Intermittent fasting (IF) has gained significant attention as a popular dietary approach for weight management and overall health. This eating pattern involves cycles of fasting and eating within specific time windows. There are many different forms of intermittent fasting, some more beneficial than others. The most popular forms of intermittent fasting involves skipping breakfast or having a late morning meal. This is the type of intermittent fasting referred to in this post. In this blog post, we will explore seven key hormones affected by intermittent fasting and delve into the scientific findings from relevant studies.
- Cortisol: Cortisol is a stress hormone that plays a vital role in regulating metabolism and stress response. Skipping breakfast, a common practice in intermittent fasting, may lead to a transient rise in cortisol levels in women, even in the absence of chronic stress (1). This is likely due to the need for the body to increase cortisol due to low blood sugar following a prolonged fast. Cortisol is needed to help release sugar into the blood stream in the absence of fuel.
- Insulin: One of the best known benefits to intermittent fasting is blood sugar control and a potential to prevent and reverse insulin resistance, however, women react differently to men. A study found that women actually had a worsening of blood sugar response after 3 weeks of intermittent fasting, whereas men showed improvement (2). Again, this is likely due to the very strong connection between cortisol and insulin. In the absence of food for prolonged periods of time cortisol increases to help increase blood sugar, cortisol then further disrupts insulin leading to unstable blood sugar levels and a more sensitive stress response.
- Testosterone: Research suggests that intermittent fasting can lower testosterone levels in both men and women. This may prove to be valuable to women struggling with PCOS who may wish to utilise intermittent fasting to help lower elevated testosterone levels. However, this effect was seen mostly with fasting in the evenings, not mornings (3). In otherwise healthy women, intermittent fasting may lead to lower than optimal testosterone levels, however this is not yet proven and only a deduction in reasoning based on the research we have.
- DHEA: Dehydroepiandrosterone (DHEA) is a precursor hormone that contributes to the production of estrogen, progesterone and testosterone in women. Intermittent fasting has been associated with a decrease in DHEA levels in women. This decrease may potentially contribute to symptoms such as vaginal dryness and other low estrogen and low progesterone symptoms (3). It is important to note that more research is needed to fully understand the implications of decreased DHEA levels due to intermittent fasting.
- Sex Hormone Binding Globulin (SHBG): SHBG is a protein that binds to sex hormones, including estrogen and testosterone. Intermittent fasting has been shown to increase SHBG levels in women. This binding of free testosterone may be beneficial for women with PCOS, as it helps reduce excess testosterone which may be contributing to facial hair, weight gain and acne (3). However, in otherwise healthy women, increased SHBG is a major cause of low libido (a decrease in SHBG is the main reason most women experience low libido while on hormonal birth control).
- GnRH: Gonadotropin-releasing hormone (GnRH) plays a crucial role in regulating the menstrual cycle and fertility. Intermittent fasting combined with calorie restriction down-regulates kisspeptin in the brain, this then down-regulates GnRh which then interferes with ovulation. This causes a myriad of disturbances in the menstrual cycle and may impact fertility in some women (4).
- Thyroid Hormones: One study found that fasting affects the hypothalamus, pituitary thyroid axis (HPT axis) and can actually have the opposite effect of weight loss in women of reproductive age. Fasting can create a state of stress by increasing the hormone cortisol which can directly affect the thyroid by down-regulating the thyroid response. Cortisol also interferes with the conversion of T4 hormone to active T3 hormone in the liver. This slows down metabolism and can actually lead to weight gain instead of weight loss (5).
To conclude, there is little evidence suggesting estrogen and progesterone levels are impacted by intermittent fasting alone, however these hormones are often impacted by calorie restrictions through low GnRH. Because the practice of intermittent fasting and calorie restriction has shown to significantly increase cortisol (even in the absence of persevered or chronic stress) this leads to a range of different effects on other hormones. In real life, intermittent fasting is likely safe when performed short term for most women, however, prolonged intermittent fasting may look like a delay in ovulation, irregular cycles or a shortening of the luteal phase which may then affect fertility. It may also look like a loss of menstrual period, energy deficiency, low libido, vaginal dryness, exercise intolerance and an overactive stress response. Intermittent fasting is especially harmful in women with a history of eating disorders, children, adolescents or women in pregnancy and post-partum due to increased energy demands and a sensitive hormonal system during those times. Post-menopausal women are less sensitive to intermittent fasting and may wish to attempt to try a version of fasting that works for them. Most research favours a more gentle “time restricted eating” where there is less calories in the evening and the bulk of calorie consumption occurs during the day.
For more reading on intermittent fasting on women of reproductive age, including some tips on how to get the benefits of intermittent fasting without the risks, you can read my other article here: https://talidavoinea.au/intermittent-fasting-for-women-of-reproductive-age/
1.Witbracht, M., Keim, N. L., Forester, S., Widaman, A., & Laugero, K. (2015). Female breakfast skippers display a disrupted cortisol rhythm and elevated blood pressure. Physiology & behavior, 140, 215–221. https://doi.org/10.1016/j.physbeh.2014.12.044 https://pubmed.ncbi.nlm.nih.gov/25545767/
2. Heilbronn, L. K., Civitarese, A. E., Bogacka, I., Smith, S. R., Hulver, M., & Ravussin, E. (2005). Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obesity research, 13(3), 574–581. https://doi.org/10.1038/oby.2005.61
3.Cienfuegos, S., Corapi, S., Gabel, K., Ezpeleta, M., Kalam, F., Lin, S., Pavlou, V., & Varady, K. A. (2022). Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients, 14(11), 2343. https://doi.org/10.3390/nu14112343
4.Meczekalski, B., Katulski, K., Czyzyk, A., Podfigurna-Stopa, A., & Maciejewska-Jeske, M. (2014). Functional hypothalamic amenorrhea and its influence on women’s health. Journal of endocrinological investigation, 37(11), 1049–1056. https://doi.org/10.1007/s40618-014-0169-3