Should runners fuel according to distance or time? That simple question recently launched a lively debate on Reddit’s running thread. “I've seen here several times that you shouldn't need to fuel anything under a half marathon distance,” wicked_lion posted. But “wouldn't it matter more time vs distance?”
We posed the question to Matt Fitzgerald, author of The New Rules of Marathon and Half Marathon Nutrition. The short answer, he says, is you can run fuel-free without hurting performance for “somewhere around an hour, an hour 15.” Meanwhile, 10 miles is the shortest distance studied where carbs aid performance. So if you’re running more than around an hour or 10 miles—whichever comes first—you should plan to ingest carbs during your workout. (At 7:30 pace, you’ll run 10 miles in 1 hour 15 minutes.)
However, there has been some evidence that periodically rinsing your mouth with sports drink for five to 10 seconds can improve performance in exercise lasting 30 minutes to an hour. Researchers believe the solution may somehow trick the brain into believing it received the extra fuel, causing you to adjust your output accordingly. Researchers also saw similar performance benefits if athletes ingested the drink instead of swishing it, but noted that at high intensities, ingestion could cause GI issues that swishing shouldn’t.
As for how many carbs runners should get in if they’re going longer than an hour, “the magic number is 60 grams per hour,” Fitzgerald says, regardless of bodyweight or intensity in general. That’s about the max rate intestines can absorb glucose, although some studies have shown that the body can absorb even more carbs per hour when you add fructose and maltodextrin to the mix, whether those carbs are in liquid, gel, or low-fat, low-protein, low-fiber energy bar form.
But absorption is also dependent on how well you’ve trained your gut to handle carbs. If you really want to reap the performance benefits of carb ingestion on race day, “dedicate at least some time to training with a relatively high carbohydrate intake,” wrote renowned sports nutrition researcher Asker Jeukendrup in a 2014 summary of research conducted on carb intake during exercise. “Anecdotal evidence in athletes suggests that the gut is trainable and that individuals who regularly consume carbohydrate or have a high daily carbohydrate intake may also have an increased capacity to absorb it.”
At this point, however, it’s unclear exactly how much of a performance boost carb-trained athletes will actually get on race day. But Jeukendrup and his colleagues believe maximizing carb absorption is important enough that it will play a role in an athlete’s ability to break the two-hour marathon barrier.
If you’re not gunning for a PR, don’t fret too much about nailing that 60 grams per hour mark. “Your fueling doesn’t have to be perfect,” Fitzgerald says. “You just want to basically do the right things.” If you are going for glory, carb intake comes down to what you can tolerate over the distance you’re running.
“When I was at my best, I ran the half marathon a little under an hour 15 minutes,” Fitzgerald says. “I went back and forth saying, ‘Am I’m gaining more than I’m losing by not taking in carbs,’ and I finally settled on not needing them. You have to experiment.”