One of the more esoteric but much beloved tools in the paleo dieter’s tool-kit is intermittent fasting.

What is intermittent fasting? I.F. is the practice of maintaining overall caloric intake while consuming those calories in fewer meals or in reduced time windows throughout the day. The goal is to create conditions of fasting in the body, but not for extreme lengths of time.

Some examples of intermittent fast strategies include 10, 8, or 5 hour eating windows throughout the day, or perhaps eating just two meals each day: one in the morning, and one at night. The evolutionary premise — the argument that proponents of intermittent fasting make — is that humans evolved to optimize their health under less-than-optimal conditions. Intermittent fasting, they say, is a natural and perhaps even necessary part of being human.

The modern-day scientific correlate appears promising, too:

Most people are nowadays aware that a calorie-restricted diet has the ability not just to decrease body weight but also to lengthen a human life. Emerging research is beginning to show, however, that intermittent fasting is just as effective as calorie restriction in ensuring these health benefits! Amazingly enough, this happens without any of the psychological crippling side effects of cravings and food obsession that practictioners of calorie-restriction often experience.

Intermittent fasting, proponents say, also may benefit the fight against cancer, diabetes, and autoimmunity. Here is an excellent, up-to-date review of the “benefits” of fasting. It is wholly understandable that fasting is all the rage these days.

Sort of.

Intermittent fasting women is a specific interest of mine because of what I have witnessed both in myself and in working with literally thousands of women in the PfW community.

Many women report to me (read more about that in this awesome book) that intermittent fasting causes sleeplessness, anxiety, and irregular periods, among many other symptoms hormone imbalance, such as cystic acne.

I have also personally experienced metabolic distress as a result of fasting, which is evidenced by my interest in hypocretin neurons. Hypocretin neurons have the ability to incite energetic wakefulness, and to prevent a person from falling asleep, in reaction to the body detecting a “starved” state. Hypocretin neurons are one way in which intermittent fasting may dysregulate a woman’s normal hormonal function.

After my own bad experience with IF, I decided to investigate intermittent fasting. I looked into both a) the fasting literature that paleo fasting advocates refer to, and b) the literature that exists out in the metabolic and reproductive research archives.

Intermittent Fasting Women: Problems in the Paleosphere

What I found is that the research articles cited by Mark’s Daily Apple (and others), focus on health benefits such as cancer-fighting properties, insulin sensitivity, and immune function.

However. I was struck by what seemed like an egregious sex-based oversight in that MDA post I linked to above. MDA cites this article as a “great overview” of the health benefits of intermittent fasting. This startled me because the article MDA cited was for me one of the strongest proponents of sex-specific differences in response to fasting.

Sex differences were relevant in two striking areas:

1) women in studies covered by the review did not experience increased insulin sensitivity with IF regimes and

2) intermittent fasting women actually experienced a decrease in glucose tolerance.

These two phenomena mean that women’s metabolisms suffered from IF. The men’s metabolisms on the other hand improved with IF across the board. Recall that the review was reported by MDA as “a great overview of benefits [of IF].”

Secondly, in another fasting post at MDA, of which there are many, the health benefits of fasting are listed and reviewed, but the sex-specific aspects of the hormonal response go unmentioned, and reproduction/fertility/menstrual health isn’t mentioned at all.

This is not to say that Mark is not attentive to who should and who should not be fasting. He knows very well and cautions people against the dangers of fasting while stressed. Still, the mere fact of being more sensitive to fasting simply by being a woman is, I would assert, pretty important for a woman who is contemplating or already practicing IF.

This goes nearly unmentioned in the blogosphere.

Intermittent Fasting Women: Problems in the Literature

Beyond reporting biases in the blogosphere, there remains an even greater problem of a significant testing bias in the fasting literature. Searching “men” + “intermittent fasting” in a Harvard article database yields 71 peer-reviewed articles. Searching “intermittent fasting women” yields 13, none of which are a) solely about women b) controlled studies or c) about more than body weight or cardiovascular benefits.

The animal studies are more equitable, but also a bit less applicable to human studies.

It is well-known in both the research and the nutritional communities that caloric restriction is horrible for female reproductive health. This is not news. There is an infertility condition – called hypothalamic amenorrhea – that millions of women suffer from due to being overly restrictive. But what of fasting?

Intermittent Fasting Women: Should we Fast?

The few studies that exist point towards no.

It is not definitive, since the literature is so sparse, and it necessarily differs for women who are overweight versus normal weight (and who have different genetic makeups), but when it comes to hormones, women of reproductive age may do well to err on the side of caution with fasting.

What follows first is a brief review of what can be gleaned in sex-specific responses to fasting in animal studies. Afterwards I talk about what has been concluded by the few relevant human studies.

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Mice & Rats

First up is a study that demonstrates the hippocampal changes of calorie restriction and intermittent fasting for both male and female rats. In this study, they do alternate day fasting, which entails free eating on one day and a fast day on the next.

The study found that brain states while fasting were different for male and female rats. For male rats the change in hippocampus size, hippocampal gene expression, and ambulatory behavior was the same no matter what kind of restricted diet they were on – but for female rats, the degree of change in brain chemistry and in behavior was directly proportional to degree of calorie intake, demonstrating the unique sensitivity of female rats to the starvation response.

” The organization of the females’ response to the energy restricted diets is suggestive of some underlying mechanism that may allow for an organized, pre-programmed, response to enhance survival in times of food scarcity. Comparatively, the males’ genetic response was less specific, suggesting that the males respond to a general stressor but they seem to lack the ability to discriminate between a high energy and low energy stressor.” Moreover, “IF down-regulated many gene pathways in males including those involved in protein degradation and apoptosis, but up-regulated many gene pathways in females including those involved in cellular energy metabolism (glycolysis, gluconeogenesis, pentose phosphate pathway, electron transport and PGC1-α), cell cycle regulation and protein deacetylation.” In this study, both male and female rats gained small amounts of weight on IF diets.

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For female rats, even in the most innocuous form of restriction–intermittent fasting–significant physiological changes take place. Male rats do not experience as dramatic hippocampal and general brain chemistry change as female rats do, and their behaviors, specifically their cognition and their dirunal and nocturnal activity, do not change.

Female rats, on the other hand, “masculinize.” They stop ovulating and menstruating. They become hyper-alert, have better memories, and are more energetic during the periods in which they are supposed to be sleep. Theoretically, according to these researchers, this is an adaptive response to starvation. The more the female rats need calories– or at least the more their bodies detect a “starvation” state– the more they develop traits that will help them find food. They get smart, they get energetic, they get active, and they stop sleeping.

In a follow-up study conducted by the same researchers who explored the masculinzation of female rats, the researchers analyzed the gonadal transcription of male and female rats subjected to IF regimes.

This study found that male reproductivity up-regulates in response to metabolic stress. Female reproductivity down-regulates.

Completely opposite to the female rats becoming infertile while fasting, male rats become more fertile. In the researchers’ own words: “our data show that at the level of gonadal gene responses, the male rats on the IF regime adapt to their environment in a manner that is expected to increase the probability of eventual fertilization of females that the males predict are likely to be sub-fertile due to their perception of a food deficient environment.”

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In the final relevant IF rat study I could find, researchers subjected rats to the same diets– to 20 and 40 percent Calorie-Restricted (CR) diets, as well as to alternate-day fasting diets, and monitored them over the long term for hormonal responses. The results were striking. Below is the abstract in full because it’s so powerful:

Females and males typically play different roles in survival of the species and would be expected to respond differently to food scarcity or excess. To elucidate the physiological basis of sex differences in responses to energy intake, we maintained groups of male and female rats for 6 months on diets with usual, reduced [20% and 40% caloric restriction (CR), and intermittent fasting (IF)], or elevated (high-fat/high-glucose) energy levels and measured multiple physiological variables related to reproduction, energy metabolism, and behavior. In response to 40% CR, females became emaciated, ceased cycling, underwent endocrine masculinization, exhibited a heightened stress response, increased their spontaneous activity, improved their learning and memory, and maintained elevated levels of circulating brain-derived neurotrophic factor. In contrast, males on 40% CR maintained a higher body weight than the 40% CR females and did not change their activity levels as significantly as the 40% CR females. Additionally, there was no significant change in the cognitive ability of the males on the 40% CR diet. Males and females exhibited similar responses of circulating lipids (cholesterols/triglycerides) and energy-regulating hormones (insulin, leptin, adiponectin, ghrelin) to energy restriction, with the changes being quantitatively greater in males. The high-fat/high-glucose diet had no significant effects on most variables measured but adversely affected the reproductive cycle in females. Heightened cognition and motor activity, combined with reproductive shutdown, in females may maximize the probability of their survival during periods of energy scarcity and may be an evolutionary basis for the vulnerability of women to anorexia nervosa.

They also found this:

The weight of the adrenal gland was similar in rats on all diets; however, when normalized to body weight CR and IF diets caused a relative increase in adrenal size, the magnitude of which was greater in females, compared with males.

And this:

The testicular weight was unaffected by any of the diets. In contrast, both CR diets and the IF diet caused a decrease in the size of the ovaries.

And this, bearing in mind that “daytime” for nocturnal rats is “nighttime” for humans:

The daytime activity of females was doubled in response to IF, whereas the IF diet did not affect the activity level of males. Nighttime activity levels of males and females were unaffected by dietary energy restriction.

And this:

Uterine activity was monitored daily with vaginal smear tests; cyclicity was scored as regular, irregular, or absent. The mild energy-restriction diets (20% CR and IF) significantly increased the proportion of animals displaying irregular cycling patterns, whereas the 40% CR animals displayed an almost complete loss of estrous cyclicity.

And this:

In males, corticosterone levels were elevated only in response to the 40% CR diet, whereas in females corticosterone levels were significantly elevated in response to all three energy-restriction diets, suggesting a relative hyperactivation in females of the adrenal stress response to reduced energy availability.

For lipids, all the rats did well: “Collectively, these data suggest that atherogenic profiles of both males and females are improved by dietary energy restriction.” Interestingly, too, as they pointed out in the abstract, human females also perform cognitively much “better” (memory and alertness) on CR and IF diets than on normal feeding schedules.

There are of course some caveats to this study: A) They are rats. B) They are somewhat “metabolically morbid” rats, which may make them more susceptible to disease. C) The rats were allowed to eat ad libitum on the IF days, but they simply did not meet their caloric requirements this way. So while it is a somewhat natural form of IF, it is still calorically reduced, such that that must be taken into account when gasping in horror at the hormonal responses of IF-ing female rats.

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The Few Human Studies

I mentioned above that through the same review that MDA used as a “great overview” of IF benefits for all sexes, I found harmful metabolic effects for women subjected to alternate-day fasting regimes.

This is the study:

Heilbronn et al found that with IF, insulin sensitivity improved in men (21 participants) but not in women (20 participants): after three weeks of alternate day fasting, insulin response to a test meal was reduced in men. Women experienced no significant change. “It is interesting that this effect on insulin sensitivity occurred only in male subjects,” they report.

With respect to other health markers female health actually declined, specifically with respect to glucose tolerance:

“Another diabetes risk factor that has shown a sex-specific effect is glucose tolerance. After 3 weeks of ADF, women but not men had an increase in the area under the glucose curve. This unfavorable effect on glucose tolerance in women, accompanied by an apparent lack of an effect on insulin sensitivity, suggests that short-term ADF may be more beneficial in men than in women in reducing type 2 diabetes risk. ” The opening line of their discussion reads: “Alternate day fasting may adversely affect glucose tolerance in nonobese women but not in nonobese men.”

In a follow up study, Heibron et. al studied the effects of alternate-day fasting on cardiovascular risk. When human subjects fasted on alternate days for another three week period, circulating concentrations of HDL cholesterol increased, whereas triacylglycerol concentrations decreased. This is a good thing. However, the shifts in lipid concentrations were shown to be sex specific: ie, only the women had an increase in HDL-cholesterol concentrations, and only the men had a decrease in triacylglycerol concentrations.

The most recent review of IF agrees with my conclusion: sex-specific differences in metabolism exist and need to be studied further.

This study of alternate day fasting included 12 women and 4 men. In eight weeks, body weight decreased by about 10 pounds, and body fat percentage decreased from 45 to 42. Blood pressure decreased, total cholesterol, LDL cholesterol, and traicylglycerol decreased. These people were significantly obese, which limits the results of this study to an obese population. However, “perimenopausal women were excluded from the study, and postmenopausal women (absence of menses for >2 y) were required to maintain their current hormone replacement therapy regimen for the duration of the study.” (Their words, my emphasis)

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The one, big study of intermittent fasting conducted on men and women looked at differences between isocaloric feeding schedules: 3 meals/day feeding versus 1 meal/day.

The study focused on body weight composition, blood pressure, and body temperature in subjects. Subjects were fed isocalorically either one meal each day or three meals each day. All subjects were between 40 and 50 years old (excluding women of reproductive age), and between BMIs of 18 and 25. They ate, so far as I can tell, a healthy diet with 35 percent fat, PUFA < MUFA < SFA. Only 15 of the original 69 completed the study (which goes to show just how fun everyone thought fasting was). As for the results,

“Systolic and diastolic blood pressures were significantly lowered by ≈6% during the period when subjects were consuming 3 meals/d than when they were consuming 1 meal/d. No significant differences in heart rate and body temperature were observed between the 2 diet regimens. Hunger was enormously larger in the one meal/day than in the three meals/day group. “The 1 meal/d diet was significantly higher for hunger (P = 0.003), desire to eat (P = 0.004), and prospective consumption (P = 0.006) than was the 3 meals/d diet. Feelings of fullness were significantly (P = 0.001) lower in the 1 meal/d than in the 3 meals/diet.” Body weight dropped only four pounds after several months. Cortisol dropped, but Total, LDL, and HDL cholesterol were 11.7%, 16.8%, and 8.4% higher, respectively, in subjects consuming 1 meal/d than in those consuming 3 meals/d.

In sum: patients on the one meal/day regiment were unhappy, hungry, lost a little bit of weight, increased cholesterol. This was a small sample, included somewhat menopausal women, and all people of normal body weight.

Intermittent Fasting Women: In Conclusion

All that being said, that’s it. That’s all that exists! Women don’t have much to go on.

There are a few rodent studies. They found that when alternate-day fasting,female rats and found significant negative hormonal changes occurring in the females.

There are even fewer human studies. Human studies on alternate day fasting have not been conducted on women of reproductive age at all, nor have any studies analyzed reproductive responses to fasting.

Moreover, the few studies that have been conducted on non-obese women have demonstrated that their metabolic responses are not nearly as robust as those of men, and may in fact be antagonistic to their health.

This post has focused on sex-specific responses to fasting, specifically intermittent fasting women. Another important distinction to make is between different body weights. Overweight and obese patients appear to experience significant improvements with IF regimes, but normal weight patients do not show the same across-the-board benefits. For women this may be a particularly sensitive issue. Overweight women may experience metabolic benefits, whereas normal weight women do not. I suspect that that may roughly be the case, but who knows. Honestly, no one at this point.

The practical solution, then, I believe, is to look at options, to be honest about priorities, and to listen to one’s body with awareness and love.

Is fasting worth trying if a woman is overweight and trying to improve her metabolic markers, and so far hasn’t had much success? Perhaps. Should it be undertaken if a woman is of normal weight? What if she is a light sleeper? What if her periods begin to dysregulate? Or stop? What if she starts getting acne, getting a stronger appetite, or losing her appetite altogether? These things happen, and I see them in women who fast and contact me time and time again.

We women (people!) should be honest with ourselves about our priorities, and act constantly with our mental and physical health foremost in our minds. All women are different. But the literature is so sparse in this area that we cannot make any real statements or predictions about the effects of fasting, other than that we just don’t know, and that we should continue to emphasize the centrality of awareness, caution, and loving nourishment in moving forward.

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IF is one realm in which the female body has unique characteristics and needs that demand attention. There are boatloads of others. If you’re interested in reading about the collective set of them and learning how to optimize female skin, weight loss, and hormone balance, for a few examples, you could do worse than my best-selling book, Sexy by Nature, here.

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And that’s a wrap! What do you think?