The exercise dose curve for fertility
30–60 minutes/day is the sweet spot
Exercise and fertility follow a U-shaped curve:
• Each hour/week of vigorous activity was associated with 7% lower ovulatory infertility risk (Nurses’ Health Study II, n=116,671).
• Physical activity interventions improved pregnancy rates: ~17% vs ~11% in controls across a meta-analysis.
• But >1 hour/day of vigorous exercise was associated with approximately 6× infertility risk in a case-control study.
The sweet spot: 30–60 minutes/day of moderate-to-vigorous activity, or roughly 1–5 hours/week of vigorous exercise.
A foundational 1985 NEJM experiment proved that strenuous exercise disrupts ovulation within 2 cycles in untrained women. The IOC’s 2023 RED-S consensus reaffirms that low energy availability — burning more than you’re eating — impairs menstrual function, bone health, and fertility.
If your period disappears or becomes irregular with heavy training, the first-line treatment is restoring energy balance: more calories, potentially reduced intensity.