Fasting Tracker

Quick answers

Intermittent fasting and your period

Timing isn't what your cycle notices. Eating too little is

What fasting does to your hormones

Researchers at the University of Illinois reviewed the human trials on fasting and reproductive hormones. In premenopausal women with obesity, fasting tended to lower androgens such as testosterone and to raise SHBG, a protein that keeps them in check. It had no effect on estrogen, on the brain hormones that control ovulation, or on prolactin 1.

The authors stress that very few studies exist, so firm conclusions aren't possible yet, and most were in women with obesity 1. We know much less about lean, very active women. Fasting Tracker's guide fasting for women covers this in more detail.

Energy is what your cycle notices

Key study

Cycles depend on having enough energy

  • Controlled study
  • 29 young women
  • 5 days at each level

Women exercised daily while researchers set exactly how much energy they had left after exercise. The brain's pulses of LH, the hormone that drives the cycle, stayed normal at moderate levels and were disrupted only when energy fell well below what the body needed.

Loucks et al., J Clin Endocrinol Metab 2003

That's the key point. A daily fasting window with enough food inside it is very different from eating too little. The risk grows when a short window, a big calorie cut and hard training come together.

PCOS

For women with polycystic ovary syndrome, fasting may even help. In a small pilot study, 15 women with PCOS who weren't ovulating ate within an 8-hour window for five weeks. Their testosterone and insulin resistance fell, and 11 of the 15 had more regular cycles. There was no comparison group, so larger trials are needed 3. If you have PCOS, plan any changes with your doctor.

Fasting around your cycle

A gentle approach

  1. Start with 12 to 14 hours and see how you feel over a full cycle before going longer.
  2. If fasting feels harder at certain times of your cycle, shorten it on those days. Flexibility is fine.
  3. Eat enough in your window, with protein at every meal, especially if you train 4.
  4. Expect your weight to go up and down with your cycle, and look at the trend, not single days.
  5. Keep an eye on your cycle while you fast.

If your period changes or stops

Treat it as a signal, not something to push through. Eat more, ease off long fasts and hard training, and see a doctor. If there's any chance you could be pregnant, take a test. See a doctor too if your periods become much heavier or more painful, or if you feel cold and tired all the time or your hair is shedding.

A history of eating disorders is another reason for caution. In adults with obesity, alternate-day fasting didn't increase disordered eating, and binge eating fell 6, but those studies didn't include people with a history of eating disorders. If that's you, don't fast without support from a professional. Around menopause, see intermittent fasting for women over 40.

Sources

  1. Cienfuegos S, Corapi S, Gabel K, et al. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022;14(11):2343. doi.org/10.3390/nu14112343
  2. 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. doi.org/10.1210/jc.2002-020369
  3. Li C, Xing C, Zhang J, et al. Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. J Transl Med. 2021;19(1):148. doi.org/10.1186/s12967-021-02817-2
  4. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. doi.org/10.1136/bjsports-2017-097608
  5. Glazier JD, Hayes DJL, Hussain S, et al. The effect of Ramadan fasting during pregnancy on perinatal outcomes: a systematic review and meta-analysis. BMC Pregnancy Childbirth. 2018;18(1):421. doi.org/10.1186/s12884-018-2048-y
  6. Hoddy KK, Kroeger CM, Trepanowski JF, et al. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44. doi.org/10.1186/s12937-015-0029-9
  7. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873

A note on health

Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.