Fasting and your menstrual cycle: what we know

Little evidence and a lot of marketing. What you can do: listen to yourself and keep track.

What studies say

There are few studies on intermittent fasting in women that look at the menstrual cycle, and most are small and short. In healthy women, 12- to 16-hour fasts did not show major changes in the cycle. Strong, prolonged energy restriction, however, can disrupt it.

"Cycle syncing" your fasts

Shortening or lengthening fasts by phase is trendy. No trials show it improves outcomes. If it helps you as an organizing rule, it does no harm. But it is not established science.

What you can do

• Log hunger, energy and mood: many people notice more hunger in the days before their period (luteal phase).
• If a day feels harder, stopping early is a valid choice.
• If your cycle becomes irregular or stops while fasting or losing weight, see a professional.

About the app's prediction

The app estimates your phase from a calendar of your previous cycles. It does not measure hormones or temperature: it cannot be used as contraception or to plan a pregnancy.

Sources

• Cienfuegos S et al. Effect of intermittent fasting on reproductive hormone levels in females and males: a review of human trials. Nutrients. 2022;14(11):2343.
• Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab. 2003;88(1):297-311.

General information; not a substitute for a health professional.