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

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.

“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 fasting, especially if they are already at a healthy weight / BMI. It’s unlikely that a few days of 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, in essence ‘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.”

Dr. Woitowich was trained as a biochemist, with a focus on neuroendocrinology. In other words, she has studied how the brain communicates with the ovaries and testes.

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.

“Things like stress, or your emotional and behavioral state, can play a role in your menstrual cycles and reproductive health,” Dr. Woitowich said. “Also, we don’t often think about it but another external signal that can affect your reproductive cycle is the circadian light-dark cycle. 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.”

“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 cortiscosterone, 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.” – Emerging insights into hypothalamic‐pituitary‐gonadal axis regulation and interaction with stress signalling, 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 – 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.

How much can I fast without affecting my period?

It would be difficult to develop any universal guidelines with regards to how many days of 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.

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.

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 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, women who work nights are more likely to report menstrual irregularity and longer cycles.

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 tumor cell proliferation. 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.”

“Alterations or disruptions of biological rhythms, as commonly occurs in shift work, jet lag, sleep deprivation […] are linked to significant disruptions in reproductive function. These impairments include altered hormonal secretion patterns, reduced conception rates, increased miscarriage rates and an increased risk of breast cancer. Female health may be particularly susceptible to the impact of desynchronizing work schedules as perturbed hormonal rhythms can further influence the expression patterns of clock genes. Estrogen modifies clock gene expression in the uterus, ovaries, and suprachiasmatic nucleus [in the brain], the site of the primary circadian clock mechanism.” – Shift Work, Jet Lag, and Female Reproduction, 2010

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.

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

Researchers are still trying to determine whether ketogenetic diets are beneficial in clinical settings – we know little yet about the impact of these diets, if any, on women’s reproductive cycles. However, we also know that 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 you are fasting each month, particularly if you are trying or planning to become pregnant. An open question is how 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.