclassification of PA to be non-differential given the prospective study design. Additional weaknesses include the narrow range of PA levels evaluated, reduced precision when stratifying the data by selected covariates, and limited generalizability due to the large percentage of non-Hispanic White participants and restriction of the cohort to pregnancy planners. WIDER IMPLICATIONS OF THE

findingsFurther evaluation of the potential deleterious effects of bicycling on male fertility, with additional consideration of the influence of bike seat type and BMI, may be warranted. STUDY FUNDING/COMPETING INTEREST(S): This study was funded by NICHD Grants R21-HD072326, R01-HD086742, R01 HD105863, and R03-HD094117. These funding bodies had no involvement in the: study design; collection, analysis, and interpretation of data; writing of the report; or decision to submit the article for publication. L.A.W. serves as a paid consultant for AbbVie, Inc. and the Bill and Melinda Gates Foundation. She also receives in-kind donations for primary data collection in Pregnancy Study Online (PRESTO) from Swiss Precision Diagnostics (home pregnancy tests) and Kindara.com (fertility apps). All of these relationships are for work unrelated to this manuscript. M.L.E. is an advisor for and holds stock in Legacy, Doveras, VSeat, Hannah, Illumicell, HisTurn, & Next. The other authors have no competing interests to declare. TRIAL REGISTRATION NUMBER: N/A.