Updated: Apr 3
Navigating the world of nutrition recommendations can be overwhelming with the constant influx of new information available on the internet. In recent years, Intermittent Fasting (IF) has gained widespread attention for its positive impact on metabolic health, reversal of disease and longevity. But, as with many diet fads, intermittent fasting research is limited in the female population and most studies involving females are still the majority male. The other variable is that IF is an umbrella term for various ways to perform fasting so the research is not conclusive to one type of fast being more beneficial than another which can cause confusion for consumers if they attempt to implement these strategies. For reference, the three main times of IF are alternate day fasting, the 5:2 diet (two, 24-hour periods of fasting a week), and Time-Restricted Feeding (TRF or fasting for 12-20 hours every day) .
What are the benefits of Intermittent Fasting (IF)?
The research, mostly in the male population with some extension to females, has shown IF to improve insulin sensitivity, reduce blood glucose levels, lower cholesterol and LDL levels, reduce body weight, inflammation, oxidative stress and improve the gut microbiome [1,2,3,4]. IF has been shown to reduce gut permeability (or symptoms associated with ‘leaky gut syndrome’), decrease inflammation in the gut and improve the diversity and richness of gut bacteria which has been associated with improvements in chronic disease and overall health .
However, all the aforementioned benefits are not without risk. Most individuals who see this new information venture out to intermittent fast on their own accord without professional guidance which can be a slippery slope. Often, without appropriate and individualized intervention, IF inevitably results in caloric restriction which could be in part why research has seen significant benefits in initial weight reduction in obese individuals . The risks of caloric restriction on a woman’s health and hormones far outweighs the benefits of a fast approach to weight loss via IF.
Intermittent Fasting and Calorie Consumption
Restriction of calories reduces ovulation and subsequently reduces fertility via two potential mechanisms: reduction of the Follicle Stimulating Hormone, which is required to grow a follicle and release an egg for ovulation, or by impacting the response of a region in the brain called the hypothalamus which helps to regulate hormones produced for ovulation and thyroid function .
Calorie restriction also causes an elevation of serum cortisol levels and a decreased ability to suppress cortisol . Perceived stress, known as an internal stressor which can result from calorie counting or increased focus on the number of calories consumed, also has been shown to cause elevated cortisol levels . Chronic, elevated cortisol levels can have system-wide impacts on the body (see previous article Cortisol: Friend or Foe?) but to highlight a few it can cause increased glucose production, increased fat storage and interruption of the Hypothalamic-Pituitary-Axis (the stress regulating network) [5,6].
The exact connection between IF and thyroid function has not been extensively studied however the impact of calorie restriction on thyroid function has been. Calorie restriction decreases levels of T3 and increases levels of Reverse T3 . T3 is the active thyroid hormone, it acts at the peripheral level in your cells to carry out the functions of the thyroid gland. Reverse T3 further blocks the conversion of T4 to T3 which manifests thyroid symptoms and in a negative feedback response interferes with normal thyroid functioning . Ultimately, without adequate T3 the thyroid gland is essentially having little to no impact on the body to do its job.
Technically yes, intermittent fasting could be accomplished without restricting calories but in alternate day fasting that would require consuming upwards of 4,000 calories per day and in time-restricted fasting that would result in cramming a minimum of roughly 2,000 calories in a short 4-6 hour window (of course, this is extremely individualized and not the exact calories required; use as reference). Consuming this significant amount of calories in a short window is difficult for many people without feeling like they are overindulging rather than eating intuitively. In most studies, over 50% of women who participated in intermittent fasting trials reported feeling hungry most, if not all, of the days probably due to inadequate nutrient consumption which can be dangerous territory potentially leading to disordered eating .
Intermittent Fasting and PCOS
The most frequent and up to date research on IF in women is on those with PCOS. IF has shown to be beneficial for hyperandrogenism (or elevated testosterone levels) and insulin resistance [2,8]. Please note, not all women with a PCOS diagnosis have elevated testosterone and not all women experience insulin resistance so having a PCOS diagnosis alone does not mean IF is the right choice for you. Nevertheless, let’s uncover what the research says about it.
In women of reproductive age, IF has shown to decrease androgens (testosterone and free testosterone), decrease DHEA-S (a hormone which testosterone and estradiol are derived from) while increasing sex-hormone binding globulin (SHBG) [2,8]. SHBG is a protein responsible for binding testosterone and essentially inactivating it therefore decreasing the levels of available free testosterone from exerting its impact on the body. Lowering testosterone levels to a level within normal limits in women with PCOS also leads to improvement in insulin resistance and the return of normal menstrual cycles .
In a study that looked at Time-Restricted Feeding on women with PCOS who were not obese, it showed significant benefit from consuming the majority of their daily caloric intake at breakfast and the lightest meal of the day being dinner . Specifically, when roughly 980 calories were consumed for breakfast, 640 calories for lunch and 190 calories at dinner, testosterone levels were reduced by 50% and SHBG levels increased by 105% (again, caloric numbers are for reference and should be based on individual needs). While this may be defined as a form of intermittent fasting, it is not calorie restricting or extensive fasting but still provides the gut with adequate rest. Conversely, when lower calories were consumed at breakfast and higher calories at dinner there were no reported changes in hormone levels and, actually, estradiol levels were shown to increase by 50% when more calories are consumed later the day . This information is imperative as many women who may have tested the intermittent fasting waters often fast through the night and morning until lunch time therefore skipping breakfast which proves to eliminate the benefits IF provides.
Furthermore, low testosterone is not the goal for everyone. IF has been shown to lower testosterone and DHEA-S levels in all women and men, not just those with PCOS . So while temporarily you may strive to bring testosterone levels back to within normal limits, individuals with already normal levels of testosterone or DHEA-S who choose to IF could develop levels of testosterone and DHEA-S below normal limits. Low testosterone and DHEA-S levels can lower metabolic function, lower muscle mass, impair mood, memory function, libido and the menstrual cycle .
Increasing SHBG is also not the goal for everyone. Evidence shows oral contraceptives increase SHBG and may remain elevated long after discontinuation of medication. Implementing IF for some women who may already have elevated SHBG could potentially only cause significantly high numbers and further decrease active testosterone . Lab testing for SHBG is not common practice in a primary care setting so most women do not actually have an idea of where their levels range. Therefore caution should be taken in IF with the potential of raising levels of SHBG as increased SHBG will cause low circulating levels of testosterone and estradiol.
Ultimately, as with all nutritional interventions, following mainstream advice without appropriate, in-depth analysis of where you stand with your health, hormone levels and relationship with food can do more harm than good. If Intermittent Fasting is something you still decide to venture into, first and foremost learn to always listen to your body. If something begins to feel off, it’s most likely because it is. Intermittent fasting has the potential to provide benefits to certain individuals temporarily but as the female body is a unique entity with complex hormone systems, it’s imperative to seek professional guidance when venturing into drastic nutrition alterations in order to keep your body in a place of safety.
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 Apr 13;19(1):148. doi: 10.1186/s12967-021-02817-2.
Cienfuegos S, Corapi S, Gabel K, Ezpeleta M, Kalam F, Lin S, Pavlou V, Varady KA. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022 Jun 3;14(11):2343
Khan MN, Khan SI, Rana MI, Ayyaz A, Khan MY, Imran M. Intermittent fasting positively modulates human gut microbial diversity and ameliorates blood lipid profile. Frontiers in Microbiology. 2022;13. doi:10.3389/fmicb.2022.922727
Sun J, Shen X, Liu H, Lu S, Peng J, Kuang H. Caloric restriction in female reproduction: is it beneficial or detrimental? Reprod Biol Endocrinol. 2021 Jan 4;19(1):1. doi: 10.1186/s12958-020-00681-1.
Tomiyama AJ, Mann T, Vinas D, Hunger JM, Dejager J, Taylor SE. Low calorie dieting increases cortisol. Psychosom Med. 2010 May;72(4):357-64. doi: 10.1097/PSY.0b013e3181d9523c. Epub 2010 Apr 5. PMID: 20368473; PMCID: PMC2895000.
Kim BH, Joo Y, Kim MS, Choe HK, Tong Q, Kwon O. Effects of Intermittent Fasting on the Circulating Levels and Circadian Rhythms of Hormones. Endocrinol Metab (Seoul). 2021 Aug;36(4):745-756.
Fontana L, Klein S, Holloszy JO, Premachandra BN. Effect of long-term calorie restriction with adequate protein and micronutrients on thyroid hormones. J Clin Endocrinol Metab. 2006 Aug;91(8):3232-5
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 (Lond). 2013 Nov;125(9):423-32.
Vann MR, Garrard C, Rapaport L, et al. Low testosterone in women. EverydayHealth.com. https://www.everydayhealth.com/low-testosterone/guide/women/. Accessed November 22, 2022.
Zimmerman Y, Eijkemans MJ, Coelingh Bennink HJ, Blankenstein MA, Fauser BC. The effect of combined oral contraception on testosterone levels in healthy women: A systematic review and meta-analysis. Human Reproduction Update. 2013;20(1):76-105. doi:10.1093/humupd/dmt038