Nobody is saying women are more delicate than men.
But it’s possible that intermittent fasting affects men and women differently. We could do with a lot more research in this area and by no means is this a blanket generalization; plenty of men dislike fasting and plenty of women thrive on it.
That said, there is evidence to suggest that women might be more susceptible to negative effects, so we interviewed a weight loss focused physician and looked at the research to address these issues.
- What Is Intermittent Fasting?
- Why Fasting Can Be Harder for Women
- Insulin Sensitivity
- Autophagy
- Cholesterol
- Is “Crescendo Fasting” Better?
(Editor’s note: The content on BarBend is meant to be informative in nature, but it shouldn’t take the place of advice and/or supervision from a medical professional. The opinions and articles on this site are not intended for use as diagnosis, prevention, and/or treatment of health problems. Speak with your physician if you have any concerns.)
What Is Intermittent Fasting?
To recap, we’re basically talking about meal skipping. Intermittent fasting, or “IF” is an increasingly common dietary protocol in which adherents go without food for extended periods of time, while still able to drink water and, usually, black coffee or plain tea. There are a few popular models:
- 16/8: You fast for 16 hours per day, including your sleeping hours. For most people, this is just skipping breakfast or dinner.
- Eat Stop Eat: You fast for 24 hours once or twice a week.
- Alternate Day Fasting (ADF): Every other day you go without food, with no calorie restriction on your feeding days.
- 5:2: Your “fast days” are really just low calorie days of about 500 calories. It’s debatable as to whether this should actually be considered IF.
Note that there are plenty of folks who fast for days at a time, but here we’re focusing on the more popular methods that don’t usually involve fasting for more than thirty-six hours.
Contrary to established “wisdom,” skipping meals does not appear to slow your metabolism or cause you to increase your body fat, nor does eating more frequently “stoke your metabolism” and increase fat loss.(1)(2)(3)(4)(5)(6)
[Get more in depth in our complete introduction to intermittent fasting!]
Debate rages as to whether or not fasting is superior for fat loss or if it’s about as good as just eating the same number of calories across the same timespan, as studies conflict. Evidence is leaning toward the fat loss being about the same, but many find that eating one or two larger meals is more satisfying and less mental work than smaller ones throughout the day, though this depends on personal preference.(7)
So what about the sex differences?
Why Fasting Can Be Harder for Women
“Some of it could be related to hormones,” says Dr. Aastha Kalra, a New York-based physician who often prescribes fasting to her patients. “In general, there’s something called gonadotropin releasing hormone in everyone. The gonads to respond to it, so for females it causes the ovaries to release progesterone and in male the testes to produce testosterone.”
The process is highly regulated in women as it’s involved in ovulation, which is reliant on cycles and schedules. It’s possible that among women, gonadotropin releasing hormone is more easily disrupted by changes to one’s habits and routines, so skipping one’s usual meal can sometimes cause more discomfort among women than men.
“Women have been shown to have a higher level of a protein called kisspeptin, which causes a greater sensitivity to fasting,” adds Dr. Kalra. “While more research is needed to be done on this, it makes sense to logically conclude that the hormonal shift (from fasting) can definitely affect the metabolism.”
This is just one possible explanation and as Dr. Kalra notes, more research is needed. However, some rodent studies have found fasting to negatively affect reproductive hormones in females, and limited research on humans have suggested women may have tend to have greater difficulty suppressing hunger during fasts.(8)(9)
A couple of studies have also found interesting differences in the nervous system: men became more parasympathetic from fasting, meaning their nervous system was less agitated, whereas women became more sympathetic, meaning their bodies were more stressed, more in the “fight or flight” state.(10)(11)
[Learn more about how to stimulate your parasympathetic nervous system.]
Insulin Sensitivity
One reason why some people like IF better for fat loss than your standard gradual calorie restriction is that abstaining from food entirely means no insulin gets released. That means you may get more sensitive to the effects of insulin (or less “insulin resistant”) because you’re not secreting as much of it, and since insulin helps with nutrient absorption and processing carbohydrates, a lot of people see insulin sensitivity as important for ideal weight loss.(12)(13)
Some very limited research has suggested a different effect on women, like a 2005 study on eight men and eight women that found alternate day fasting to adversely affect glucose tolerance, which is associated with insulin sensitivity, in the women and not the men.(14)
However, larger, higher quality studies refute this. One randomized controlled trial of a hundred overweight or obese women found that six months of IF did indeed decrease insulin levels by about 29 percent and improve insulin sensitivity.(15)
“In another study, about eight to twelve weeks of IF was shown to decrease insulin levels by twenty to thirty-one percent and blood glucose levels by six percent in individuals with prediabetes,” says Dr. Kalra.(16) “In this particular study, they did not see much difference in insulin sensitivity between men and women.”
We need more research here and while it’s not conclusive, these studies (especially the randomized controlled trial) were of higher quality than the smaller one from 2005, so differences in insulin sensitivity may not be as big a concern as some people thought.
Fasting and Autophagy
Another big benefit linked to fasting is autophagy, which means “self eating”: it refers to when the body eats up and recycles dead or damaged cells. Fasting has been linked to increased autophagy, especially in the brain, which might be why it’s sometimes linked to lower risks of some neurodegenerative diseases.(17)(18)(19)(20)
Again, there’s very little data here, but a 2009 study on mice found that males experience significantly more autophagy in the brain when fasting than females.(21) But then again, those are mice.
“So there’s more ongoing research in the differences in autophagy in men versus women,” adds Dr. Kalra. “We do understand there is autophagy benefit, but the timings in men and women might be variable.”
Put simply, there’s currently nowhere near enough evidence to make the claim that human males and females experience different levels of autophagy.
Fasting and Cholesterol
Finally, one study suggests fasting affects men and women differently with regard to heart health — but women may have the upper hand here.
Now, a 2010 study published in Obesity had twelve women and four men undergo ten weeks of alternate day fasting and found little difference between the two sexes.(22)
“But a study published in Clinical Nutrition ESPEN showed improvements in HDL, or ‘good’ cholesterol from fasting and women actually showed greater improvements than the men,” says Dr. Kalra.(23) “Similarly, women showed slightly lower levels of triglycerides in this study than the men in the Obesity study, so there might be some correlation with women doing slightly better as far as heart disease and risk of strokes.”
Once again, though, more research is needed, especially as some research has found fasting to not affect cholesterol or triglycerides at all.(24)
Crescendo Fasting
The fact is that everyone, not just men and women, are different in how they respond to fasting.
That’s why if it’s something you’re interested in trying — and remember you really don’t have to do it if you don’t want to — Dr. Kalra recommends what she calls “crescendo fasting” for people who have trouble with it.
“I start with short fasts once a week, such as between twelve and sixteen hours, and I will recommend that for two weeks, then after two weeks I might add another day,” she says. “I recommend light exercise such as walking or gentle yoga on the days that they are fasting.”
Wrapping Up
The vast majority of data does suggest benefits for insulin sensitivity and body composition among men and women. That said, it isn’t recommended if you’re pregnant, lactating, or having irregular periods, nor is it recommended if you have diabetes or insulin resistance. Many find success with it but remember that it’s individual: there’s no reason to try fasting if it’s something that you feel will add anxiety to your life or if you feel it will disturb your relationship with food.
Even if you’re perfectly healthy, it’s smart to talk to your physician before embarking on a new weight loss regimen.
References
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2. Zauner C, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000 Jun;71(6):1511-5.
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11. Solianik R, et al. Two-day fasting evokes stress, but does not affect mood, brain activity, cognitive, psychomotor, and motor performance in overweight women.Behav Brain Res. 2018 Feb 15;338:166-172.
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20. Uchiyama Y, et al. Autophagic neuron death. Methods Enzymol. 2009;453:33-51.
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