-Written for Kion by Kelsey Hess, CTNC
Fasting continues to make headlines. Every day more research is published revealing its application to everything from heart health to neurodegenerative diseases, cancer prevention, autoimmune disorders, and nearly everything in between. For these reasons and many more, we’re big fans of having a regular fasting practice.
But there’s one glaring truth that seems to fly under the radar as fasting gets more popular…
Most of the studies on fasting have been conducted on rodents or men. Which means there’s very little research on how fasting impacts women.
As fasting gets more mainstream, many females are starting to adopt it as a regular practice without fully understanding how it may impact them differently.
Women see men thriving on daily 18-hour intermittent fasting (IF) or one meal a day (OMAD), with their shredded abs and endless energy, and think it’ll work for them, too.
And while fasting can absolutely be helpful for some women, there’s no doubt that others are experiencing negative side effects from their fasting practice.
So in Part 1 of this two-part series on fasting for women, we're taking a deep dive into:
- What the (limited) research tells us about how women respond to fasting
- How and why fasting affects women differently than men
- How to determine if fasting is right for you
And because we haven’t seen this topic covered in great detail before, we wanted to do it justice. Hence, our first ever two-parter! (Part 2 here.)
So buckle up, it's gonna be a doozy...
Caution: Fasting is not for everyone, especially those who are pregnant or breastfeeding, under the age of 18, or have diabetes, low blood pressure, or a history of eating disorders. Please consult your doctor before starting a fasting practice.
Fasting for Women: Is It a Good Idea?
The main question we’ll attempt to answer in this article is the obvious one: Should women fast?
The answer, unfortunately, is not so obvious. Put simply: It depends.
When you pose this question to the average health expert, you’ll get some pretty polarizing, absolute-sounding answers...
- Either a resounding “Yes, everyone should fast!” with no qualifiers that females should consider a different approach.
- Or, you’ll be knocked over with a booming “ABSOLUTELY NOT. WOMEN SHOULD NEVER, EVER FAST. THAT'S DIET CULTURE BS!”
However, we’re inclined to take a more balanced approach. The answer is not black and white. Fasting can work wonderfully for some women, and for others it can be a really, really bad idea.
Like most health-related topics, bio-individuality is king (or should we say, queen). This is especially true when it comes to how fasting affects different sexes.
Yes, This Article is Written by a Woman...
In case your gender equality radar is already going off, we feel the need to disclose a few things...
Yes, the author of this article is indeed a female. Every word is written with a deep understanding and compassion for other women and the unique health issues we experience.
(And now, please excuse my not-so-graceful switch to first person to make this less awkward.)
More importantly, I have personally practiced fasting for over a decade. I've used it as a tool to improve my gut health, immune system, skin health, and as a way to manage an autoimmune disorder as naturally as possible.
But I've also found out the hard way that more fasting is not always better, especially as a female.
As Dr. Stacy Sims says so beautifully: “Women are not small men.”
Women are different than men in so many wonderful ways, and therefore we require different approaches to our nutrition, fitness, and yes—even how we fast.
Before getting into the details of how women should fast, let’s talk about why they should consider a different approach.
The (Few) Studies on Women and Fasting
While there are admittedly not that many human studies on fasting, there are even fewer on human women, and many of those studies are conducted on specific groups, such as overweight and/or post-menopausal women.
When it comes to healthy, pre-menopausal women, we don't have much scientific ground to stand on when it comes to the benefits of fasting.
This is not a trend limited to fasting, by the way.
Most health-related studies are conducted on animals or men, not young women. To put it simply, we are very complicated to study in an n=1 environment. We are cyclical creatures, with hormones that fluctuate daily. To control for every variable—every woman is in the same phase of her cycle at the same time, and has the same length of cycle—is quite a difficult and expensive task.
Therefore, finding a definitive, science-backed answer to the question of whether or not women (especially young, healthy women) should fast is nearly impossible at this point in time.
But we can certainly make some hypotheses based on the research.
Here’s a synopsis of the small amount of female animal studies on fasting:
- One study found that in response to 3-6 months of alternate day fasting (ADF), female rats became emaciated, ceased cycling, underwent endocrine masculinization, and exhibited a heightened stress response . The male rats, on the other hand, maintained a higher body weight than the females and did not change their activity levels as significantly.
- Another study on rats showed that 3-6 months of ADF led to a reduction in ovary size and irregular reproductive cycles in female rats .
- A study on male and female rats found that dietary restriction via intermittent fasting caused adverse effects on fertility and the suppression of reproductive capabilities in both sexes .
- Another investigation on male and female rodents noted a negative effect on reproduction from intermittent fasting, indicated by significant changes in body weight, blood glucose, estrous cyclicity and serum estradiol, testosterone and LH .
As for human studies, a few highlights include:
- A study on a small group of menstruating females noted that after a 72-hour fast, the women had decreased thyronine levels, increased cortisol secretion, and amplified nocturnal secretion of melatonin (resulting in a circadian phase shift of 81 minutes) .
- One final study highlights the gender discrepancy in glucose response as a result of fasting. After three weeks of ADF, the women had a slightly impaired glucose response to a meal while men showed no change in response .
OK, It's Not ALL Negative...
While most of those results seem negative, keep in mind that these are experimental studies, mostly done on animals. They don't paint a full picture, but they should make us stop and think about the gender differences with fasting.
On the contrary, it should be noted that other studies on fasting as a therapy for cancer and neurodegenerative diseases show promising results for both males AND females .
In his article on fasting and women, Mark Sisson sums up this dichotomy quite nicely:
“As it stands right now, I’d be inclined to agree that pre-menopausal (and perhaps peri-menopausal) women are more likely to have poor—or at least different—experiences with intermittent fasting (at least as a weight loss tool). That said, it appears to be a potentially gender-neutral therapeutic tool for chemotherapy, cancer, and age-related neurodegeneration patients.”
While the science is limited, from these studies we can at least start to make an argument that women have a different (sometimes more negative) response to fasting than men.
So... why is that?
Why Fasting May Affect Women Differently
The Ancestral Explanation
The idea that fasting—or any shift in calorie intake—affects women differently makes a little more sense from an evolutionary perspective. One of the primary roles of females in evolution has been to grow and nourish the next generation. Even if you’ve made the decision not to have children (which we fully respect!) this is a pretty incredible thing women can do.
Like it or not, this evolutionary role comes with some baggage. Procreation is a very 'expensive' process.
Because of the energy and nutrients required to conceive, grow, and nurture a child, female bodies are naturally much more sensitive to external stressors.
For men, on the other hand, procreation is... ahem... not nearly as energy-demanding.
When the female body senses too much stress, whether that's from calorie restriction, infrequent meals, not enough sleep, too much exercise, or something else, reproduction is the first process that gets shut down. The last thing your body wants to do when it's stressed out is give all your precious energy to a growing baby.
But if you're not satisfied with the ancestral explanation, here's a slightly more scientific (complicated) answer of why fasting affects women differently.
The Scientific Explanation
While the exact mechanism is a bit unclear, women seem to have a more severe response to calorie restriction or fasting due to negative impacts on several hormones including gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and estradiol—all of which are required to be in balance for proper reproductive function .
One explanation is outlined in a study titled Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats :
"These findings support the current data suggesting that increase in estradiol level in IF-DR (Intermittent Fasting-Dietary Restriction) female rats may lead to down regulation of GnRH and LH level via negative feedback. The reduced gonadotropin secretion may be due to the inhibitory effect of estrogen at the pituitary level and androgens at the hypothalamic level."
In other words, by disrupting upstream hormone precursors and signaling molecules, fasting may down-regulate reproductive hormones in females (rats, to be fair).
When these reproductive hormones are disturbed, a whole host of issues can occur depending on that woman's sensitivity to hormonal imbalances, the most common being:
- Menstrual cycle irregularities
- Infertility or trouble getting pregnant
- Complete loss of menstruation (amenorrhea) 
- Hypothyroidism, or thyroid-related symptoms
- General hormone imbalance symptoms (PMS, acne, poor mood, hair loss, weight issues, low libido, etc.)
And listen, infertility isn't just a problem for women who want children. Even if you never plan to get pregnant, the health of your reproductive system is still critically important to your overall well-being as a female.
Your sex hormones are vital for strong bones, protection against cancer, mood, energy and vitality, sex drive, skin and hair health, and so much more .
These hormones are—in essence—what make you a healthy, vibrant, bad-ass woman. Babies or no babies.
How to Know If Fasting Is Right for You
So, does all of this mean women should never fast? Not necessarily...
At Kion, we believe in bio-individuality: that each person's health needs are unique. The perfect diet, fitness plan, or fasting practice simply doesn't exist because we all have different genetics, activity levels, health conditions, and more.
Whether or not it's a good idea for a specific woman to fast depends on a number of factors.
How old are you? Why are you fasting? What are your goals? How active are you? Do you have a history of hormone issues or imbalances? Are you planning for pregnancy soon?
While it's impossible to address every scenario, what we can do is provide general guidelines for women who should probably think twice about their fasting practice.
There are certain populations already on the fine line of hormonal imbalances, and caloric shifts via fasting may tip them over the edge into full-blown hormone dysfunction.
Fasting may not be ideal for women who:
- Are athletes, or very active
- Are extremely lean (<18% body fat)
- Have sleep disorders or poor sleep
- Have low reproductive hormone levels
- Are pre-menopausal (typically under age 40)
- Have subclinical hypothyroidism (low thyroid hormone)
- Struggle with infertility, irregular periods, or amenorrhea (unless associated with PCOS or obesity, in which case fasting may benefit)
The following groups of women should definitely avoid regular extended fasting (more than 12-14 hours), and should instead focus on getting in a surplus of calories and nutrients:
- Young women and teens
- Women planning for pregnancy
- Pregnant or breastfeeding women
- Females with a history of eating disorders
Additionally, if you currently practice any form of regular fasting and have experienced any of the following, you may want to consider a different approach (check out Part 2 of this series for tips):
- Poor sleep
- Low libido
- Thinning hair
- Acne or dry skin
- Always feeling cold
- Irregular heartbeats
- Slow recovery or healing
- Negative changes to mood
- Decreased athletic performance
- Binge eating or strong food cravings
- Inability to lose weight or unexplained weight gain
- Irregular menstrual cycles or loss of period (amenorrhea)
Ki Points on Fasting for Women
But we're also not blind to the fact that female-specific studies (or lack thereof) are often left out of the discussion.
Yes, there's still research to do. Until then, if you choose to practice fasting as a female—especially a young woman of reproductive age—pay attention. Listen to your body.
If it's telling you something, don't ignore it for the sake of self-discipline, or 'autophagy', or abs. You'll likely do more harm than good.
On the other hand, if you find fasting to be a beneficial practice in your life and only experience positive results, then continue on your journey! You know your body best.
Martin, Bronwen et al. “Sex-dependent metabolic, neuroendocrine, and cognitive responses to dietary energy restriction and excess.” Endocrinology vol. 148,9 (2007): 4318-33. doi:10.1210/en.2007-0161
Kumar, Sushil, and Gurcharan Kaur. “Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis.” PloS one vol. 8,1 (2013): e52416. doi:10.1371/journal.pone.0052416
Fernandez-Fernandez R, Martini AC, Navarro VM, Castellano JM, Dieguez C, et al.. (2006) Novel signals for the integration of energy balance and reproduction. Mol Cell Endocrinol 254–255.
Berga, S. “Endocrine and Chronobiological Effects of Fasting in Women.” Fertility and Sterility, vol. 75, no. 5, 2001, pp. 926–932., doi:10.1016/s0015-0282(01)01686-7.
Heilbronn, Leonie K et al. “Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting.” Obesity research vol. 13,3 (2005): 574-81. doi:10.1038/oby.2005.61
Safdie, Fernando M et al. “Fasting and cancer treatment in humans: A case series report.” Aging vol. 1,12 988-1007. 31 Dec. 2009, doi:10.18632/aging.100114
Alirezaei, Mehrdad, et al. “Disruption of Neuronal Autophagy by Infected Microglia Results in Neurodegeneration.” PLoS ONE, vol. 3, no. 8, 2008, doi:10.1371/journal.pone.0002906.
Marques P, Skorupskaite K, George JT, et al. Physiology of GNRH and Gonadotropin Secretion. [Updated 2018 Jun 19]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279070/
Meczekalski, B et al. “Functional hypothalamic amenorrhea and its influence on women's health.” Journal of endocrinological investigation vol. 37,11 (2014): 1049-56. doi:10.1007/s40618-014-0169-3
Baker, E, and L Demers. “Menstrual Status in Female Athletes: Correlation with Reproductive Hormones and Bone Density.” International Journal of Gynecology & Obstetrics, vol. 28, no. 4, 1989, pp. 391–391., doi:10.1016/0020-7292(89)90624-3.
Chuffa, Luiz Gustavo De Almeida, et al. “The Role of Sex Hormones and Steroid Receptors on Female Reproductive Cancers.” Steroids, vol. 118, 2017, pp. 93–108., doi:10.1016/j.steroids.2016.12.011.
Steiner, Meir et al. “Hormones and mood: from menarche to menopause and beyond.” Journal of affective disorders vol. 74,1 (2003): 67-83. doi:10.1016/s0165-0327(02)00432-9
Charkoudian, Nisha, et al. “Influence of Female Reproductive Hormones on Local Thermal Control of Skin Blood Flow.” Journal of Applied Physiology, vol. 87, no. 5, 1999, pp. 1719–1723., doi:10.1152/jappl.19220.127.116.119.