Female health and fasting

For women, the guidance here distinguishes fasting from simply keeping meals within the daytime: long fasts and fasted workouts may be a poor fit, especially for active women, while individual responses and circumstances matter.

• Long fasting windows: Stacy Sims describes fasting until noon or very low-calorie days as potentially detrimental for active women, citing added stress and possible disruption to thyroid and reproductive-hormone signaling. She favors eating in alignment with daytime circadian rhythms rather than extending a fast into the morning. 1 2
• Morning training: Sims says fasted training can raise stress and cortisol and make it harder to reach the intensity needed for high-intensity or resistance-training sessions; she says this concern may increase during perimenopause. 3 4
• Not a universal rule: Rhonda Patrick describes choosing based on how she feels and her goals; she sometimes trains fasted, but notes longer sessions can carry a performance cost. 5
• Fertility context: Regular menstrual cycles while using time-restricted eating may suggest it is not disrupting fertility, but low calorie intake or excessive activity can stop periods. During pregnancy, the discussion advises against starting time-restricted eating without consulting a doctor. 6

In short, the transcripts support caution with prolonged or morning fasts—particularly alongside exercise—and emphasize adequate fueling and individual context.