If you are a woman over 40, how long should you fast?

Sara Szal GottfriedMD · board-certified OB/GYN by her own account · precision medicine

Start at 13 hours, get baseline labs first, and extend only if sleep, mood and cycle stay stable. She fasts herself and is not arguing against fasting.

What it rests onShe reports three months of daily 16:8 producing no weight change, heavier periods, falling ferritin, rising cortisol and falling DHEA, and a recent DHEA-S of 35 that 'got my attention clinically'. Her mechanism is that the female reproductive axis monitors energy availability closely, that elevated cortisol suppresses GnRH pulsatility and disrupts LH and FSH signalling, and that in perimenopause the HPA axis is already more reactive, so a 16-hour window 'may land not as a gentle metabolic intervention but as a strong physiological stressor'. Her cited human data: greater perceived stress and higher plasma norepinephrine in women than men under an identical 48-hour fast, significant cortisol rise in women fasted 72 hours, and an oestradiol drop in women not seen in men. Her conclusion is explicitly not abstinence: 'Despite that result, I still fast.'

Mindy Hetzer PelzD.C. · chiropractor, author and fasting educator

Time the fast to the cycle phase, and in the low-hormone phase go long: 13 to 72 hours, with 24-hour and 36-hour fasts recommended as the window to experiment.

What it rests onHer Fasting Cycle divides the menstrual cycle into four phases with different fasting lengths. Power Phase 1, days 1 to 10, carries 'Fasting length: 13 to 72 hours', on the reasoning that sex hormones are at their lowest so 'your body tolerates fasting beautifully. There's no hormonal interference', and that fasting lowers insulin, creating the low-insulin environment in which oestrogen rebuilds. Her instruction: 'If you've been wanting to experiment with a 24-hour fast, a 36-hour fast, or even a longer extended fast, this is the window to do it', and 'The more aggressive you go with fasting during this phase, the more healing potential you unlock.' She caps the ovulatory Manifestation Phase, days 11 to 15, at 13 to 15 hours maximum, warning that 'Long, stressful fasts raise cortisol.' For perimenopausal women still cycling she says to keep timing fasts to the period, and to switch to the moon when cycles become irregular.