Why Intermittent Fasting Can Be Dangerous for Women, and How to Get It Right

Intermittent fasting is officially more than a diet trend. The eating regimen has exploded in popularity, largely because of what it doesn't require: counting calories, tracking macros, labeling foods as "good" or "bad."

This type of fasting — which restricts eating to certain hours of the day or certain days of the week — has also received praise from the scientific community because of budding research on its benefits, including effective weight loss and lowered disease risk, per Harvard Health Publishing.

But when it comes to intermittent fasting for women, there are also some risks to consider.

Indeed, some groups of women should avoid practicing IF or at least consult a health professional first. And all women may benefit from easing into a fasting pattern and following a protocol that reduces the risk of negative side effects.

Here, we'll lay out everything you need to know about intermittent fasting for women.

Most women can safely partake in intermittent fasting, Pamela Nisevich Bede, RD, dietitian and author of ‌Sweat. Eat. Repeat.,‌ tells LIVESTRONG.com.

"Women find weight-loss success on intermittent fasting for a plethora of reasons, [including] a forced reduced-calorie intake, a low-energy state that prompts ketosis and a thoughtful connection with food in which individuals become more mindful of intake," Bede says.

IF also opens the door to a wealth of potential health benefits for women specifically, according to a review published in the April-June 2016 issue of the ‌Journal of Mid-Life Health‌, which connected periodic fasting to:

• Decreased biomarkers associated with certain cancers;
• Improved reproductive health in women who were overweight before fasting, by way of decreasing their weight; and
• Protective effects on heart and metabolic health.

However, it's important to keep in mind that the research looking at intermittent fasting for women specifically is extremely limited, Amy Rothberg, MD, an endocrinologist and associate professor at Michigan Medicine, tells LIVESTRONG.com. Most of the currently available information on IF and female physiology comes from rodent studies, she says.

"There is better evidence from studies of time-restricted feeding, [which means] limiting the window of eating during the day, which lengthens the period of fasting at night" Dr. Rothberg says, "and also studies that align eating with our circadian clock."

One such study, published March 2015 in ‌Cell‌, notes that time-specific food intake has profound effects on our physiology, and especially our hormones and metabolism. When we disrupt the natural cycle of eating during daylight hours, the study author notes, that can throw our other bodily processes out of whack, increasing our risk of disease.

‌Read more:‌ 12 Dos and Don'ts of Intermittent Fasting

So far, Dr. Rothberg notes, studies have primarily focused on how IF affects body weight, metabolism (meaning markers like insulin and blood lipids) and mortality. Limited evidence suggests some hormonal changes, but such changes are almost always associated with overall caloric restriction, so they don't seem to be unique to IF.

However, young women should be aware that, "When food supply is insufficient to meet metabolic demands, puberty is delayed and ovulation and the normal menstrual cycle is suppressed," Dr. Rothberg says.

In other words, women run the risk of developing amenorrhea (loss of their menstrual period for at least three consecutive months) if calorie intake becomes too low, according to a June 2017 review in ‌Seminars in Reproductive Medicine‌. That's because a lack of calories can cause changes to the hormones estrogen and progesterone, which regulate the menstrual cycle. It can also potentially cause a loss of bone density and do potential harm to a developing fetus if a woman was to get pregnant, Dr. Rothberg says.

If your calorie intake remains adequate, though, intermittent fasting alone should not put you at risk for amenorrhea or these other conditions — it's all about overall calorie and nutrient intake.

Other non-hormonal changes that women may experience due to fasting or calorie restriction include cramping, fatigue, headaches, fainting spells and — if severe — nausea and vomiting, says Dr. Rothberg. These symptoms typically occur due to shifts in electrolyte balance.

"The studies that exist on intermittent fasting suggest that both sexes respond to fasting and have similar rates of weight loss. That being said, both men and women succeed and struggle with it."

"There is not much research currently to show how intermittent fasting affects men and women differently," Natalie Allen, RD, dietitian and instructor of biomedical sciences at Missouri State University, tells LIVESTRONG.com. "As a relatively new diet trend, we are still gathering credible information on intermittent fasting."

Bede argues that the difference isn't sex-specific, but genome-specific. That is, everyone responds differently to fasting regardless of sex because our genes differ.

"The studies that exist on intermittent fasting suggest that both sexes respond to fasting and both have similar rates of weight loss," says Bede. "Anecdotally, I know many more women who experiment with intermittent fasting than men. That being said, both men and women succeed and struggle with it."

Across both sexes, it's important to note that fasting is not necessarily easy, says Bede. "We humans are hardwired to seek food for survival, and these innate signals are put in place to drive us to refuel."

So if you experiment with intermittent fasting and continuously feel poorly, seek advice from your health care provider and reconsider your approach — intermittent fasting is certainly not the only way to achieve the health or wellness benefits you seek, says Bede.

‌Read more:‌ 6 Things to Know About Fasting Before You Try It, According to Nutritionists

If you're interested in intermittent fasting, the best way to start is to ease into it, Dr. Rothberg says. " I would suggest that most ease into intermittent fasting gradually to make sure that it's something they think they can adopt and to which they can also adapt," she says.

So, if you'd like to try fasting for 16 hours each day, for example, start by fasting for 10 hours, then — assuming you feel well — slowly bumping up your fasting time every few days until you hit 16.

If you try IF and notice any of the following effects, stop and talk to a doctor or dietitian:

• Sudden irregularity in your period or complete loss of your period
• Rapid weight loss despite your total calorie intake remaining similar to before you started intermittent fasting
• Extreme mental or muscular fatigue that persists more than a couple of weeks into intermittent fasting (feelings of lethargy are normal at first, Bede notes, but should subside as your body adjusts)
• Menopause-like symptoms if you aren't of menopausal age (hot flashes, sudden difficulty sleeping, night sweats, etc.)

There simply isn't enough data looking at the effects of intermittent fasting on women specifically, Dr. Rothberg notes, so women need to look out for symptoms that indicate that fasting — or their overall calorie or nutrient intake — is messing with their physiology, particularly their hormones. The scenarios above are rare-but-serious effects that can occur when your body doesn't adjust to a fasting protocol.

‌Read more:‌ 7 Strategies for Smarter Intermittent Fasting

Despite the potential benefits of intermittent fasting, some women should steer clear of this eating pattern. For some, IF can cause more harm than good. According to Bede and Allen, those groups include:

• Women with a ‌history of eating disorders‌ or disordered eating, because intermittent fasting can perpetuate dysfunctional eating habits, such as a binge/purge cycle, restrictive eating and emotionally charging food.
• Women who are ‌pregnant or breastfeeding‌ should not try intermittent fasting without first discussing with a doctor, because there is not enough data to support the idea that fasting while pregnant is healthy for mom or baby. Very low-calorie diets, lack of balance of both macro and micronutrients and fasting can result in abnormal fetal growth and lifelong consequences for the offspring, including a higher risk of having overweight or obesity, says Dr. Rothberg.
• If you are ‌trying to conceive‌, talk to your doctor first to make sure that intermittent fasting won't affect your likelihood of getting pregnant.
• ‌Female athletes‌ who perform high volumes of training or are preparing for competition shouldn't start intermittent fasting without talking with a sports dietitian, coach, trainer or doctor, as they may be at risk for amenorrhea or female athlete triad disorder.
• Women with ‌diabetes‌ shouldn't try IF without clearance from a doctor because there isn't enough data available to support fasting with diabetes.
• Women with certain ‌autoimmune conditions or genetic disorders‌ should consult with a doctor or dietitian before trying intermittent fasting to ensure that their condition is not a contraindication for fasting.
• Women who are otherwise already ‌at risk for nutrient deficiencies‌ shouldn't attempt intermittent fasting without working with a registered dietitian or physician who can ensure adequate nutrient intake.

‌Read more:‌ When Does Intermittent Fasting Become Disordered Eating?

There are many different versions of IF, which range from fasting for 12 hours each day to completing a 24-hour fast once or twice per week. There's also the Warrior Diet, which calls for 20 hours of daily fasting, and alternate-day fasting, during which you restrict calories to 500 per day every other day, then eat normally the other days.

"An individual needs to think about her routine, eating patterns, workout plans and weight-loss goals as she chooses which plan to follow," Allen says.

Bede recommends the 16:8 protocol for women, especially for those who are new to intermittent fasting, because it's the most researched of the IF protocols. The 16:8 protocol involves eating all of your food during an eight-hour window (10 a.m. to 6 p.m., for example) and fasting for the other 16 hours. It's often considered the easiest IF protocol because most people are sleeping for seven to nine of their fasting hours.

"It also appears to be the most reasonable and achievable" says Bede. "This approach is as simple as shutting down the kitchen after an early dinner, which prevents mindless snacking and grazing while watching late-night TV and also forces one to recognize hunger signals before diving into lunch."

Additionally, the 16:8 protocol allows for fasted training or "training low" for those who exercise early in the morning, says Bede. Exercising at the tail end of a fast allows your body to work off of its fat stores and burn additional calories. Bede cautions, however, that you should utilize fasted training only occasionally and never before a competition or race, as you'd put yourself at risk for low glycogen and low blood sugar. Low glycogen means your muscles are running low on energy stores, which can affect your physical performance, while low blood sugar (hypoglycemia) can result in dizziness, shakiness, rapid heartbeat and fainting.

Dr. Rothberg notes that there's not really enough evidence to promote one intermittent fasting protocol over another, but based on the info available, the 16:8 protocol makes sense. "I think the best evidence, again limited, is for time-restricted feeding," she says. "That is, eating during the day for eight hours and fasting in the evening and overnight for 16 hours, and eating the larger meals of the day in the beginning of the day."

‌Read more:‌ What Really Happens to Your Body When You Fast?