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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.