Hello, ladies! That’s not meant to alienate you, fellas. I know there are plenty of guys interested in this complex, heavily debated question as well. Intermittent fasting is one of the hottest dietary trends. And unlike many dietary fads, this one is actually supported by a significant and growing body of evidence—both in the lab and the real world. But what about intermittent fasting for women? Does it actually work? Or, is it harmful?
There’s a lot of “noise” out there, so let’s sort through it and unpack the evidence!
Intermittent Fasting for Women Can Work—Except When It Doesn’t
You’ve likely heard about intermittent fasting (IF) and its various forms. [Note that IF is an umbrella term. It encompasses alternate-day fasting, periodic fasting, and time-restricted feeding. While there are important nuances, for the purposes of this article, we’ll lump them all together. Keep in mind, however, that the differences are significant enough that you may find that one form suits you better than another. It’s worth experimenting to find what works best for you.]
On one hand, you may have heard IF is the best thing since sliced bread. All you have to do is worry about when you eat. You don’t have to be overly restrictive with what you eat. You don’t have to count calories. And you’re afforded quite a bit more dietary flexibility—as long as you eat within a pre-determined window of time.
On the other hand, you may have heard horror stories, particularly from women who’ve dabbled with IF and suffered from unbearable consequences. These can include insatiable appetite, no weight loss (or even weight gain), poor blood glucose regulation, fatigue/energy drain, brain fog, disrupted menstrual cycles, low libido, and thyroid issues to name a few.
That’s not meant to scare you off. If nothing else, these case reports provide evidence that there’s no one-size-fits-all diet. There are inter- and intra-individual differences. In other words, nothing works for everyone. What’s more, what “works” for you now may not be the right fit for you at another point in your journey.
And when it doesn’t seem to be the right fit and goes awry, it has less to do with IF than it does with the female body’s perception of energy availability. Let me explain.
Women’s Bodies Are Smarter, Stronger, More Stubborn, and More Complicated
Example 1: Women & hunger Hormones
From the onset of puberty all the way through menopause, women, on average, maintain a greater percentage of body fat than men despite less caloric intake (relative to lean body mass) and preferential use of fat as a fuel during exercise compared to men. 1 In other words, despite eating less and burning relatively more fat during exercise, women tend to store more fat than men.
And despite burning relatively more fat during exercise, women tend to store fat more efficiently than men when they’re not exercising, which is a pretty hefty chunk of time for most. Even more, female bodies seem to “defend” body fat more aggressively than males, often through hormonal sex differences that affect appetite and caloric intake as well as energy expenditure.
Take the “energy-sensing” hormone leptin, for instance. Leptin seems to regulate or be intricately related to many factors tied to energy balance, such as caloric expenditure, appetite, activity of food reward systems, reproductive hormone output, sympathetic nervous system tone, thyroid output, and more.
While there’s a lot to leptin, something I want to point out is that leptin levels typically drop in the face of caloric restriction (i.e., dieting). However, this effect seems to be much stronger in women than in men. This provides evidence that women are much more sensitive to the threat of energy restriction.
For example, in a study published in the journal Metabolism, researchers found leptin levels dropped significantly in both men and women following just 7 days of a reduced-calorie diet. But here’s the kicker. While the guys’ leptin levels decreased 36%, on average, the women’s leptin levels plummeted a massive 61%—nearly twice the reduction of the men. 2 Yes, the women had higher levels to begin with, but that difference is normal and is null when adjusted for circulating concentrations of sex hormones. 1
Another example of the stubbornness of women’s bodies can be found in the notorious “hunger hormone” ghrelin. Ghrelin stimulates appetite and turns on reward centers in the brain (driving you to eat calorie-dense, highly-palatable foods…and lots of them). Considering that ghrelin is leptin’s counter hormone, it may not surprise you that, when faced with reduced energy availability (i.e., caloric restriction), women demonstrate robust compensatory appetite, food intake, and hunger hormone responses (e.g., ghrelin, PYY). 3
Here’s something else interesting about ghrelin: It’s an “entrainable” hormone. What I mean by that is the body releases it at “normal,” anticipated meal times. This explains why you likely tend to feel hungry around the same times of day—regardless of what you have (or haven’t) eaten prior. In other words, ghrelin is not just a “hunger signal.” It also rises “in anticipation of food intake based on learned, habitual feeding patterns.” 4
In my experience, women tend to be more sensitive to alterations in meal timing. And intermittent fasting for women certainly involves deviating from one’s “normal” eating schedule in some way or another. Those elevated levels of ghrelin at anticipated meal times (i.e., ghrelin entrainment) may be in part to blame.
Example 2: Women, Energy and Glucose Intolerance
This may also be due to gender differences in metabolic responses—namely blood glucose—to fasting and changes in meal timing. More precisely, women tend to demonstrate glucose intolerance and greater swings in glycemic variability in response to fasting. Regardless of the mechanism, women, in general, seem to have a harder time adjusting to new feeding schedules and/or going longer periods of time without eating.
Practically speaking, women’s bodies rebel when they perceive energy is scarce. They get hungrier, they eat more, and it’s not their fault. Their hormones are working against their efforts, often with their best interests in mind. In addition to winding up with more ghrelin and lower concentrations of leptin (and insulin), all of which lead to a greater appetite and drive to eat, energy deficiency can also have a profound impact on reproductive hormones, which makes sense from an evolutionary standpoint.
And this is a frequent concern, not just when it comes to intermittent fasting for women but marked caloric restriction in general, which may perturb the hypothalamic-pituitary-gonadal axis in women and alter the frequency and length of menstrual cycles. For instance, a long-term diet (i.e., chronic energy deficit) suppresses ovulation, inhibits gonadotropin-releasing hormone secretion, reduces pulsatility of luteinizing hormone, and lowers libido in women. In men, however, energy deficiency seems to have no major impact on reproductive success.
Such effects are likely to be related to the starting weight of the individual, overall energy balance, and the number of consecutive restricted days with IF. 5 There are also well-documented differences in metabolic responses to periods of fasting and marked energy restriction between pre-menopausal women (i.e., increased ketones and free fatty acids) compared to men and post-menopausal women suggest possible different metabolic responses, and perhaps better tolerance to IF within certain populations.
Example 3: Hormone Balance & Intermittent Fasting
In general, women’s hormonal mechanisms seem to be more sensitive to real or perceived energy deficits than men’s. There is also a complex set of mechanisms that sense and regulate energy availability, including:
- Hypothalamic sensors
- Pituitary sensors
- Adrenal sensors
- Adipose sensors (such as leptin)
- Gonadal sensors
- Thyroid sensors
- GI sensors (such as ghrelin)
Beyond appreciating the beauty and complexity of the system, what’s important to reiterate here is that women tend to be more sensitive than men to the effects of nutrient depletion and energy deficiency. More simply put, it’s a lot easier to affect women’s hormones with dieting. And factors like starting weight, degree of caloric restriction, and activity levels can all play a role. For example, leaner women, more severe energy deficits, and very active women seem to command greater sensitivity to dieting.
Here’s another key point: It’s not intermittent fasting for women that’s problematic per se. It’s the body’s perception of insufficient energy availability, and fasting is one such threat. In other words, dieting is hard on women’s bodies. Problems arise with caloric restriction, not IF specifically (although fasting can obviously be perceived by the body as a sign of energy deprivation).
With all that said, somewhat surprisingly, IF actually works quite well for women.
The Evidence: Intermittent Fasting for Women
While we can’t ignore key physiological differences, we can’t simply make assumptions. Instead we have to make informed decisions based on the evidence (both published research and real-world experience).
Along those lines, in a recent systematic review with meta-analysis (a critical review of existing research often regarded as the highest form of scientific scrutiny) published in the journal Obesity Science & Practice, researchers from Johns Hopkins Bloomberg School of Public Health compared results from alternate-day intermittent fasting (ADF) trials to those of very-low-calorie dieting (VLCD), a common and effective (albeit quite extreme at 800 calories per day) weight-loss strategy. 6
As far as this form of intermittent fasting for women, the researchers reported, “While these differences, on a physiologic level, are important in determining which diet may be most appropriate for a given patient, it is important to note that no gender differences in weight loss have yet been shown in human studies.”
What’s more, the researchers also concluded, “ADF is an efficacious dietary method and may be superior to VLCD for some patients because of ease of compliance, greater fat‐mass loss, and relative preservation of fat‐free mass.” As an added bonus, VLCD tend to increase the risk of headache, fatigue, dizziness, hair loss, constipation, and dehydration—not to mention VLCD limits you to 800 calories per day while ADF allows you to essentially eat freely every other day.
Also of interest, of the studies that were included in this review, women represented 92% of the participants in the IF trials. This is important to point out because, in general, IF (particularly alternate-day fasting) is heavily studied in women with overall positive results. Particularly as “a useful strategy to promote weight loss, without the concern for caloric compensation.” 5 Women are such a focal point of IF research, in fact, that leading fasting researchers have called for further study “in diverse populations including men.”
While several studies comparing IF to daily caloric restriction have demonstrated comparable reductions in body fat, multiple randomized trials have found intermittent fasting leads to greater loss of body fat. 5,7,8 In other words, at worst, IF is at least as effective as traditional dieting. And there’s some evidence suggesting it may be superior—at least for weight loss and markers of overall health.
In a recent review study published in the journal Obesity Research & Clinical Practice, a group of leading intermittent fasting researchers set out to examine the existing body of evidence on alternate-day fasting (one popular version) to see if they could identify what characteristics predict weight-loss success. Outside of age and race (Caucasian subjects tend to achieve greater weight loss), they found baseline body weight and baseline body mass index (BMI) did not predict weight loss. And as it pertains to the present topic, they found that “males and females achieved similar weight loss” with this variation of IF. 9
Interestingly, they did report that folks aged 50 – 59 years of age achieved greater weight loss than other age groups. Although they did not specify gender/sex playing a role, I don’t think it’s out of bounds to infer that this provides some evidence that pre-menopausal women may be more sensitive than post-menopausal women to IF (and caloric restriction, in general).
In a study published in the journal Frontiers in Physiology, a group of researchers explored the effects of a high-protein, IF, low-calorie diet plan on weight loss and other health markers among obese men and women. 10 At the end of the experiment, their results revealed concomitant reductions in body weight, BMI, and blood lipids. And they observed “no sex-specific differences in responses.” The researchers concluded, “A high-protein, intermittent fasting, low-calorie diet is associated with similar reductions in BMI and blood lipids in obese men and women.”
While alternate-day fasting is the most heavily researched application of intermittent fasting, it’s not the most heavily practiced. That crown belongs to time-restricted feeding (TRF), which typically involves daily fasts ranging from 16 – 20 hours interspersed with feeding “windows” of 4 – 8 hours. Despite its popularity, TRF hasn’t been studied quite as extensively. Plus, among the well-controlled human trials, a number of them have involved only men.
Having said that, in two time-restricted feeding studies that did include women, the results have been quite promising. For example, in a trial published in the journal Nutrition and Healthy Aging, 23 participants (20 of whom were women) who participated in an 8-hour TRF protocol (eating as much as they wanted from 10:00 am to 6:00 pm) lost, on average, 2.6% more body weight than a control group over 12 weeks. 11 Although that may not seem like a tremendous amount of weight lost, keep in mind that participants only restricted when they ate, not how much (i.e., no conscious caloric restriction).
Intermittent Fasting For Women ISN’T One Size Fits All [SHOCKER]
Although individual differences apply (as always), intermittent fasting for women CAN work. It’s not, however, a one-size-fits-all approach. Generally speaking, women tend to be more sensitive to…
- Changes in caloric intake (i.e., caloric restriction)
- Changes in meal timing/frequency (e.g., hunger hormones, glycemic variability
In other words, when problems arise, it may simply be a consequence of the circumstances. In general, pre-menopausal women may be more sensitive to IF than post-menopausal women. Other factors—such as degree of caloric restriction, level of leanness, and activity levels—may also contribute to tolerance (or lack thereof).
It’s also worth reiterating that there are different forms of IF you can “try on.” Although time-restricted feeding is the most commonly practiced, alternate-day fasting is actually the most heavily researched. And participants in those studies heavily favor women.
Finally, while intermittent fasting for women can be an effective tool for weight management and overall health, there are several populations for whom it is not advised. These include pregnant and nursing mothers, children and adolescents, folks who are severely underweight, and people with a history of or propensity toward disordered eating to name a few.
What about you? Have you experimented with IF? How has it worked for you? Let me know, and if you have any questions, be sure to share them below.