Some women who try intermittent fasting experience missed periods, metabolic disturbances, and even early-onset menopause. Sure, it can work for some women. But here’s why intermittent fasting could be bad — even counterproductive — for your goals.
For as long as I can remember, my dad has eaten once a day. Once.
In the morning he gets up and drinks black coffee. That’s it — no eggs, no cereal, no muffin.
Lunch: more nothing.
For supper, he eats a decent-sized meal at home.
The whole thing is even more impressive considering that Dad owns a restaurant!
As a kid, I shrugged it off. Dads do crazy stuff (at least he didn’t play the accordion). He was always in great health, and at 74, he still is today.
My mom couldn’t be more different. She’s at the breakfast table within an hour of waking up, never willingly misses lunch or supper, and if the time between meals gets too long, she fixes herself a snack. She’s the kind of person who keeps almonds in her car just in case she starts to feel peckish while she’s out and about.
I won’t say how old Mom is, but she’s also in great health, and always has been.
Years ago, as I began to immerse myself in a career dedicated to the study of nutrition and sport, I started to wonder:
How can two people who eat so differently be so similarly healthy?
Back then, I had no idea that my father was decades ahead of his time — a kind of pioneer of a now super-buzzworthy diet called intermittent fasting.
Intermittent fasting and your health
Intermittent fasting (IF) is the practice of going for prolonged periods without eating.
There are lots of ways to do it, including meal skipping, alternate-day fasting, Eat Stop Eat, and others (PN’s free e-book on intermittent fasting offers an excellent rundown).
There’s evidence that IF, when done properly, might help regulate blood glucose, control blood lipids, reduce the risk of coronary disease, manage body weight, help us gain (or maintain) lean mass, reduce the risk of cancer, and more.
We even published an article suggesting it could help us live longer and stronger. So, naturally, a lot of people are trying it.
An accompanying trend that’s emerged: While some women who try IF say it’s the best thing that’s happened to them since grapefruit, others report serious problems, including binge eating, metabolic disruption, lost menstrual periods, and early-onset menopause. This has happened in women as young as their mid-20s.
Maybe my mom was on to something. Maybe IF is totally different for women than for men.
Fasting and female hormones
In the grand scheme of your life’s health decisions, experimenting with IF seems tiny, right? Unfortunately — for some women, at least — it seems like small decisions can have big impacts.
It turns out that the hormones regulating key functions like ovulation are incredibly sensitive to your energy intake.
In both men and women, hypothalamic-pituitary-gonadal (HPG) axis — the cooperative functioning of three endocrine glands — acts a bit like an air traffic controller.
- First, the hypothalamus releases gonadotropin releasing hormone (GnRH).
- This tells the pituitary to release luteinizing hormone (LH) and follicular stimulating hormone (FSH).
- LH and FSH then act on the gonads (a.k.a. testes or ovaries).
- In women, this triggers the production of estrogen and progesterone — which we need to release a mature egg (ovulation) and to support a pregnancy.
- In men, this triggers the production of testosterone and sperm production.
Because this chain of reactions happens on a very specific, regular cycle in women, GnRH pulses must be very precisely timed, or everything can get out of whack.
GnRH pulses seem to be very sensitive to environmental factors, and can be thrown off by fasting.
Even short-term fasting (say, three days) alters hormonal pulses in some women.
There’s even some evidence that missing a single regular meal (while of course not constituting an emergency by itself) can start to put us on alert, perking up our antennae so our bodies are ready to quickly respond to the change in energy intake if it continues.
Maybe this is why certain women do just fine with IF while others run into problems.
Why does IF affect women’s hormones more than men’s?
We’re not totally sure.
But it might have something to do with kisspeptin, a protein-like molecule that neurons use to communicate with each other (and get important stuff done).
Kisspeptin stimulates GnRH production in both sexes, and we know that it’s very sensitive to leptin, insulin, and ghrelin — hormones that regulate and react to hunger and satiety.
Interestingly, females mammals have more kisspeptin than males. More kisspeptin neurons may mean greater sensitivity to changes in energy balance.
This may be one reason why fasting more readily causes women’s kisspeptin production to dip, tossing their GnRH off kilter.
Putting it all together: The study
It would be nice to find a human study to illustrate the science I’ve been describing here, but, as I mentioned, there are none. So instead, we’ll look at a recent study on rats:
The subjects included 10 male and 10 female normal-sized rats.
- Half the rats ate whenever they wanted.
- The other half ate only every second day. In between feeding times, their food was removed and they fasted.
This went on for 12 weeks, which is the equivalent of about 10 years in a human life.
By the end of the 12 weeks, the fasting female rats had lost 19 percent of their body weight, their blood glucose levels were lower, and their ovaries had shrunk.
Overall, the experiment affected the female rats’ hormones much more significantly than the males’.
While kisspeptin production went down in both male and female fasting rats, in the females, LH absolutely plummeted, while estradiol, a hormone that inhibits GnRH in humans, skyrocketed to four times higher than the normal level.
The appetite hormone leptin was six times lower than in a normally fed female rat.
It only took 10-15 days for the experiment to disrupt their reproductive cycle.
In other words, the female rats’ hormones — both reproduction- and appetite-regulating — were totally out of whack.
What does this mean for humans?
It’s hard to say. But based on what we do know about the HPG axis, kisspeptin, the relationship of hormones to appetite, and women’s sensitivity to environmental factors, it’s plausible that fasting could have a similarly dramatic effect in human females.
Fertility, meet metabolism
You might be thinking: So, what’s the big deal if kisspeptin drops off and I miss a few periods? I’m not having kids anytime soon, anyway.
Here’s the thing.
The female reproductive system and metabolism are deeply intertwined. If you’re missing periods, you can bet that a bunch of hormones have been disrupted — not just the ones that help you get pregnant.
Take this snapshot.
In general, women tend to eat less protein than men. Fasting women, obviously, will consume even less.
Consuming less protein means taking in fewer amino acids.
Amino acids are needed to activate estrogen receptors and synthesize insulin-like growth factor (IGF-1) in the liver. IGF-1 triggers the uterine wall lining to thicken and the progression of the reproductive cycle.
Hence, low protein-diets can reduce fertility. (Not to mention sexytime.)
And importantly, estrogen isn’t just for reproduction.
We have estrogen receptors throughout our bodies, including in our brains, GI tract, and bones. Change estrogen balance and you change metabolic function all over: cognition, moods, digestion, recovery, protein turnover, bone formation…
When it comes to appetite and energy balance, estrogen works in a few ways.
First, in the brainstem, estrogens modify the peptides that signal you to feel full (cholecystokinin) or hungry (ghrelin).
In the hypothalamus, estrogens also stimulate neurons that halt production of appetite-regulating peptides.
Do something that causes your estrogen to drop, and you could find yourself feeling a lot hungrier — and eating a lot more — than you would under normal circumstances.
Estrogens are thus key metabolic regulators.
Yes, estrogens, plural. Because the ratios of the estrogenic metabolites (estriol, estradiol, and estrone) change over time. Before menopause, estradiol is the big player. After menopause, it drops, while estrone stays about the same.
The exact roles of each of these estrogens remain unclear. But some theorize that a drop in estradiol may trigger an increase in fat storage. Why? Because fat is used to make estradiol.
This may partly explain why some women find it harder to lose fat after menopause. And it might serve as a reason to care about your reproductive health — even if you’re not focused on making babies.