Physical Activity and Sex Steroid Hormones in Women: A Prospective Study

Female reproductive function is sensitive to energetic stress and reacts to negative energy balance with reproductive suppression. This sensitivity is important for understanding human reproduction from an evolutionary perspective and also for the prevention of chronic diseases, especially hormone-dependent cancers such as breast cancer. High estradiol and progesterone levels increase cancer risk, and regular exercise may lower these hormones and cumulative hormonal exposure. This study examined the effects of increased physical activity during consecutive menstrual cycles on estradiol, progesterone, and the frequency of ovulation in healthy women.

Data were collected from 135 healthy, urban women who participated in the project for three consecutive menstrual cycles. In cycle 1, participants maintained their habitual activity; in cycles 2 and 3, they were requested to engage in at least 180 min per week of moderate physical activity. Physical activity was measured with wristband accelerometers. Estradiol and progesterone were assessed from daily saliva samples in cycles 1 and 3. Ovulation was monitored using urinary luteinizing hormone tests.

Average daily step counts increased by ~14% from cycle 1 to cycle 2, with no further rise in cycle 3. Greater increases in daily steps from cycle 1 to cycle 3 were significantly associated with lower progesterone levels in cycle 3, after adjusting for baseline activity, baseline hormone levels, and body fat. Physical activity did not have a significant effect on estradiol. The occurrence of ovulation declined from 70% in cycle 1 to 39% in cycle 3, but it was not related to step counts or their changes.

This study confirmed that energy expenditure alone, even when not leading to negative energy balance, is sufficient to cause modest changes in ovarian function. Even moderate increases in physical activity can lower progesterone levels, reducing the chance of conception in a cycle and also the cumulative lifetime hormonal exposure. These findings highlight the importance of encouraging regular physical activity, particularly in premenopausal women, as a strategy to modulate hormone levels, maintain reproductive health, and potentially reduce the long-term risk of hormone-related cancers.