Your Menstrual Cycle on Intermittent Fasting

As intermittent fasting and the ketogenic diet become more popular, women who fast regularly might have questions about how intermittent fasting could impact their reproductive cycle, hormones and reproductive health. While there is limited research on how intermittent fasting or ketogenic diets may impact women’s reproductive cycles, we can take some clues from research involving other metabolic and lifestyle traits and behaviors such as weight, exercise and caloric restriction.

Jump to see our FAQ on fasting and your period 

Extreme calorie restriction could lead to missed periods

Whether it’s not considered worthy of research or it’s considered “old news”, there is a dearth of clinical research on how ketogenic diets and intermittent fasting may impact women’s reproductive cycles and reproductive health, says Dr. Nicole Woitowich, Associate Director for the Women’s Health Research Institute at Northwestern University. However, we do know that caloric intake, exercise and weight can affect the reproductive hormone cycle. For example, extreme forms of caloric restriction, weight loss, exercise and nutritional deficiencies can all cause amenorrhea, or irregular or skipped periods.

Caloric restriction, for example, is a “stressor” that is processed in the brain and can modify the release of sex hormones through what is called the hypothalamic-pituitary-gonadal (HPG) axis. Reproduction and fertility are regulated via hormones of the HPG axis.

“It’s unlikely that a few days of intermittent fasting each month will throw off your menstrual cycle”

“With regards to intermittent fasting, it could be possible that individuals who aren’t getting enough caloric intake to support the hypothalamic–pituitary–gonadal axis could experience irregular menses,” Dr. Woitowich said. Simplified, this means that if a woman does not have enough nutritional or metabolic energy to support carrying a pregnancy, the body will signal the brain to shut off the reproductive cycle. For this reason, women who are trying to become pregnant may be wary of fasting for longer than 24 hours at a time or meaningfully restricting their caloric intake through intermittent fasting, especially if they are already at a healthy weight / BMI. It’s unlikely that a few days of intermittent fasting each month will throw off your menstrual cycle, Dr. Woitowich says, although substantial caloric restriction might.

Negative energy balance or caloric restriction in very young women might also have an effect of delaying puberty through effects on the HPG axis and neurohormones.

“We know that weight loss, and even excessive exercise, can cause irregular or absent menstrual periods,” Dr. Woitowich said. “Even if you aren’t thinking about getting pregnant at this point in your life – your body is. Our reproductive systems are capable of supporting a pregnancy from the time we experience our first period until we reach menopause. Women need to have a certain amount of energy and nutrients, obtained from food and stored as fat, in order to support a health pregnancy.

Our bodies have the ability to recognize when these energy reserves are low and can ‘turn off’ our reproductive cycles so that pregnancy does not occur. The mechanism by which this happens is actually quite complex and requires a delicate balance of signals to be shared between the brain, pituitary gland and ovaries. Typically, once a woman regains weight and/or resumes a nutrient-rich diet, normal menstrual cycles will return.”

How the HPG axis works

Reproductive health and menstrual cycles are governed by a range of both external and internal inputs into the brain. These inputs can include things like your energy state, or your nutritional and caloric intake and expenditure, your stress levels and even external inputs into your circadian rhythm. These various inputs work through the HPG axis by affecting the activity of gonadotropin-releasing hormone (GnRH). This hormone is responsible for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) into the bloodstream from the anterior pituitary gland in the brain. FSH and LH, once released, travel to the ovaries to promote the release of ovarian follicles (which contain egg cells) and the production of estrogen, progesterone and testosterone. GnRH is itself considered a neurohormone and is released from special GnRH neurons in the hypothalamus.

Your hormones, stress levels and circadian rhythm affect your menstrual cycle

“Things like stress, or your emotional and behavioral state, can play a role in your menstrual cycles and reproductive health,” Dr. Woitowich said.

“Because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signaling, including corticosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. […] Stress, including psychological stress, can negatively impact reproduction in many mammalian species, including humans. In females, stress can disrupt ovarian cyclicity, as well as upstream gonadotrophin synthesis and secretion.” – Rodriguez-Acevedo, et al. 2018

Of interest, healthy levels of moderate exercise, sleep, mindfulness and time spent outdoors can reduce physiological stress and chronic levels of cortisol, a stress hormone that can affect the reproductive cycle. Fasting is complicated when it comes to stress levels – intermittent fasting is often considered to be a hormetic or “good” stress, like exercise, meaning that it may temporarily raise levels of stress and cortisol but ultimately have a positive, anti-inflammatory effect if practiced regularly.

Your menstrual cycle and the circadian rhythm

“Another external signal that can affect your reproductive cycle is the circadian light-dark cycle,”says Dr. Woitowich. While most of us live in an area where we get approximately 12 hours of sunlight, twelve hours of darkness, people who live at the poles and experience extremely long days or extremely long nights tend to have more irregular menstrual cycles, for example.”

Other factors that disrupt your circadian rhythm, such as eating late at night, can elevate levels of estrogen, particularly in women with polycystic ovarian syndrome (PCOS). Too much estrogen is responsible for the loss of ovulation in women with PCOS. Therefore, eating most of your calories earlier in the day may be best to prevent elevation of estrogens if you suffer from PCOS. Women with PCOS show improvements in menstrual cycle irregularity when they practice time-restricted eating and limit their calorie intake to daylight hours.

FAQs on Fasting and your period

How much can I fast without affecting my period?

Extreme caloric restriction, through extended fasting (36+ hours) for example, could lead to missed periods. However, It’s unlikely that fasts of shorter durations would affect your menstrual cycles. However, it would be difficult to develop any universal guidelines with regards to how many days of intermittent fasting per month, for example, would be safe for a woman trying to become pregnant or to prevent any menstrual cycle changes. For one, there’s a lot of inter-person variability in menstrual cycles. While moderate time-restricted feeding (around 12-14 hours per day or less) or occasional fasting periods under 24 hours are probably safe, diet quality, caloric intake and BMI are likely to determine the impacts of intermittent fasting and ketogenic diets on reproductive health.

“If your intermittent fasting practice leads to nutrient deficiencies or causes prolonged low-blood sugar or hypoglycemia, it will likely impact the hypothalamic–pituitary–gonadal axis and disrupt the production of reproductive hormones,” Dr. Woitowich said.

There is some research in animal models (young rats) suggesting that dietary restriction via intermittent fasting can negatively impact the hypothalamus-hypophysial-gonadal axis and thus reproduction. In one study, rats fasted every other day, representing up to a 40% reduction in caloric intake (a day of fasting for a rat is more like a week of fasting for a human!), experienced significant changes in body weight, blood glucose, estrous cyclicity and serum estradiol, testosterone, luteinizing hormone level and GnRH expression.

While these findings are indeed concerning, it should be noted that the female rats were very young (3 months old), which corresponds to a human aged 9 years old. Intermittent fasting is not recommended for children under the age of 12, since it has the potential to negatively impact puberty and growth. Thus, data from the Kumar et al. study, though valuable to the field, do not provide evidence for how fasting may impact reproductive hormone levels in adult females. Cienfuegos, et al. 2022

How does Ramadan fasting impact menstrual cycles?

As we described above, factors known to affect menstrual cycles include weight loss, extreme exercise and psychological stressors. But some research studies have also identified abnormal eating patterns, abnormally low levels of leptin (related to energy deficiency) and Ramadan fasting as factors that can affect menstruation. Ramadan fasting and other forms of intermittent fasting are also contraindicated for pregnant women, as studies have revealed changes in reproductive hormones and poor weight gain in women who fast during Ramadan month. Pregnant women who fast during ramadan also show increased levels of the appetite-stimulating hormone Neuropeptide Y.

A 2013 study of 80 female college students at Hamedan University of Medical Sciences found that individuals who fasted for more than 15 days during Ramadan month were more likely to report menstrual abnormalities, including infrequent or missed periods, abnormal bleeding or heavy or prolonged menstruation. These effects lasted for up to three months following Ramadan month. A majority of the participants were normal weight or lean.

However, another study among women with Polycystic Ovary Syndrome showed that Ramadan fasting can have beneficial effects on the levels of stress hormones like cortisol, with limited effects on reproductive hormones such as follicle-stimulating hormone and luteinizing hormone. This highlights the need for more research on how intermittent fasting may impact different women in different ways depending on their weight and other factors. For example, overweight women suffering from inflammatory issues may stand to the benefit the most from intermittent fasting or time-restricted eating, even with it involves a suboptimal eating schedule (e.g. evening eating) such as that of Ramadan fasting.

During Ramadan, individuals abstain from food and drink from dawn to sunset, daily. This eating pattern may affect reproductive hormone cycles both directly but also indirectly through disruption of circadian rhythms and sleeping patterns, especially since abstaining from food and drink during the day runs counter to eating patterns that support healthy circadian rhythms. It is well known that menstrual cycles affect the circadian rhythm through sex hormones like estrogen. For example, diurnal rhythms of melatonin and cortisol change throughout the menstrual cycle. On the flipside, disruption of circadian rhythms is also associated with menstrual cycle disturbance. For example, shift work increases irregularities in the menstrual cycle of female nurses.

Both shift work and abnormal estrogen signaling can also influence the expression of the circadian CLOCK gene, with downstream implications for reproductive health and even breast cancer. Circadian rhythms in cell division protect our body from overzealous cancer cells. Circadian disruption helps cancer cells proliferate by letting them divide “around the clock.”

“Several positive associations between shift work and infertility have been found including increased rates of menstrual irregularities, dysmenorrhea, and increased time to pregnancy. Night work, specifically, has been hypothesized to have poorer outcomes, and 2 studies report increased time to pregnancy in women working permanent night shifts or 3 night shift rotating schedules.” Influence of sleep on fertility in women, 2016

There is still a lot we don’t know about exactly how circadian rhythm disruption affects menstruation and reproductive health, although it like has to do with changes in hormone secretion. The ovary appears to have its own circadian clock; when this clock is out of tune with circadian rhythms elsewhere in the body, reproductive issues can arise. But what does intermittent fasting have to do with this? IF, particularly time-restricted feeding, may help us maintain more robust circadian rhythms through nutrient signaling, if nutrient intake is time with normal phases of activity (e.g. during the day). On the other hand, being otherwise healthy you might not want to fast during the day and eat only late in the evenings, as this may contribute to disrupted circadian rhythms that can impact your hormone levels.

More reading:

• Environmental control of biological rhythms: effects on development, fertility and metabolism, Journal of neuroendocrinology, 2014
• Interesting read: Melatonin and stable circadian rhythms optimize maternal, placental and fetal physiology, Human reproduction update, 2013

What do we know about how ketogenic or other diets might affect our periods and reproductive health?

Obesity negatively impacts reproductive health and success rates in achieving pregnancy. From this perspective, a ketogenic diet as a weight-loss intervention prior to pregnancy is likely to benefit the reproductive health of overweight and obese individuals.

“There’s no doubt that our weight has an impact on our overall health, so it should be no surprise that this extends to reproductive health,” Dr. Woitowich said. “Women who weigh too much or too little may have irregular menstrual cycles or struggle with infertility. Interestingly, there have been a few research studies that have examined how ketogenic diets may improve the reproductive health of women with Polycystic Ovary Syndrome (PCOS), but more research is needed to see how ketogenic diets impact women without PCOS. As a general rule, I would encourage all women to speak with their physician before starting any new diet, but especially caution those who are pregnant, or trying to become pregnant, as proper nutrition is essential to maintain and support a healthy pregnancy.”

“There is convincing evidence that reducing carbohydrate load can reduce circulating insulin levels, improve hormonal imbalance and result in a resumption of ovulation to improve pregnancy rates. […] The findings of this review suggest that low carbohydrate diets [less than 45% carbohydrate] may optimize fertility in some clinical groups, particularly for overweight and obese women with PCOS.” – The Effect of Low Carbohydrate Diets on Fertility Hormones and Outcomes in Overweight and Obese Women: A Systematic Review, 2017

Can diet contribute to more cramping during periods / heavier flows?

“If you do a quick internet search related to menstrual cramps and diet, you will find a whole host of information suggesting that various diets, foods or supplements both cause and relieve menstrual cramping!” Dr. Woitowich said. “There is some peer-reviewed data, though preliminary, suggesting that low-fat vegetarian diets and 3-4 servings of dairy products per day may be helpful in reducing menstrual cramps.”

“A low-fat vegetarian diet was associated with increased serum sex-hormone binding globulin concentration and reductions in body weight, dysmenorrhea [painful periods] duration and intensity, and premenstrual symptom duration. The symptom effects might be mediated by dietary influences on estrogen activity.” – Diet and sex-hormone binding globulin, dysmenorrhea, and premenstrual symptoms, 2000

Track It

To be a healthy female intermittent faster, be aware and monitor your reproductive cycle. Some of our LIFE Fasting Tracker app users’ favorite reproductive health and period tracking apps include Clue (period tracker), Glow (a fertility calculator), NaturalCycles (track your cycle), and the Ava cycle tracking wearable device. If you notice anything that’s different than your normal pattern, contact your physician.

Tracking disruptions to your menstrual cycle may be a good way to determine whether you should reduce the number of hours or days in which you practice intermittent fasting each month, particularly if you are trying or planning to become pregnant. An open question is how intermittent fasting may impact the effectiveness of the birth control pill – ask your gynecologist if you have any questions about this, as research is currently limited in this area.