Intermittent fasting boasts many benefits, but it could do more harm than good for women. We review the pros and cons of intermittent fasting for women, plus tell you how to get started.

Intermittent fasting is one of the lastest trendy diets. Fans claim it can help you lose weight, burn fat, reduce risk of diabetes and lower blood pressure. With all those supposed benefits, people—and particularly women—may be wondering if they should give intermittent fasting a try. Historically, people have mostly fasted for religious reasons, but recently many have jumped on the intermittent fasting bandwagon for these proposed health benefits. There is some science to back these, but intermittent fasting can also negatively impact women, due to the effect of caloric restriction on female hormones, fertility and bone health (learn more about intermittent fasting and weight loss). 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. 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 pm) 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, referred to as modified fasting, 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 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., R.D., L.D.N. Integrative Functional Dietitian at Prevention Pantry Nutrition. "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. However, the structure of IF makes it easier for some to reduce caloric intake.

Intermittent fasting can boost fat burn too. When we eat, blood sugar (glucose) rises and insulin is released to take glucose to our cells for energy. Extra glucose is stored as fast. If you don't eat for 10-16 hours, your body will start to use its fat stores for energy.

A study 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 are in animals and more studies in women are needed.

There's also emerging research that eating in sync with circadian rhythm keeps chronic diseases at bay. In other words, 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," says 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 explains, "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." (Here are 6 foods to ditch to stress less—and what to eat instead.)

"Fasting can also lead to undereating, which we know negatively influences female hormones in a variety of ways," Greaves says. The caloric restriction caused by intermittent fasting could lead to loss of menstrual cycle and interfere with fertility (learn more about how what we eat impacts our hormones).

Fasting can increase hunger and obsession with food, leading to overeating or a cycle of restricting and bingeing. 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 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 Get Started

For starters, 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." Studies are lacking on long-term adherence to intermittent fasting. (Learn more about intermittent fasting and if it's right for diabetes.)

If you have the go ahead from your doctor or dietitian, start slow. "Some studies 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 (get healthy meal and snack ideas from our meal plans).

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 workout regularly may find it difficult to stick to intermittent fasting. If it's that time of the month and you're ravenous, it will also be hard to fast.

We know what you're thinking: 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 caramel latte, is not the best way to start the day.

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.

Lainey Younkin, MS, RD, LDN is a Boston-based weight loss dietitian who helps women ditch diets and change habits for a healthy lifestyle that lasts. She helps frustrated women, who feel like they eat healthy and workout but still can't lose weight, work smarter not harder to lose weight and keep it off. Follow along on Instagram at @weight.loss.dietitian.