What You Need to Know about Intermittent Fasting for Women

Intermittent fasting has been around in various forms for ages, but it really hit popularity status around 2012 with the documentary Eat Fast, Live Longer, and the book The Fast Diet. Fans claim it can help you lose weight, burn fat, reduce the risk of diabetes and lower blood pressure.

Historically, people have mostly fasted for religious reasons, but many have jumped on the intermittent fasting bandwagon for these proposed health benefits. But are the benefits real? And are they safe?

There is some science to back the claims, but intermittent fasting can also negatively impact women, due to the effect of caloric restriction on female hormones, fertility and bone health. The restrictive nature of the diet can also spiral into disordered eating.

Here we take a closer look at the science behind intermittent fasting, including what it is, what the benefits are, what any potential health consequences are and how to get started.

What Is Intermittent Fasting?

Intermittent fasting (IF) is a pattern of eating which alternates periods of fasting with periods of eating. There are three main methods: time-restricted, modified and alternate day fasting. A 2022 study in JAMA Internal Medicine explains that time-restricted IF includes only eating in a certain time window, such as the 16:8, where you eat during an 8-hour window (usually 12-8 p.m.) and then fast for the other 16 hours of the day. Some people may adjust and fast for 12 hours, then eat during a 12 hour window.

The 5:2 method referred to as modified or periodic fasting, per a 2021 review in Experimental Gerontology, involves restricting calories to 20-25% of energy needs on two nonconsecutive days per week (this could be as low as 500 calories per day) with no restrictions on calories or timing the other five days of the week.

Complete alternate-day fasting, as described in a 2020 review in Frontiers in Nutrition, involves alternating fasting days (not eating at all) with non-fasting days (eating anything you want).

Pros of Intermittent Fasting for Women

"There is substantial research to support the therapeutic benefits of fasting," says Jillian Greaves M.P.H., RD, LDN, an integrative functional dietitian. "Some potential health benefits include improved cellular health, improved metabolic markers and weight loss."

Studies show that intermittent fasting leads to weight loss in women, but doesn't lead to any more weight loss than a calorie deficit overall, per a 2022 study in Nature Reviews Endocrinology. However, the structure of IF might make it easier for some to reduce caloric intake.

Intermittent fasting might boost fat burn too, although the same 2022 study in JAMA Internal Medicine states that more research needs to be done on this topic. When we eat, blood sugar (glucose) rises and insulin is released to take glucose to our cells for energy. Extra glucose is stored as fat. If you don't eat for 10-16 hours, your body will start to use its fat stores for energy, per a 2022 review in Nutrients.

A 2019 review in the New England Journal of Medicine found that in a fasted state, cellular repair occurs, which is linked to increased longevity, reduced risk of cancer, lower inflammation and improved metabolism. However, many of the studies were on animals and more studies on women are needed.

There's also emerging research, like the 2018 review in Current Obesity Reports, that eating in sync with circadian rhythm keeps chronic diseases at bay. In other words, they're suggesting eating during a 6-10 hour window during the day, when it's light outside, and reducing nighttime eating.

Cons of Intermittent Fasting for Women

"Despite the benefits found in research, it's important to consider the context and remember that it's not appropriate for all people at all times," warns Greaves. "Women of reproductive age need to be particularly careful with intermittent fasting as their bodies are more sensitive to stressors like prolonged fasting and caloric restriction."

Greaves goes on to explain, "Intermittent fasting itself is a stressor on the body, and in the context of our modern-day life that's already filled with chronic emotional, physiological and environmental stressors IF might do more harm than good. Fasting increases cortisol which can lead to blood sugar dysregulation, increased insulin resistance, lean muscle loss, fatigue and disruptions to thyroid function over time. In the short-term, fasting may lower thyroid stimulating hormone, but elevated cortisol on a persistent basis can reduce the conversion of thyroid hormone."

"Fasting can also lead to undereating, which we know negatively influences female hormones in a variety of ways," adds Greaves. The caloric restriction caused by intermittent fasting could lead to loss of the menstrual cycle and interfere with fertility.

Fasting can also increase hunger and obsession with food, leading to overeating or a cycle of restricting and bingeing, per a 2022 review in Eating Behaviors. This same study states that in addition to disordered eating, intermittent fasting can also negatively impact mental health, especially in younger people. This is particularly detrimental for women with eating disorders or chronic dieters who have a history of restricting foods or disordered eating.

When the body goes for prolonged periods without food, hunger hormones are released that increase appetite. "I often see women applying IF incorrectly by skipping breakfast and eating late into the evening which we know can disrupt circadian rhythms and contribute to hormone imbalance," says Greaves. "I also see women practicing IF and completely ignoring their body's biological hunger cues altogether which is not health-promoting, physically or mentally."

How to Safely Intermittently Fast

It's important to note that IF is not beneficial—or safe—for everyone. Greaves says she does not recommend IF to women who aren't sleeping enough, don't eat enough or don't eat consistently, have irregular or absent cycles, experience thyroid issues, have a history of current or past disordered eating, are under a lot of stress or have blood sugar issues.

"I'm an advocate for eating consistently which is a really supportive way to reduce stress on the body and balance blood sugar," says Greaves.

If you have the go-ahead from your healthcare practitioner or dietitian, Greaves recommends starting slow, pointing out studies, like the 2020 report in Cell Metabolism, that have shown that fasting for just 12 to 14 hours overnight can yield metabolic benefits. "So it's important to remember that you don't necessarily have to fast for 16 or 18 hours to experience the benefits," emphasizes Greaves.

We recommend starting with the time-restricted method versus the 5:2, which restricts calories two days per week, and sets people up to overeat the other days.

First, count how many hours you currently go from when you stop eating at night to when you start eating the next day. Consider extending your fast by one hour to start, then increase by two hours, etc. Time-restricted IF has no calorie restrictions. We recommend eating three balanced meals with protein, high-fiber carbohydrates and healthy fats, spaced evenly throughout the eating window.

Those who have never been big breakfast eaters may find it easy to not eat until 10 or 11 am, while others are hungry when they wake up. The most important thing is to listen to your body and if you're hungry, eat. Women who work out regularly may find it difficult to stick to intermittent fasting. And if it's that time of the month and you're ravenous, it will also be hard to fast.

You may be wondering: Does coffee break the fast? If there's anything in it, then technically, yes. Black coffee has zero calories. But consider your goals—are you doing this for weight loss? If so, remember that IF doesn't lead to any more weight loss than a calorie deficit overall, so you are probably fine to add a little creamer to your coffee. Are you doing it for blood sugar control? If so, then spiking your sugar with a flavored latte is not the best way to start the day.

The Bottom Line

"Fasting is a tool, and should never be treated like a rigid diet," says Greaves. For women specifically, there's a lot to consider before starting intermittent fasting. IF is not recommended for women with diabetes, eating disorders or women who are pregnant or breastfeeding. Start slowly if you do try and pay close attention to your hunger and satiety.

If you find yourself starving throughout the day or week, it may be best to resume a more regular eating pattern. And definitely keep tabs on symptoms such as fatigue, mood swings, hunger, reduced energy, lack of concentration and loss of menstrual cycle. You should feel fueled, energized and satisfied throughout the day, not groggy and hungry.