Many differences exist between the two sexes. We look different. We sound different. We dress differently from each other. We like different things. Different genres of movies cause men and women to cry (differently). And although society, media, and culture drive and/or inform many of our differences, some are inherent and physiologically-driven. For example, men and women have different biological equipment – both external and invisible to the naked eye – that change how we interact with and respond to our environments, our exercise, our sleep, and our eating habits. Nowhere are these gender differences more evident than in the realm of health and nutrition, and yet it seems that I’ve overlooked a big one: different sex responses to intermittent fasting.
Let’s take a look at a couple recent reader emails:
I’m a woman (28 years old) who followed your recent fasting series with great interest, gave it a shot, but had mixed results. Then I read this post , which mentioned your series and questioned the suitability of intermittent fasting for women. Is it true? Do we respond differently than men? What do you think of that post? Thanks!
Paleo for Women blog says that fasting may not be for women: that it’s more suited for male physiology. I have been fasting for three years and never experienced any missed periods/sleeplessness, etc. Moreover I got a handle on my mindless eating. Can you give your word on IF for women?
Thanks for writing in with your questions.
First of all, I really, really liked Stefani’s post. I should say “posts,” actually, since Stefani Ruper (who wrote the post linked in the reader question) also just did a guest post  on Free the Animal, in which she discussed the treatment of women’s issues in the community at large. While I don’t agree with everything she said, both were quite well done.
Even though her articles – for lack of a better phrase – “called me out” (in a completely non-confrontational way), I was actually quite happy to read them. Heck, I was happy to read them because of it. After all, I’ve always encourage people to be critical about what they think they know about nutrition and fitness, and to be skeptical about what they read on the Internet  – my articles included. The beauty of MDA is that it isn’t one-sided. I get constant feedback from readers that send me down new paths of inquiry, and it’s through this kind of crowd-sourced effort that the Primal Blueprint message grows and becomes stronger than it already is.
I also appreciated Stefani’s articles because they do highlight a blindspot – not just in my own series of posts , but in nutritional science as a whole. In the push to eliminate the confounder known as inherent endocrine gender differences, they’ve forgotten that real life is a series of confounding variables all pushing, pulling, poking, and prodding at the results we get. They’ve forgotten that while their results may represent fodder for publishing and accolade accumulation and hypothesis confirmation (or rejection), real live humans in normal living situations are not placebo-controlled. That women are not the same as men and respond differently to stimuli and stressors isn’t a “confounder”; it’s a fact deserving of further study! Because what are we ultimately trying to do here – put together nice, neat, peer-review-ready trials, or help real people living real lives?
Since I’m trying to do the latter, I happily accept constructive criticism. So should we all.
So, what did Stefani’s research find?
Fasting has different endocrine effects on male and female rats.
In male rats:
No matter the duration or degree of nutritional stress, male rat brain chemistry responds with similar changes. Nocturnal activity and cognition stay fairly stable, regardless of the intensity of the fast. If you push the fast long enough, males will get a little wonky and frantic, but overall they maintain pretty well. It’s like they’re equipped with the ability to handle nutritional stressors.
In female rats:
Any degree of nutritional stress (fasting or mere caloric restriction) causes increased wakefulness (during the day, when they normally sleep), better cognition (for finding food), hyper alertness, and more energy. In short, female rats become better at finding and acquiring food when they fast, as if their bodies aren’t as well-equipped to deal with the stress of going without food. They also become less fertile, while the males actually become hornier and more fertile (probably to account for the females’ plummeting fertility). Ovary size drops (bad for fertility), adrenal gland size increases (which in rats indicates exposure to chronic stress ), and menstrual cycles begin to dysregulate in proportion to the degree of caloric restriction.
In humans, the male-female fasting literature is quite scant, but Stefani also found considerable differences beween the sexes, when data was available:
- One study , which I’ve cited before as evidence of a benefit to fasting, found that while IF improved insulin sensitivity in male subjects, female subjects saw no such improvement. In fact, the glucose tolerance of fasting women actually worsened. Ouch.
- Another study  examined the effect of alternate day fasting on blood lipids . Women’s HDL improved and their triglycerides remained stable; men’s HDL remained stable and their triglycerides decreased. Favorable, albeit sex-specific results.
- Later , both obese men and women dropped body fat, body weight, blood pressure, total cholesterol, LDL cholesterol, and triglyercides on a fasting regimen. These people were obese, however, and perimenopausal women were excluded from the study, so the results may not apply to leaner people or women of reproductive age.
I figured I’d look through my other recent fasting posts for data on female (preferably pre-menopausal) responses to fasting. Here’s what I found:
- In the only heretofore extant human study  on fasting and chemotherapy, seven females (including a 44-year old woman who was likely premenopausal, given when menopause usually onsets, though it wasn’t explicitly stated) and three males found that IF improved their tolerance to and recovery from chemotherapy. Takeaway: male and female (mostly middle aged, though that’s the population that generally gets cancer and undergoes chemotherapy) chemotherapy patients appear to benefit equally from IF.
- Although both men and women displayed greater increases in VO2 max and resting muscle glycogen concentration in response to fasted cycling training, only men showed greater skeletal muscle adaptations when fasted. Women had better muscle adaptations when fed . Takeaway: fasted endurance training, then, may work better for women than fasted weight training .
As it stands right now, I’d be inclined to agree that pre-menopausal (and perhaps peri-menopausal) women are more likely to have poor – or at least different – experiences with intermittent fasting, at least as a weight loss tool . That said, it appears to be a potentially gender-neutral therapeutic tool for chemotherapy, cancer , and age-related neurodegeneration  patients.
As I alluded to earlier, this is what I love about this open forum we call the Internet: the fact that if you leave something out, or overlook a key point, someone will call you out on it, most likely publicly. When that happens, you grow despite yourself. If not for Stefani’s posts, I may never have taken a closer look at the inherent differences in men’s and women’s metabolic responses to fasting. I certainly receive enough feedback from female readers for whom fasting has been helpful, so it’s good to see another side.
To sum things up – if such a thing can even be done – and answer the questions in the intro, men and women have inherent metabolic and hormonal differences, and it’s evident that these differences in part determine how we respond to a stressor like intermittent fasting. I’ve never prescribed intermittent fasting as a requisite piece of the Primal lifestyle, but rather as an adornment, a choice, a potentially therapeutic strategy that each individual must test for him or herself . Although my recent series on fasting  might have thrown some people off, I want to reiterate that I am not a huge IF guy. For myself, I generally fast when it makes sense – if I’m traveling and good food isn’t available, if I’m just not hungry, stuff like that. I periodically do 16/8 or 14/10 (i.e. eating in an 8 or 10 hour window) and find it works great for me because I am fully fat-adapted. But even I don’t hold rigidly to that. It’s not for everyone. And that hasn’t changed.
So who should and shouldn’t consider fasting? Have my recommendations changed?
If you haven’t satisfied the usual IF “pre-reqs,” like being fat-adapted, getting good and sufficient sleep, minimizing or mitigating stress, and exercising well (not too much  and not too little), you should not fast. The pre-reqs are absolutely crucial and non-negotiable, in my opinion, especially the fat-adaptation. In fact, I suspect that if an IF study was performed on sugar-burning women versus fat-adapted women, you’d see that the fat-burning beasts would perform better and suffer fewer (if any) maladaptations.
I would also caution against the already lean, already calorie-restricted woman jumping headfirst into IF. I mean, fasting is ultimately sending a message of scarcity to your body. That’s a powerful message that can get a powerful response from our bodies. If you’re already lean (which, depending on the degree of leanness, arguably sends a message of scarcity) and restricting calories (which definitely sends a message of scarcity), the response to fasting can be a little too powerful.
I’d also say that daily fasts, a la 16/8  or even 14/10, run the risk of becoming chronic stressors  and should be approached with caution by women. Same goes for ultra-long fasts, like a 36 (or even 24) hour marathon. Most of all, though, I’d simply suggest that women interested in fasting be cautious, be self-aware, and only do so if it comes naturally. It shouldn’t be a struggle (for anyone, really). It shouldn’t stop your cycle or make it harder for you to get pregnant. It should improve your life, not make it worse. If you find that fasting has those negative effects, stop doing it. It should happen WHEN (When Hunger Ensues Naturally), if it happens at all.
I’m not going to say that women should or shouldn’t fast. I’ll just echo Stefani’s advice “to look at options, to be honest about priorities, and to listen to one’s body with awareness and love.” Frankly, everyone should be doing that, but with regards to fasting, it looks like women should probably hew a little closer to her words.
Of course, if I had to make one minor quibble with the content of Stefani’s otherwise outstanding posts, it would be her source for the number of unique visitors Mark’s Daily Apple gets each month. Nowadays, we’re actually getting closer to 1.5 million monthly uniques, not 250-300,000… but who’s counting?
That’s it for me, today. What about you? If you’re a woman who has tried fasting, or know someone who fits the description, let us all know about your experiences. I’m intensely curious to hear from as many of you as I can. Thanks for reading.