Intermittent Fasting in Cycling Women: The Effects of Hormones and the Menstrual Cycle
Dr Jaclyn Smeaton
Intermittent fasting has become popular lately, with evidence growing that shows benefits for weight management, metabolic health, longevity, and more1. Unfortunately, most of this research has been performed in animal models and men, leaving women to wonder if intermittent fasting is right for them as well.
WHAT IS INTERMITTENT FASTING?
Intermittent fasting is an eating pattern that involves having a defined period within a day when all food is consumed (the eating window), and a defined period of fasting each day.
There are several types of intermittent fasting. The most popular pattern is 16:8, where food may be eaten during an 8-hour period each day, followed by a 16-hour fast. For example, someone may begin fasting at 8pm and have their first meal the next day at noon. There are other fasting regimens, including 12:12, where there is a 12-hour fasting period, or even formats where people fast for 24 hours once or twice per week.
RESEARCH ON INTERMITTENT FASTING IN WOMEN
There have been concerns raised on the potential negative impacts of intermittent fasting on women and their reproductive cycles. These worries began with the findings from a study in young female rats2. In this experiment, rats fasted for 24 hours every other day for 12 weeks. In the fasting group, rats experienced an increase in estradiol along with a decrease in luteinizing hormone compared to controls, as well as menstrual cycle irregularities.
It should be noted that these rats were only three months old, which would correlate to nine years of age in humans. However, this study raised concerns about the potential negative impact of this or other fasting regimens on cycling women.
INTERMITTENT FASTING AND HORMONE LEVELS
ESTROGEN
To date, no studies have directly evaluated whether intermittent fasting affects estradiol levels. There is one female study looking at meal timing in women with polycystic ovarian syndrome (PCOS). This study found that when women with PCOS consumed more than 50 percent of their daily calories at dinner (versus at breakfast), there was an increase in estradiol3. No fasting was involved in the study, only an intentional shift in calories to later in the day.
ANDROGENS
A few studies have evaluated the effect of intermittent fasting on androgen levels in women. One study evaluated premenopausal women who followed a 5:2 diet (where two days per week, women fasted and consumed only 500 calories, and five days per week they ate freely) and found that after 24 weeks their free androgen index decreased, but there was no impact to DHEA-S, testosterone, or androstenedione4.
Two studies completed in women with PCOS demonstrate that intermittent fasting may reduce various androgen measures (free androgen index, total testosterone, DHEA-S, and androstenedione). Reductions in inflammatory markers, insulin resistance and body weight were also observed in these studies5 6. These research articles suggest that intermittent fasting may reduce androgens in women, with potential clinical benefit in management of PCOS.
The studies mentioned above also looked at additional hormonal markers. Some trials have shown that intermittent fasting may increase concentrations of sex hormone binding globulin (SHBG), which has been associated with improved insulin sensitivity, independent of sex hormone concentration. One trial5 also assessed LH and FSH levels and found them unchanged.
INTERMITTENT FASTING FOR CYCLING WOMEN
For most cycling women, intermittent fasting may lower circulating androgen levels, which might be desirable in circumstances of PCOS without impacting other hormones such as gonadotropin hormones (LH & FSH) and estradiol.
The rat study by Kumar does show that with more restrictive intermittent fasting regimens (or perhaps for younger women) there could be a risk of menstrual cycle disruption. More research is needed to determine whether intermittent fasting is good for women, and to establish the best way to practice it.
SHOULD I START INTERMITTENT FASTING?
Be sure to take on eating patterns like intermittent fasting in a gentle way, and in partnership with your provider. They can monitor your hormone levels to ensure you reap the benefits without any unwanted effects to your cycle.
If you are interested in intermittent fasting, a DUTCH Test can be a great tool to evaluate your hormone health. To understand how this type of testing can support you when you are considering adopting intermittent fasting, talk to a healthcare provider with knowledge of DUTCH Test interpretation.
- Varady K.A., Cienfuegos S., Ezpeleta M., Gabel K. Cardiometabolic Benefits of Intermittent Fasting. Annu. Rev. Nutr. 2021;41:333–361. doi: 10.1146/annurev-nutr-052020-041327.
- Kumar S., Kaur G. Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: A study of hypothalamo-hypophysial-gonadal axis. PLoS ONE. 2013;8:e52416.
- Jakubowicz D., Barnea M., Wainstein J., Froy O. Effects of caloric intake timing on insulin resistance and hyperandrogenism in lean women with polycystic ovary syndrome. Clin. Sci. 2013;125:423–432.
- Harvie M.N., Pegington M., Mattson M.P., Frystyk J., Dillon B., Evans G., Cuzick J., Jebb S.A., Martin B., Cutler R.G., et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: A randomized trial in young overweight women. Int. J. Obes. 2011;35:714–727.
- Li C., Xing C., Zhang J., Zhao H., Shi W., He B. Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. J. Transl. Med. 2021;19:148
- Jakubowicz D., Barnea M., Wainstein J., Froy O. Effects of caloric intake timing on insulin resistance and hyperandrogenism in lean women with polycystic ovary syndrome. Clin. Sci. 2013;125:423–432.
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