Intermittent fasting is gaining in popularity for everyone from celebrities to your co-workers — and even healthcare professionals.
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“I’m a big fan of fasting, for the right people. It can have some impressive results,” says registered dietitian Julia Zumpano, RD, LD.
But. There’s a catch.
A catch that affects a lot of people.
Intermittent fasting can have a big (and bad) effect on the female sex hormones progesterone and estrogen. So, it’s not necessarily a quick-and-easy fix for women, specifically females of childbearing age and pre-menopausal people assigned female at birth (AFAB). That’s to say, people who menstruate.
“Fasting can affect ovulation. It can affect your menstrual cycle. And even if you’re not trying to get pregnant, those hormones and that cycle still have effects across your body,” Zumpano explains. “Intermittent fasting can still be effective for women, but you want to be thoughtful to do it the right way.”
Zumpano explains how intermittent fasting affects females and how to fast in a healthy way.
Females can get many of the same benefits from intermitting fasting as men and people assigned male at birth (AMAB). That includes things like:
The effects of intermittent fasting for females, though, usually aren’t as dramatic as the results in males. And the reasons are entirely based on hormones.
“Fasting can make estrogen and progesterone take a nose-dive,” Zumpano explains.
It’s important to note that these effects are based on research in cisgender men and women. The effects of intermittent fasting for people who are undergoing feminizing hormonal therapy as part of their gender-affirming care haven’t been studied.
Throughout your menstrual cycle, your estrogen and progesterone levels change. The rise and fall of your hormones is chiefly regulated by gonadotropin-releasing hormone (GnRH).
“GnRH can be very sensitive to environmental factors,” Zumpano states. “Things like fasting can keep it from doing its job and releasing the chemicals needed to stimulate estrogen and progesterone.”
One reason for that may be evolutionary. A rise in certain hormones triggers ovulation, which is the window of your cycle during which you can become pregnant. Fasting, the theory goes, could make your body behave as if food is scarce and that you’re at risk for starvation. Not the ideal conditions for a healthy pregnancy. So, your body keeps ovulation from happening to prevent pregnancy.
That effectively lowers the estrogen and progesterone in your body, which can cause a cascade of symptoms, including:
Menopause is the time of your life when you haven’t had a period for a year. It effectively ends your child-bearing years. Typically, it happens around the age of 50.
Your estrogen and progesterone levels don’t fluctuate much in your postmenopausal years. They stay relatively constant and at a low level.
Because of that, Zumpano says that intermittent fasting may be more effective for postmenopausal females in their 50s, 60s and beyond. But you still should be cautious.
“After menopause, ovulation and menstruation aren’t going to be affected by intermittent fasting,” she says. “But some people still experience symptoms of having low hormone levels after menopause. So, you still want to be smart about it and be aware of whether fasting is causing additional symptoms.”
If you’re pregnant, breastfeeding (chestfeeding) or trying to conceive, intermittent fasting isn’t recommended. But for other females who are pre-menopausal, you may be able to reap some of the benefits of intermittent fasting without major impact on your hormones. But you should consider a few precautions first.
“It’s not that women shouldn’t try intermittent fasting ever. It’s that they’ll be better off if they tread lightly,” Zumpano reiterates.
There are several different approaches people take to intermittent fasting — some more intense than others.
Some people will restrict their eating to certain hours per day. Others will eat on a normal schedule for certain days of the week and severely restrict their calories on other days.
Zumpano says pre-menopausal females may be best served with a low-intensity intermittent fasting schedule, especially at first.
“I recommend starting with a 12-hour fasting schedule. That’s a pretty safe entry point for most people.”
For example, you could start by fasting between 8 p.m. at night and 8 a.m. in the morning.
And if that goes well for you after a week, Zumpano says you can stretch it out by two hours — adding an hour of fasting on to either side. So, if you were fasting 8 p.m. to 8 a.m., you could now fast 7 p.m. to 9 a.m.
Zumpano suggests working up to a 16-hour fast overnight, leaving your eating to within an eight-hour window. Be sure to time it correctly to your cycle if you’re menstruating (read on!).
If you haven’t gone through menopause, there are certain times of the month that are better than others to try intermittent fasting.
“Fasting is going to be more effective and cause less hormonal imbalance if you time it up with your cycle,” Zumpano advises.
Better times to try fasting are a day or two after your period begins and a week or so after. You’ll want to limit your fasting times during the two weeks before your period is due. You’re most likely to be ovulating two weeks before your period. So, during that time, your hormones are most likely to be affected by fasting.
Avoid fasting the week before your period. That’s when your body is most vulnerable to stress. Estrogen drops during that time, which leads to cortisol (stress hormone) sensitivity. That’s why the week before your period you may experience mood swings, low energy and increased appetite or food cravings.
Intermittent fasting tends to focus on when you eat. But what you eat is still very important.
Zumpano suggests cutting down on processed and packaged foods, and sticking with lean proteins, fruits, vegetables and whole grains.
And that first meal after fasting should be carefully chosen.
“You want to break your fast with a high-protein, high-fiber meal with healthy fats,” she adds. “That will help you avoid a big blood sugar spike.”
And don’t inhale it. Take your time and savor your food after fasting.
Planning and following an intermittent fasting eating plan can be tricky. And you’ll want to make sure your efforts are helping you reach your goals without major effects to your hormones and the rest of your body.
Zumpano advises consulting a healthcare provider, like a registered dietitian, beforehand. They can help you understand your health goals and offer healthy strategies to reach them.