“So what’s with this fasting thing. Should I be doing it?”
If I had a nickel for every time I’ve been asked this question this year, I’d be able to buy enough bacon to feed all the fasted cardio people on the step mills.
Well, that might be an exaggeration.
But have you heard?
Outside of the ketogenic diet, FASTING is the next big thing!
“Intermittent fasting improves metabolic rate”1, 2, 3, 4
“Intermittent fasting improves growth hormone and testosterone”5, 6
“Intermittent fasting improves will power, sleep, and even sex drive”7, 8
A quick internet search will yield 3,130,000 (at the time of this writing) hits most touting the amazing benefits of going without food for 8-30 hours at a time and gobbling up your meals in 1-2 large sittings.
AND many of those articles are backed by ample, sound, and positive research to boot as cited above.
So what’s my deal? Are you ready for the hammer?
Do a similar search of the research and the craze is leaving out an amazingly important caveat.
Ladies beware. If you Google, “how does fasting affect me as a woman?” You’ll get very few answers. Let’s explore.
Men and women are different...
It pains me that I even have to type these sentences.
Politics, economics, and cultural arguments aside, biology has formulated men and women differently at a genetic (and thus phenotypic) level – and the differences go beyond the simple X and Y chromosomes. These differences are typically fairly basic, such as men are generally taller 9, and women have smaller lungs 10.
What does this have to do with intermittent fasting?
Well, the roles dictated by biology has had a part to play in shaping male and female metabolic responses to exercise, carbohydrates, sleep deprivation, and yes – you guessed it – fasting!
You see, pre-agricultural man was a hunter-gatherer society meaning that there were times of plenty and times of scarcity (unlike today where calories always abound) to which men and women adapted differently. For our purposes, understand “scarcity” to be “fasting”.
So what happens in a time of fasting to the pre-agricultural man and woman?
Well, males with their generally larger physical size respond to fasting with a giant boost in metabolic rate to fuel a hunt. “Go get food for everyone,” says the genetic makeup of a male when he hasn’t had much to eat.
And the research bears that out. During short periods of fasting, (12-24 hours) men’s metabolisms increase up to 14% 13, testosterone utilization increases (depending on a number of factors) 10%-200%, growth hormone increases 100%-2000%, and blood lipids improve to support the increased hormonal production and decrease risk factors for cardiovascular disease. 20
Yay manhood. Yay fasting. If you have male clients, intermittent fasting can be a great way to give them a shove toward better body composition!
Hey wait, I thought we were going to talk about women?
In a stroke of gender bias, out of 71 studies found in Harvard’s Database for intermittent fasting, only 13 include women at all and guess how many controlled studies were about the female population in general?
Zero. Zip. Zilch. Nada. There were no controlled studies that allow us to draw intelligent conclusions about how intermittent fasting affect the female population.15
For example, 1 of the 13 female studies I found was on PREGNANT women fasting for RAMADAN. They found no improvement in insulin sensitivity and an increase in blood lipids which is the opposite of most articles touting the practice of fasting.16 However, EVERY woman who has been there knows her body is completely different when pregnant vs. not pregnant! (though it should be noted that even these imperfect studies didn’t get any press for their counter-current findings.)
When you sift through all of the precious little data on women in a fasted state you find something fascinating: Women don’t respond to fasting like men do.17
In fact, instead of the sunshine-and-roses of manly metabolic boosts, women might find a 50% increase in cortisol21 and a decrease in insulin sensitivity that could lead to obesity and diabetes!18
Women's Body Composition Suffers from IF
You heard me. While fasting appears to be a great practice for some men to undertake, it seems that it could be a VERY BAD IDEA for most women if they have any kind of body composition goals.19
And we can easily account for this by exploring biology again.
In pre-agricultural society, while the tribe was starving and men were out hunting to fix the issue (with the metabolic boost from a lack of food), what’s going on with the women?
Well, they were back at the camp, growing a baby, breastfeeding, or otherwise caring for the children (who need to eat too, even if that means mom can’t) and their metabolism was wired to slow down, conserve energy, store fat, and buckle down for the potential long-term famine.
What that means for your female clients is that due to inherent biological factors, intermittent fasting practices may not be right for them.
It appears that while men might benefit from skipping breakfast and/or the occasional day without food, women who want to optimize body composition, have consistent and brimming energy, and continually improve their workout performance would do best to maintain an unchanging flow of high-quality calories during their day.
The old guideline “eat 4-6 small meals per day” seems to apply more consistently for ladies than for gentlemen.
Bottom Line: Research is Lacking
As a trainer and hard-core research junkie, it fascinates me that so many of the disparities in studiable health and fitness practices (High carb vs. Low carb; Weight training vs. Cardio training; Volume vs. Intensity) could be chalked up to a failure to study different client types in a controlled way. This is one of the reasons our profession exists: One Size Does Not Always Fit All.
If you aren’t already, I encourage you to be skeptical in the field of “personal” training when the next diet or exercise FAD claims to be true for EVERYONE.
Inherently, we know that men and women need different things from us as trainers (in the same way people of different ages, training experience levels, and somatotype need different things from us as trainers) and I hope one day the science of nutrition and exercise will catch up to our intuition.
The takeaway: science suggests that some males can skip breakfast and receive possible benefits.
However, it seems that “IF” for most females isn’t currently supported by research, thus it is better for most women to eat on a consistent and frequent schedule lest the metabolic disturbances result in a decreased level of overall health for women.
Coach your clients accordingly.