One Dirty Magazine

Fasted Training May Have Long-Term Risks, Especially For Female Athletes

Training in a depleted state could have different effects on female athletes and male athletes. But for everyone, it comes with potential risks.

David Roche January 6th, 2020

Fasted Training May Have Long-Term Risks, Especially For Female Athletes

Female athletes and male athletes respond to some training interventions differently. And the discussion around glycogen depletion might be one of the biggest offenders when it comes to failing to fully account for sex differences.

Let’s take a step back. Why do I think this issue is important enough to warrant an article? Here’s a scenario that plays out more often than you may think. A female athlete will ask me if she should eat to fuel activity, spurred by a male training partner that swears by depleted runs. That male athlete may have heard that Kilian Jornet can do long runs without fuel, or remember reading Anton Krupicka blogs about doing the same. Maybe they even read some of the scientific articles pointing out potential benefits of fasted training. And based on the physiology, it may have some benefits for the male athlete in moderation.

But for the female athlete, it could be lighting a fuse on athletic stagnation and hormonal crisis.

Or it might not. Physiology is fascinating because it’s so highly variable. But no matter what you do with specific nutrition practices, every athlete needs to make sure they are eating enough always. That’s the essential point to remember in all nutrition discussions.

Physiology is fascinating because it’s so highly variable. But no matter what you do with specific nutrition practices, every athlete needs to make sure they are eating enough always. That’s the essential point to remember in all nutrition discussions.

Nutritionist Kylee Van Horn of Fly Nutrition supports athletes I coach on these issues, and she has two pieces of advice that will underpin the discussion below. First, Van Horn says, “Athletes should really be focusing more on timing and composition of pre- and post- workout fueling that we know produce real benefits.” Second, “These approaches often end up being more stressful than trying to just focus on eating enough and eating well.”

Van Horn says that the question isn’t whether or not you should do fasted training, but instead how can you make sure you are always eating plenty to fuel activity, since that is essential for long-term growth. Let’s delve into some of the science to see how this particular issue overlaps with performance and health.

The Case For Fasted Runs

Here, we are defining fasted runs as training in a depleted state, usually meaning in the morning without pre-exercise fuel. Fasted runs could have a few benefits. First, some studies point toward strategic low-glycogen availability during training sessions possibly increasing the capability for the body to oxidate fat to fuel exercise. A strong summary of these concepts is in this 2015 article in Nutrition and Metabolism.

Fat oxidation is important because of how the body produces energy during running. Aerobic threshold is the intensity range when the body switches primarily from fat oxidation to primarily relying on carbohydrates (glycogen). That is a barrier for how hard an athlete can go in longer events, since glycogen stores burn off relatively rapidly, but refill at a drip. As summarized by this 2010 article in Computational Biology, that equates to up to a couple hours at marathon pace. Improve fat oxidation just a bit, and an athlete can excel at longer races by pushing closer to the limit from the starting gun. 

Second, there is some evidence that biomarkers associated with adaptation could be increased when training in a glycogen-depleted state. For example, a 2015 article in the European Journal of Sports Sciences found that training in depleted states could enhance activation of key cell signaling kinases, transcription factors and transcriptional co-activators, causing changes in mitochondria and fundamental aerobic adaptations.

It only gets more complicated from there, as outlined in this 2018 article in Cell Metabolism and summarized further in my nightmares about biology class. That could be one reason that two runs in one day can be effective. Since glycogen stores refill slowly, the second activity often involves training in a low-glycogen state even with adequate fueling in between, possibly without the potential breakdown associated with intentional depletion.

Put it all together, and there are likely benefits to periodic fasted runs for some athletes. … they need to be done strategically, with purpose and planning, because fasted running may come with long-term risks.

Third, proponents argue that approaches like intermittent fasting can improve body composition. I’m not getting into the studies here because as reviewed in this 2019 article in the International Journal of Sport Nutrition and Exercise Metabolism, caloric restriction can be fundamentally bad for athletes. Make sure you’re eating enough food. And this 2014 study in the Journal of the International Society of Sports Nutrition found no changes in body composition with fasted or non-fasted exercise when controlling for total caloric intake. Bonus: that study involved female athletes exclusively, which is somewhat uncommon and totally awesome.

Put it all together, and there are likely benefits to periodic fasted runs for some athletes. There are amazing success stories from athletes who do fasted runs, and I am not discounting those. Either way, they need to be done strategically, with purpose and planning, because fasted running may come with long-term risks.

The Case Against Fasted Runs

Even if fasted runs in moderation may improve fat oxidation and cellular signals related to adaptation, if you zoom out a bit, the evidence can be feel like a house of cards in a hurricane. Yes, fat oxidation may increase, but total output likely has to decrease relative to absolute potential.

For example, a 2008 study in the Journal of Applied Physiology found that even moderate depletion lowered performance in training sessions. Multiple studies back-up that finding. It’s possible that the performance detriment from depletion can be reduced with training, but it will likely undermine maximum performance in a training session except during low-intensity easy runs and long ultra events no matter how fat-adapted an athlete is.

Even if fasted runs in moderation may improve fat oxidation and cellular signals related to adaptation, if you zoom out a bit, the evidence can be feel like a house of cards in a hurricane. Yes, fat oxidation may increase, but total output likely has to decrease relative to absolute potential.

On top of that, low energy availability can impact health even in windows shorter than a day. A 2017 article in the Scandanavian Journal Of Sports Medicine found that female athletes with menstrual dysfunction and metabolic disturbances spent more time in a low-energy state, even when controlling for energy availability in a 24-hour period. A 2018 article in the International Journal of Sport Nutrition and Exercise Metabolism had a similar finding for male athletes, with decreased muscle breakdown, worse hormone balance and metabolic disturbances from within-day deficits. 

At its best, indiscriminate fasted running risks making athletes more efficient at being slightly inefficient. At its worst, fasted running can cause health crises.

Zoom out and you see the potential problem. Long-term adaptation for most athletes requires efficient maximum performance and health. At its best, indiscriminate fasted running risks making athletes more efficient at being slightly inefficient. At its worst, fasted running can cause health crises.

Why Female Athletes May Want To Avoid Planning Fasted Runs

Here’s where the rubber hits the road in this article. Dr. Stacy Sims has led the way in this field, so check out her book Roar and consume all of the content she puts out if you can. Her work and the research generally refutes the potential benefits of planned fasted runs for female athletes.

There is a theory that female athletes will not improve fat oxidation substantially with fasted runs. According to Dr. Sims, the hormonal state of female athletes results in higher rates of fat oxidation naturally, which is one reason that women can often improve performance relative to men in longer events. 

Instead, there are cascades of hormonal responses to low-energy states for some female athletes. Concentrations of the stress hormone cortisol go up, and some research indicates that female athletes can be more sensitive to its effects. In moderation, cortisol isn’t bad, but with chronically elevated levels like those from consistent fasted running, overall hormone balance can go haywire. Reproductive health may be affected, possibly contributing to amenorrhea.

Hormone perturbations can also contribute to fatigue, bone stress injuries, and (possibly counter-intuitively) weight gain from reduced metabolic rate. Recovery and adaptation may slow down too. For example, a 2010 study in the Journal of Science and Medicine in Sport found markers for muscle adaptations responded better in non-fasted female athletes, with male athletes responding more to fasting. 

That’s not to say that female athletes should never do fasted runs, or need to have a gel on every run over an hour. While female athletes and male athletes have different physiology, all are homo sapiens, and some of the benefits may still be present in small doses. In addition, for transgender athletes, these considerations could be slightly different, since a transgender/nonbinary person taking hormone-replacement therapy receives hormones through a different mechanism than a cisgender person (though the responses to stimuli are likely similar).

The overall point: there’s a good chance that the strict approaches to fasting and glycogen depletion that may work for some male athletes in moderation will work for very few female athletes.

The overall point: there’s a good chance that the strict approaches to fasting and glycogen depletion that may work for some male athletes in moderation will work for very few female athletes. And as Van Horn says, “Restriction of food is generally not mentally healthy.” Instead, she advises athletes to ask the following question: What can I add to my nutrition to improve my performance and health?

Food is amazing. Make sure you are always eating enough to fuel training and life.

 

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. His book, The Happy Runner, is about chasing your potential and having fun along the way.

 

 

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Rebecca T.Kenny McFaddenBrianKarla WagnerMaybe you should read more carefully Recent comment authors
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Scott Reeves
Guest

What about the runner who is in ketosis? They always have body fuel in the form of fat. Any research on this?

Rory Lewis
Guest

Thanks for the very biased opinion against fasted running. It’s interesting how there are zero citations to white papers written by MD PhDs from Johns Hopkins, NIH, EU, NSF and the USOC, to name a few who say the exact opposite. Even anecdotally, the evidentiary proof of Bushmen, Kenyan’s and other very skinny tribes that run +30 miles without water or food has prompted the EU equivalent of the NIH to fund why fat Europeans and obese Americans insist we stuff our faces with food before our run, is better than the skinny lithe athletes such as the aforementioned Bushmen,… Read more »

Maybe you should read more carefully
Guest
Maybe you should read more carefully

Weird, none of the runners you mention are female, and that’s the focus of this article, with plenty of peer-reviewed journals cited….I find your comment is off-base and potentially damaging to female runners looking for actual advice on what fasting runs could mean for their bodies. As a female, professional runner, this is the type of article I’m grateful for. I’m sick of men citing men and anecdotal men’s experiences on how and why I should train a certain way. All the best to you. I encourage you to read the many cited peer review articles above, from my count,… Read more »

Annie
Guest

Thank you, David, for yet another reminder to just eat the tacos. 🙂 Many female runners, myself included, have a long and storied background with less-than-ordered eating habits. We’ve got to love ourselves and eat what our bodies (truly) ask, and let the rest sort out in training.

Caolan macmahon
Guest

A lot of this depends on the PACE at which these runs are done (relative to the individual runner). If the aim is glycogen depletion, that’s one possible aim. However, in many cases the aim is not glycogen depletion but rather taping into fat metabolism. Since the body will use the fuel that’s easiest to access, then easy carbohydrates will be used first. If you are doing an aerobically paced run and sucking down gels your body will use those first. If, however, you run at an aerobic pace where glycogen is not the first choice of fuel, then you… Read more »

Lee
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Lee

I wish I could have all of my teammates realize this. I am the only one that ever eats before our morning runs, or even a lot of times they will barely eat before afternoon practice. They always say it doesn’t matter because they will just eat a lot after, and that they aren’t hungry enough or don’t feel like eating before. It is very frustrating.

Todd Baum
Guest
Todd Baum

This is what drives me crazy about reporting on nutrition science. When you are in the realm of “may have long term risks” why report on it at all. Thousands of runners get up early every morning and run fasted without thinking about it (particularly women before a busy day with small kids and work). Now we just have one more thing to worry about. Clearly, just plain common sense says I’m going to be hungry and thirsty, and it’s unlikely there will be any long term risks. Really, we’re not going to eat enough?

Kenneth Posner
Guest

David With all respect, I think you’re approaching this topic from a very narrow angle. Speaking for myself, I am one of the many people whose stomachs don’t tolerate sugary stuff on long runs. Over the years, I experimented with many different fueling strategies, and eventually concluded, the less, the better. I remember how back in the day, without gels I’d predictably hit the wall at 16 miles and feel awful. But with training, I ended up running all my marathon PRs with 0 kcals and eventually ran as far as 50 miles without any kcals, too, albeit at a… Read more »

Karla Wagner
Guest
Karla Wagner

I can’t say that I appreciated any bias against fasted exercise. Clearly, individuals will respond differently to this training method and I agree there is research to support some adaptations plus a lot of anecdotal evidence. However, I think one will be pressed to find studies that examine the effect for women over an extended duration which I believe is the point of the article. Virtually all studies involve small numbers of healthy males in their 20’s (as, unfortunately, is the case for most exercise physiology articles). Thank you David for another thoughtful article.

Brian
Guest
Brian

So basically there’s a study to show that any training style can be bad. Or be good. So just choose what works and feels good for you. And don’t worry about what the others are doing. Got it!

Kenny McFadden
Guest
Kenny McFadden

So , what you’re saying from a male “intermittent fasting” standpoint is that it’s up to their individual goals of meeting their ultra-race goals yet their body composition will not change? So my “dad-bod” most likely will not lose its stomach tire even if I take in the same calories as a non-fasting ultra runner?

Rebecca T.
Guest

Sorry, this article does not make much sense to me. Why would levels of cortisol go up in fasted females but not in males? And doesn’t all moderate to high output exercises cause a temporary rise in cortisol for anyone, regardless of gender? That exercise induced cortisol production is known as the stress response, and some level of it is good as long as there is sufficient rest & repair right? And if this is about women, why no mention of sex hormones? How does fasting affect levels of estrogen, testosterone, and progesterone? I have switched from being a late… Read more »

 

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