Intermittent Fasting is taking the diet world by storm, as it can result in quick, effortless weight loss. But is it safe for women?
Despite the fact that women tend to dominate the "diet" world, intermittent fasting has maintained a bit of a male-centric status. While IF brims with potential health benefits and can truly catalyze weight loss (without calorie restriction!), people commonly assume fasting is bad for women. Seen as a hormonal "stressor" on par with relentless calorie restriction and exhaustive over-exercise, worries run rampant that intermittent fasting will upset a woman's adrenals or threaten fertility.
But does it?
It's a bit complicated.
Unfortunately, the majority of gendered IF studies feature Minnie Mouse rather than Snow White. Furthermore, both these rodent studies and the few relevant human studies often implement intermittent fasting patterns involving periods of calorie restriction. Such an approach looks at the effects of actual consistent dietary restriction, rather than brief hormetic periods of fasting followed by ample calorie intake. (We'll circle back around to this in a bit!) The time has come, my dear readers, to see what the scientific literature actually says about women, fasting, hormones, and #allthethings. The findings just may surprise you!
MEN VS. WOMEN
Since fasting inevitably instigates a state of running on body fat stores, let's first look at the foundational differences in how men and women utilize fat for energy.
As you likely know, women are typically "fatter" than men. It's just a thing. The primary (awesome) reasoning for this, is that women store fat in preparation for pregnancy, which can double a woman's energy requirements - oh my! From her body's "perspective," if a woman were to get pregnant without ample body fat hanging around (literally), it could mean not only malnutrition, but also potential death for her and/or the baby. Since pregnancy is the ultimate goal from an evolutionary perspective, and we cannot simply "tell" our bodies we're basically assured of consistent food intake (wouldn't that be nice?), a girl's body totally favors fat. Unlike temporary carbohydrate stores in the muscles, which are extremely transient and harbor no nutrition beyond potential energy, fat provides a steady, sustainable, nutrient-rich, long-term energy source.
Because of this whole fat-loving thing, women are actually pretty awesome fat users in general, typically more so than men. This may seem hard to believe, since women often bemoan their weight loss struggles, sighing at how "easy" it is for men to stay lean. But it's actually more like women simply store more fat in general, so there's more to go through to get "lean," coupled with the fact that women's body's don't particularly like being too lean, because of the whole pregnancy thing we just discussed.
But when it comes to actually using fat in the moment, women rule! After all, since body fat is the "chosen" currency of choice for women, you better believe they'll be good at using it! Studies show women are better at accessing fat for energy during times of temporary energy depletion (such as fasting and exercising) than men, who tend to favor more glucose (sugar) burning. Women also store excess fat in a more "healthy" manner, such as subcutaneously and around the thighs and hips, whereas men tend to store excess fat in more unhealthy manners - viscerally deep around organs, and around the abs.
Given this, you'd think women would actually perform pretty well in the fat-burning fasted state. But do they? And how does this affect hormones and all the things downstream?
People often references rodent studies in regards to IF, since we have many of those. The primary (and very substantial) problem with this approach, falls to the fact that "intermittent" fasting in rodents typically equates to days in adults. I'm not sure we even have rodent studies comparable to what a typical intermittent fast would be in a human female. Rodents fasted for 24 hours, for example, can lose up to 18% of their body weight! That's the equivalent of a 130 lb human losing 23.4 lbs in a day! Can you imagine the hormonal effects that could be happening with that?
With all that said, let's jump into some of the gender-specific rodent fasting studies, shall we?
Two connected rodent studies - one in 2007 and the follow-up in 2008 - analyzed the effects of various restrictive dietary protocols in male versus female rats. Both used the same diet breakdowns: 40% calorie-restriction (40% CR), 20% calorie-restriction (20% CR), alternate day intermittent fasting (IF), and a high fat/high glucose diet (HFHG). In case you're wondering, the calorie-restricted diets utilized rat chow featuring complex carbohydrates, whereas the HFHG diet used rat chow made of lots of refined fat and sugar. Seeing as how the HFHG diet was pretty much bad news all around, I'll say that's not a good testament for refined foods! But I digress...
While results varied quite a bit, some trends did materialize. The 2007 study revealed the following:
- IF significantly increased the female's brain performance, but not the males. In maze tests, the calorie-restricted and fasting female rats made fewer errors and finished faster than both the males and the other females eating normally.
- IF increased plasma (but not hypothalamus) brain-derived neurotrophic factor in females but not males. BDNF encourages the growth of new neurons in the central nervous system and positively affects memory, learning, and higher thinking. We want more BDNF around!
- 20% CR and IF both decreased ghrelin - a hormone which increases hunger - in the females only, insinuating that mild dietary restriction and/or IF may decrease appetite in females.
- 20% CR, IF, and HFHG all encouraged irregular cycles, but only 40% CR actually stopped it. So while severe restriction seems to definitely be no beuno, smaller amounts of restriction and IF may be a more complicated issue.
- All of the diets, included the HFGF, decreased ovary size in the females, but did not affect the male's testicles. So fiddling with calories at all (either adding or taking away) affected fertility status.
- In females, the primary female sex hormone estradiol was only significantly affected by extreme restriction (40%) or overfeeding (HFHG). Interestingly, all the diets affected the male's estradiol.
- On the flip side, the steroid hormone corticosterone was elevated only in response to the 40% CR diet in males, whereas in females it was significantly elevated in response to all three energy-restriction diets. While not quite as important in humans, corticosterone is a primary hormone in rodents involved in the stress response, insinuating that the female rats experienced a heightened adrenal response to all forms of restriction.
- IF doubled activity during the day for females, but not males. Since rodents are nocturnal, this may not be the best for circadian rhythms.
So basically, the study found that restricting calories - be it through typical calorie restriction or via fasting - rendered the females more active and alert. On the positive side, this made them smarter and perform better, Super-rodent-woman style. On the downside, it fiddled with circadian rhythms and hormonal cycles. (Though only severely restrictive states actually stopped fertility.) In the end, the researchers concluded that female rats are more sensitive to calorie intake, due in part to energy requirements for reproduction, and that their increased performance in times of fluctuating energy helps them better compete with other rodents for foods.
The researchers' follow-up 2008 study utilized the same types of diets, but zeroed in on the effects on the rats hypothalamus: the central governing system in the brain responsible for appetite, energy use, metabolism, fertility, and a whole host of other things! Here's some of what they found:
- IF upregulated beneficial genes which influence the metabolism and energy production of cells, resistance to oxidative stress, and protein degradation, in females, but not males.
- The calorie-restricted diets, including IF, boosted the size of the hippocampus in females, but not males, upregulating a whopping 60 pathways in the females, compared to 17 in the males. (Increased hippocampus size tends to boost performance.)
- Again, IF substantially increased maze performance in the females, especially compared to the males.
- When it comes to genetic changes, the males responded in a more "general" manner, and only to severe calorie restriction (40%) and overload (HFHG), whereas females responded much more specifically to all of the various diets. The restricted diets also affected genes specifically involved in energy deprivation in the females, but not males.
So again, the researchers found that any fluctuation in dietary intake instigated more intense results in females than males on both a genetic and performance level, increasing the females' energy metabolism, alertness, cognitive ability, and activity. By comparison, male rodents were much more chill about everything. Basically, only when calorie intakes reached extremely low or high amounts out of the ordinary, were the boy rats casually like, “Oh hey… stress. Let’s maybe do something about that?” On the contrary, even slight shifts in calories resulted in the females being like, “Oh my goodness! We are experiencing x amount of stress at this moment! We should act accordingly by precisely adjusting various genes and hormones to ensure both we and our future offspring survive this stressful time!”
The researchers concluded that the female's brains were more resilient in the face of metabolic and oxidative stress, and are much more in tune with specific dietary changes and fluctuating calorie intakes, in order to increase their survival odds. Again, this all makes sense in the context of reproductive roles. When the female rodents experience a decrease in the amount of fuel availability, their bodies respond by making them more capable of getting fuel: they become smarter and faster, and "good" with stress. Pretty cool! At the same time, their bodies also instigate changes in their reproductive rhythms, though it's hard to gauge the downstream effects of this, and is important to note that only substantial restriction (40%) seems to actually stop things.
While these studies are fascinating, there is one major issue for drawing related conclusions about IF's effect on female hormones and fertility: the implemented IF patterns created calorie restriction. This means the IF diet was indeed an extended stressor, without ample nutrition. Given the fact that metabolism and nutrient assimilation is determined by overall intake in a given day, rather than food timing per se, it is quite possible that a different IF implementation with adequate calories would yield different results.
Indeed, when it comes to women, hormones, and fertility, most of the studies which find negative effects on menstruation from diet and/or exercise, look at notably stressful manifestations of such, such as extended calorie restriction or intense exercise. While we may want to tag IF right alongside those, I'd wager that IF with ample nutrition is not exactly the same thing. After all, women are excellent at using fat for energy during the fasted state, and if they are consuming ample nutrition, calories, and macronutrients (fats, proteins, and carbohydrates) when they eat, such could likely provide more assurance to the body.
HUMAN IF STUDIES
Before we get too far down the rodent hole, what about the human side of things? While there are increasingly more studies on intermittent fasting in humans (Yey!), most of those specifically comparing health biomarkers between men and women find varying results. The same goes when comparing separate studies looking at similar factors. For example, some studies find IF majorly benefits fasting glucose in women, while others find it statistically insignificant. One study found alternate day fasting protocols (ADF) reduced insulin release post-meal for men but not women, while another found ADF did zilch for men’s insulin. Another found ADF significantly lowered glucose, insulin, free fatty acid, and LDL cholesterol more so in women than men. And all the breakfast skipping studies I've seen find no notable differences between the sexes, or simply don't address it. With such fluctuating effects, it's difficult to draw any general conclusions about the specific effects of IF in men versus women. In the spirit of personalized nutrition, this makes me think it's all extremely individual, and a person's given genetics, health history, diet, and lifestyle in general is likely uber important, with gender being an important but not defining factor.
So rather than analyze #allthethings when it comes to comparing men and women and fasting (We'd be here till the cows came home!), let's zero in on the few women-specific studies which analyze IF's relationship to women's adrenals, hormones, and the reproductive side of things.
- While not intermittent, a 1994 study looked at normal-weight women on longer 3 day fasts. Despite losing significant weight, they experienced no changes in reproductive hormones, including immune luteinizing hormone, follicle-stimulating hormone, oestradiol, and progesterone. It concluded that a "72-hour fast during the follicular phase does not affect the menstrual cycle of normal cycling women.
- A 2013 study of 15 overweight women found that an IF protocol alternating eat-all-you-want days with severely calorie-restricted days (25-30% of basal needs) reduced weight, blood pressure, and cholesterol, but did not affect menstrual cycles
- A 2010 Ramadan study (which restricts food intake to night time hours, without calorie restriction) analyzing fasting and reproductive cycles, found fasting bore no effect on menstruation. It concluded that Ramadan fasting "causes neither significant variation in the secretion of hormones around ovulation nor does it influence the occurrence of ovulation."
- Another Ramadan study, this one in 2014, specifically analyzed fasting's effects on reproductive hormones in regards to polycystic ovary syndrome (PCOS), a hormonal disorder involving cysts and abnormally sized ovaries, which can lead to psychological, metabolic, and reproductive problems. Some researchers believe this may be due to overstimulation of the nervous system activity, which in turn affects the HPA axis (which we'll look at more in a bit!), encouraging it to shut down the reproductive system. In the study with 40 women, the researchers wanted to determine how fasting affects nervous system-related hormones, including the stimulating "stress" hormone cortisol (also more on that in a bit!), catecholamines (hormones produced by the adrenals), and sex hormones. Unlike the implications of the rodent studies, the researchers found fasting actually decreased cortisol and noradrenaline, while all other stress levels and sex hormones remained unchanged. The study concluded that “Ramadan fasting can be a good pattern for reducing stress hormones in PCOS women and so sympathetic nervous system may offer a new therapeutic target for this syndrome.”
- Similarly, a 2002 study also found fasting benefited ovulatory issues in women with PCOS.
So quite unlike the restricted rodent studies, these human studies - with notably ample nutrition - found IF either doesn't affect fertility, or can even benefit the stress response in certain situations. Speaking of, let's zero in on the whole "stress" thing for a bit.
NATURAL ADRENAL AND STRESS RHYTHMS
In general, I think we engage in a skewed relationship with stress, leading lifestyles which over-encourage it to our detriment, as we simultaneously view it in the wrong mindset. We tend to view the stress response as something to be avoided at all costs, lumping together anything which causes "stress" as automatically bearing equivocally detrimental effects on the body. In reality, not all stress is equal. Studies increasingly indicate that stress perception is huge, and can literally affect the effects stress has on the body. I cannot encourage you enough to check out Kelly McGonigals' The Upside of Stress for more on all this!
The stress response itself is determined by a complex system involving the HPA axis, composed of the governing hypothalamus and pituitary gland in your brain, as well as the adrenal glands above your kidneys. We've also got the various stress hormones themselves, including the essential cortisol and aldosterone (without them - you'd die!), as well as "nonessential" hormones like adrenaline, which aid the whole process. In any case, the stress response ultimately serves a purpose of benefiting us in times when we need energy and performance - enhancing our cognition and upregualting fatty acids for fuel in the body.
Exposure to brief, punctuated stress, such as physical activity or fasting, can function as beneficial "hormetic stressors." With an adequate recovering period following this (which is key), these stressors ultimately make us grow stronger. This differs from uncontrolled, chronic stressors stemming from our cognitive reaction to perceived threats, be it traffic or annoying coworkers or the ding of text message. These chronic stressors can instigate the same physical stress response, yet constantly and when it is unneeded, leading to a prolonged stressed state without adequate recovery, wearing and tearing at our system.
When it comes to women and hormones, we definitely want to support a healthy adrenal stress response. And it looks like IF can do that, if properly implemented!
The stress hormone cortisol is a steroid stress hormone secreted by the adrenals, and involved in this whole stress thing. It serves to mobilize energy in the body, and is primarily released in response to situations where the body needs more energy (such as low blood sugar or stressful situations). From an evolutionary hunter-gather perspective, cortisol would spike in the morning to liberate fatty acids from the body for fuel, to support fasted activity as we went hunting for foods. Cortisol levels then tapered off by the evening: a time of eating, rest, and recovery (assuming your hunting was successful of course!) Healthy daily cortisol and circadian rhythms (determined by our exposure to sunlight) are therefore key for a healthy constitution and adrenal system. Cortisol should spike in the morning upon waking, causing you to be alert and active and jumpstart your day, and then taper off the by the end of the day, when you eat and sleep and night.
Common eating patterns, however, often go completely against our natural adrenal and stress rhythms. Rather than embrace our bodies natural inclination to run for bits in a fasted state and use fatty acids for fuel, we often constantly cram food into our mouths. This overabundance of ingested fuel actually raises inflammation and encourages a state of chronic stress in the body, which struggles to constantly deal with the excess. If what we're eating is processed, toxic, or otherwise inflammatory, it will further increase the stress response. In other words, common eating patterns are not good for the adrenals.
Intermittent fasting, however, can supports healthy cortisol rhythms in line with our evolutionary history. If implemented correctly, with adequate nutrition and recovery, it can encourage a daily rhythm of fat-fueled activity, autophagy (the process by which the body breaks down old proteins for growth and repair), and enhanced nutrition partitioning at night upon eating.
THE ULTIMATE DETERMINANTS OF REPRODUCTIVE STATUS
To swing back to the female/reproductive side of things, I personally believe a woman's fertility and adrenal status is likely more determined by the overall amount of chronic stressors and/or consistent dietary intake, rather than bits of fasting per se. Living in a constant, chronically stressed state is likely more detrimental than bouts of controlled fasting, which can actually serve to reduce feelings of stress. In fact, a 2013 Ramadan study of 313 fasting nurses, found fasting significantly reduced levels of stress and depression.
Following an IF approach with ample calorie consumption also departs drastically from common calorie-restricted diets, which do indeed create a state of chronic stress and threatened fertility. Like in the rodent studies discussed, a 40% reduction in calorie intake was ultimately the most detrimental to reproductive markers. But IF as I encourage it provides ample nutrition.
It has also been postulated that a lady's overall reproductive status - governed mainly by the hypothalamus - may rest more on the body's perception of long-term rather than short-term energy availability. In other words: her overall body fat percentage, rather than her transient dietary intake. This would be mostly determined by total fat on the body, rather than temporal changes in daily fasting or even in a given meal. This also means a girl with ample or excess body weight could likely safely fast longer with no negative hormonal affects, than an underweight girl.
And ironically, even if dietary restriction temporarily "pauses" the menstrual cycle, it may potentially extend fertility, possibly by preserving female eggs. In rodent studies, mice put on ADF protocols experience menstrual cycles later in life than mice who eat normally. And Dutch women who suffered extreme starvation during World War II ultimately experienced no change in fertility despite fluctuations during the war, and in fact bore children who were, in turn, more fertile. Go figure.
PRACTICAL IF ADVICE
Time for some practical implementation of all this fasting stuff! Use these tips and tricks to support a healthy IF adrenal response!
- Make sure you’re consuming enough calories in your eating window. If you're not eating enough, your hypothalamus may be more likely to interpret IF as a chronically restricted state.
- On top of that, make sure you're getting adequate nutrition. Favor whole, natural foods, while minimizing or (better yet) eliminating grains, sugar, and processed foods.
- Make sure your carbohydrate intake suits your personal health and adrenal needs. While some women can thrive on low carb and IF, others need more carbs for adrenal and thyroid health. Consider embracing healthy carbs in the forms of fruits and/or Paleo-friendly root vegetables, like sweet potatoes, yams, carrots, and rutabagas.
- Make sure you're eating enough fat for your personal nutrition needs. Some women do better on higher fat approaches, while others favor lower fat approaches. In any case, embrace healthy fats from sources like pastured animals, grass fed butter and/or egg yolks (if not allergic), avocados, fatty fish, etc.
- Pay acute attention to your overall stress levels. Lack of sleep, feelings of resentment, and other stressors can fill up your adrenal bucket. Embrace mindfulness, meditation, and social support.
- A life lived by constant fluorescent lighting and computer screens, can easily do a number on our adrenals. Studies even suggest that light exposure, rather than food intake, may be the primary factor regulating cortisol rhythms. Try to expose yourself to sunlight upon wakening, and minimize light exposure at night. Consider installing f.lux on your computer (it's free), using black out curtains, and wearing blue-blocking goggles at night. (So cheap, and seriously life changing!)
- Don't go too extreme with IF, especially if you feel like it is indeed taxing. You can always implement IF just a few days per week, rather than every day. You can also shorten your fasting window or lengthen your eating window.
- Pay close attention to overall adrenal, hormonal, and thyroid health. Get stress and thyroid panels if needed.
- Be mindful of your body, and how certain foods and eating patterns make you feel.
I believe we need to change our perspective of intermittent fasting. Saying any and all IF is automatically a harmful stressor, is like saying all eating is automatically a harmful stressor. In reality, the composition and implementation of such is what ultimately determines the cost/benefit. If you're eating processed fake foods 24/7, for example, then eating (despite its necessity) is likely doing harm to your body. On the contrary, if you eat healthy whole foods, eating overwhelmingly supports growth and renewal. In the same vein, if by "intermittent fasting" you fast extensively for hours on end without ample calories and nutrition when you eat, than IF will very yield negative ramifications. On the contrary, if you practice IF in a pattern supportive of your energy levels and activity, in tune with your natural circadian and adrenal rhythms, and with an abundance of nourishing whole foods in your eating window, IF can be a wonderful stress-slaying lifestyle indeed! In supporting a healthy body composition and wielding potent anti-inflammatory effects, IF can encourage an ultimate state of vitality.
Rather than focus on IF as a negative stressor, I encourage women to evaluate the entirety of their life stressors and, perhaps more importantly, reactions to such. In minimizing chronic stressors and embracing a meditative mindset of awareness, intermittent fasting can ultimately combat stress, rather than encourage it. So ladies, do the type of IF which makes you feel good. Eat the things which make you thrive (and plenty of them!). Love your body, knowing that it's just trying to protect you and your future. I also encourage you to be open to change, embracing the IF pattern in the moment which makes you feel best. Some times you may rock lots of fasting, while other times you may need to slow things down. Listen you your body and trust yourself. You totally got this!