Is it Safe to Try Fasted-State Running?

Molly Hurford
by Molly Hurford
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Is it Safe to Try Fasted-State Running?

Fasted training has been trending for a few years now, yet it’s still one of the most misunderstood topics in the endurance sports world. To do a fasted-state run, most people opt to get out the door ahead of breakfast so it’s been around 12–14 hours since their last meal.

The oft-misunderstood goal of running on empty is to improve your body’s efficiency and fat-burning ability, but does it really work? Here, we’re taking a look at the research, the misconceptions and what dietitians are recommending for most runners.


“It doesn’t work for everyone. It doesn’t suit everybody,” explains Gemma Sampson, a Girona, Spain, based dietitian who works with endurance athletes.

A small-scale study done on male athletes showed fasted exercisers burned 20% more fat than those who ate breakfast. In another small study published in the American Journal of Physiology, a group of overweight men with no athletic background benefited from fasted exercise for fat loss. However, these limited studies need to be taken with a grain of salt.

Alternatively, other studies more relevant to runners have shown eating breakfast pre-workout can make the body more efficient at burning carbohydrates during exercise and metabolizing food faster post-workout: Ideal for those with speed-oriented goals, and for those who hate the idea of skipping breakfast.

So while a fasted-state may train the body to burn fat more efficiently, carbohydrates are still king for intense workouts that require glycogen from carbohydrates as the fuel source.

Fasted-state running isn’t something you should do for every single workout. “While there can be value from intentionally training with low-carbohydrate and low-glycogen availability at times, you don’t want to do it all the time because if you’re doing it too much, then that could be affecting the quality of training,” Sampson adds. If every run is fasted, your body unlearns how to exercise with full glycogen stores, which will almost certainly mean a much slower time in your next race.

However, it can help with steady, ultra-distance running. “Being able to use fats as fuel is great for ultra-endurance and for multi-day races where the intensity in the pace is consistent but low,” says Sampson. “If your races are fast and hard and you need a lot of energy and effort, then that’s where carbohydrates are still king.”


It’s also important to understand training like this won’t make you lean. There’s a difference between fat-burning and burning fat, explains Sampson. Fasted training teaches your body to utilize fat as fuel, but it’s not immediately eating through that extra few pounds you’re carrying around your waistline. You’re becoming a more efficient runner in terms of using fat and carbs as fuel, not a thinner one.

Most people mistakenly believe running on empty is the quick way to lose pounds, but more often, it just leads to binging on a bigger breakfast while unduly stressing your body. “You’re actually burning a lot less energy in this situation than you would if you’re doing, say, a higher intensity effort,” she notes. “So if you look at it from a caloric perspective, you would burn more from doing a hard training session.”


If you are interested in testing fasted-state training, remember it needs to be done at extremely easy paces and low exertion. For most runners, especially those who are training at a recreational-level, Sampson notes this might mean walking versus running on an empty stomach. (That’s right: Walking your dog for 20 minutes before you have your first cup of coffee can count as fasted training!) “Most people go way too hard in fasted runs,” she says.

Women need to be particularly careful with this. Stacy Sims, PhD, author of ROAR, explains that women may not be suited for morning fasted runs because, “if you go out and exercise first thing, your body wants to pump out more cortisol but needs the right ingredients to manufacture it, which are your sex hormones — testosterone, estrogen and progesterone.” She explains, “your body steals those hormones to make more cortisol. Now your cortisol is very elevated, which stimulates fat storage. In short, you’re storing more of what you’re trying to lose.”

Sampson is quick to note that fasted training is not about calorie-cutting. When you finish a session, you should be eating your normal meals, not skimping on the post-run recovery. “Really, the most important thing out of all of this is that you don’t get back to the house starving and wanting to eat everything in sight,” she adds. Your willpower will be as low as your glycogen stores, which can send you straight for the cookie jar. “Have a recovery meal planned ahead of time.”

About the Author

Molly Hurford
Molly Hurford

Molly is an outdoor adventurer and professional nomad obsessed with all things running, nutrition, cycling and movement-related. When not outside, she’s writing and podcasting about being outside, training and health. You can follow along with her adventures on Instagram at @mollyjhurford.


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