Fasted Workouts: Why You Should Sweat on an Empty Stomach

running fasted

Stumbling straight from the bed and into your sneaks has its benefits. Not only can a good morning run leave you feeling energized but there’s the accomplishment of completing your cardio before heading off to work. (For me, that’s most about knowing I can crash on the couch when I get home.)

Still, there’s one issue that always gives me agita — how to eat enough to fuel my jog without ingesting so much I start to feel a bit sick a few miles in. So when I heard about fasted workouts — i.e. hitting the pavement without hitting the fridge — I was intrigued. The science sounded promising. Generally during exercise, your body burns through its preferred fuel source of glycogen, or stored carbs.

But when you haven’t eaten in eight to 10 hours, those levels will be low “so you’re able to use stored body fat as a fuel source,” explains North Carolina-based dietitian and USA cycling coach Tommy Rodgers, “and kind of lean out a bit.”

Plus, for those looking to conquer longer races (think: marathons, triathlons) running on empty can teach your body how to do less with more. Notes Rodgers, “Hopefully long-term, you shift your body toward being able to deal with using fat as primary fuel versus carbohydrates.”

Still, he cautions, you shouldn’t starve yourself before every sweat session. Fasted runs should be reserved for more casual outings such as basic jogs or tempo runs rather than, say, sprint repeats. “Once you start getting into the combination of high-intensity and/or long duration, you start to run the risk of bonking,” explains the pro. (Bonking translation: having your body quit on you mid-workout.)

He advises keeping all fasted sessions under 90 minutes and committing to no more than two a week. And if the rest of your seven-day schedule include intense fitness sessions, skip ‘em all together. The issue, he says, “is you run the risk of not just bonking during that workout, but being chronically low on glycogen and under-fueled.” And if you’re always making up that deficit, he explains, “Your hard workouts won’t go well.”

To ensure these sessions go swimmingly, start slow — make the first outing no more than 20 minutes if you’re running, or an hour for cyclists — and stay slow. Says Rodgers, “Keep the pace very comfortable.”

The experience as well. As a just-in-case measure, Rodgers always totes fuel with him during a workout (energy gels are his go-to) to reach for if he starts feeling hungry. And should you feel light-headed, he says, “dial back the intensity or start walking.”

Then, as soon as you get home, stroll straight to the fridge. With fasted workouts, says Rodgers, it’s crucial to refuel with a carbohydrate-heavy meal within 30 minutes of finishing. Doing so not only refills all the glycogen stores you blew through, but it allows you to gobble up some of the benefits of training on an empty stomach. Teaching your body how to rely on fat once it has used up all the carbs is a key piece of distance training, says Rodgers. And you can speed up that process with fasted workouts. Because you start your session with already lowered levels of glycogen, says the pro, you’re forcing your body to turn to fat earlier allowing you to accomplish that goal in less time.

After chatting with the pro, I was convinced to insert fasted workouts into my marathon training this go-round — going up to distances I’d previously been nervous to try without fueling. So far I’ve easily cruised through runs without feeling low on energy and I’m diligently carbing up post session. Fingers crossed it makes a difference this November when I wind 26.2 miles through New York City, but for now I’m loving the ease of the fuel-free morning workouts. Plus, spending my whole day knowing I’ve already crossed cardio off my to-do list — that’s also legit.

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About Sarah Grossbart

The first time Sarah tried running she complained the whole time — both laps. Two-plus decades later she voluntarily runs marathons, boxes, spins and is borderline addicted to megaformer classes at SLT. A graduate of Michigan State University, the Michigan native now lives in New York City where she writes for publications such as Us Weekly, Real Simple, HGTV Magazine, Martha Stewart Weddings, Mental Floss and aSweatLife. When she’s not working out or bingeing bad reality television, she can be found watching college basketball with her husband and yelling at the TV. What? They can totally hear you through the screen.