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Curious About Trying Fasted Runs? Here’s How to Do It Right.

Two nutritionists weigh in on the benefits and risks of running fasted, and provide guidelines if you're thinking about experimenting.

Do you ever wake up for an early-morning run without much of an appetite, down a cup of coffee, and head out for your run within the hour? That’s actually my MO for most of my weekday runs lasting about an hour or less. This is a form of fasted running or cardio, which just means working out after not eating for an extended period of time, usually 10-14 hours. 

Like many runners, I am intrigued by the concept of running fasted because of touted benefits such as burning more fat (versus carbohydrate) as fuel, increased weight loss, a leaner body mass, and boosted performance. While some research has shown that fasted training can help your body tap into fat stores sooner and burn more fat during those training sessions, it can backfire in the long-term when it comes to performance improvements. 

While I tend to feel fine and finish most runs feeling strong, running fasted isn’t necessarily for everyone. As with most things, there’s a time and a place for it. In fact, a 2020 study showed that high-level athletes improved their performance by doing some of their runs, specifically their second runs, in the latter part of the day without any carbohydrate intake.

That, however, is a pretty specific context. Kelsey Beckmann, RD, a sports dietitian in Jacksonville, Fla., says that the study primarily shows that if you’re going to try fasting, it is best suited for easy runs less than an hour in duration at low intensities where your heart rate is lower.

“When you exercise fasted, you’re tapping further into your energy stores to support that activity and you’re simply not going to recover as fast,” she explains. “What this research did show is that it’s such a specific group that could benefit from running fasted, and to be honest, that’s probably only 2% of my clients.” 

Besides potentially interfering with your recovery, running fasted is also going to affect your blood sugar levels during the run, which can make you weak, nauseous, and shaky. Ingested carbohydrates are going to produce glycogen, which turns into glucose — the primary fuel source for our bodies, says Starla Garcia, RD, LDN, a Houston-based dietitian who specializes in working with runners. This is why she recommends that you typically want to have a carb-focused snack in the hours before your runs, because it’s going to help your blood sugar from getting too low. 

“If you have a pre-run snack, it will keep your blood sugar at a more productive level for training purposes,” Garcia says. “Because your body has easy access to the glucose and their blood sugar is at a level that’s going to be more conducive for training.” 

If you’re still interested in running on empty, read on for tips from Garcia and Beckmann on how to do fasted runs right and determine if they’re a good option for you.

Don’t run fasted if you actually wake up hungry.

Close up of a woman getting ready for a workout in the morning without eating.
(Photo: Getty Images)

If you wake up hungry, both Garcia and Beckmann recommend just eating something without overthinking it.

“If you wake up hungry, having a snack even for runs that are less than an hour could end up making your run feel a lot better in terms of energy,” Garcia says. “You may also be experiencing low blood sugar upon waking up, so to help balance out your blood sugar throughout your run you may want to eat something small.”

Do consider your eating schedule the night before running fasted.

As far as comfort goes, you may want to consider eating dinner a bit later the night before a fasted run so that you feel fuller for a little bit longer, or having a bedtime snack if you prefer to eat dinner at a set time with your family, Beckmann says. Otherwise, you’re likely going to feel like you need a little something before heading out the door, which you shouldn’t ignore.

Do make it a priority to refuel ASAP after running fasted.

While you might feel great on your first-thing-in-the-morning run even without eating anything, don’t put off breakfast too long. Your recovery nutrition needs to be a priority following your run. Make sure to have a snack or meal that combines protein and carbohydrates within 45 minutes of completing your run, Beckmann says.

Do only run fasted for easy runs — not workouts.

Ever bonked hard when attempting a hard workout while literally running on empty? That’s no coincidence, and you likely know that that’s not something you’d try on race day, either. If you are going to try running fasted, both Garcia and Beckmann recommend it only for easier, lower-heart-rate runs lasting less than an hour.

Don’t even flirt with the idea of doing depleted long runs (unless you’re training for an ultra).

Runners have been increasingly experimenting with doing depleted long runs, in which you avoid eating or eat very little before your run and then don’t consume any gels or other forms of carbohydrates during the run, essentially running in a semi-starved state. Unsurprisingly, a lot of sports nutrition professionals don’t advise this. For runs over an hour (you might stretch it to 75 minutes), Beckmann recommends implementing calories through carbohydrates, such as through energy gels.

“We have more evidence that carbohydrates [are most effective for] fueling exercise, so with the efficacy of pre-exercise nutrition, I’m a big fan of fueling our bodies and giving it what it needs in order to fuel exercise and recovery properly,” she says.

According to Beckmann, depleted long runs have been popularized due to the fact that many people think they’re fat-adapting and thus they’re able to oxidize their fat storage. There is evidence that you can modify how your body burns fuel, but you’ll have to radically transform your entire diet and metabolism, and Beckmann feels the process is not worth the risks unless you’re focused on very long efforts at slower paces, like ultra-marathons.

Depleted runs are better suited for the ultra-running scene than road racing because carbohydrates are the body’s most efficient source of energy, so if you’re always relying on fat, it’s going to make it harder to sustainably train at faster speeds. Not only that, but if you’re planning to eat before a big goal race (as you should) but aren’t eating before most of your long runs, you’re not giving your gut the opportunity to adapt to utilizing fuel when you’re training, Beckmann says. 

“One thing I like people to understand is that it’s not just one energy system operating at a time that you can choose from, and you’re still burning carbohydrates and fat while using a different amount of each energy system based on your effort,” Beckmann explains. If you’re starving the carbs, Beckmann says, you might not be able to train at the paces you want to race at. She adds, “You might not be burning fat quick enough and you might be breaking down muscle in the process.” That can lead to injury.

Garcia echoed that sentiment, noting that it will often lead to people overthinking their food choices and struggling to replenish missing nutrients.

“I’ve found that many of my clients who’ve experimented with depleted long runs find that it takes them a very long time to feel recovered,” she says. “They also tend to experience unsteady blood sugars, bingeing habits and cravings, and they don’t feel great the next day after training. So, personally, for someone who is working on their relationship with food and wanting to have a more positive relationship with their running, I wouldn’t encourage it at all.”

Finally, doing depleted long runs also poses a risk for dehydration due to excess loss of sodium and fluid, Garcia says. 

“If the temperature is really high, you’re posing a huge risk on the body by running depleted,” Garcia says. “You could bottom-out blood-sugar-wise and could have a drop in blood pressure from the lack of sodium.” 

Do consider your health and injury history.

Young sportswoman eating an apple on running track.
(Photo: Getty Images)

It’s key to remember that running and nutrition are extremely individualized things, and your personal health and injury history is important to consider before trying out fasted runs. 

“If you are someone who is injury-prone or susceptible to injury, if you’re running fasted you are putting a lot more stress on your body,” Beckmann says. “Not only could you be making your body more at risk for injury, but you could also be compromising your immune system if you’re under-fueled and starting to go on a run.” 

“A lot of people think they can get away with not fueling, but I really encourage looking at their history of training, such as by tracking their heart rate and energy levels to help decide if a pre-run snack would be more beneficial to them,” Garcia adds. “I also encourage runners to take a look at their belief systems, like if they’re afraid of sugar or weight gain and want to avoid fueling because of that. Things like that need to be addressed prior to actually putting in recommendations that are appropriate for them.”

Don’t make fueling decisions based on what your peers are doing.

As with anything, you should never decide to try something just because seemingly everyone else is doing it. 

“There’s a large cultural issue with runners seeing their peers not fueling and thinking they don’t need to either, which is not okay,” Garcia says. “Everyone’s body and everyone’s blood sugars are different, and that doesn’t necessarily mean that something is wrong with you; it just means your body is responding to exercise and food slightly differently than your peers.” 

Finally, consult with a sports dietitian before regularly running fasted.

Even aside from breaking down the concept of fasting, working with a sports dietitian can help a runner check the boxes to ensure they’re meeting all their nutrition needs and that they’re at a level of performance that trying out fasted runs would be appropriate, Beckmann says.

“Whether or not you could benefit from running fasted really depends on your goals, as with all things related to running,” Beckman says. “I always try to start by getting a feel for what my clients are trying to accomplish if there are any gaps in their performance, any injury history or low energy levels. After navigating that, I can come up with a best-case scenario for them.”

About the Author 

Emilia Benton is a freelance journalist primarily covering running, health, and fitness. In addition to PodiumRunner, her work has appeared in Runner’s World, Women’s Running, SELF, Women’s Health, and more. Emilia is also a 10-time marathoner and lives in her hometown of Houston with her husband, Omar, and Boston Terrier rescue, Astro.