Could Switching from Simple to Slow-Release Carbs Prevent Bonking?

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For decades, runners have reached for simple carbs to fuel their runs. Think: a bagel for breakfast, sugary gels or chews every half hour or so on long runs. For many runners, it’s a winning strategy. But it’s not your only option.

Some experts suggest that reaching for carbs that are low on the glycemic index (GI)—meaning they take longer for your body to digest—could provide more sustained energy for better endurance, helping you to avoid that dreaded bonk at mile 20 of your marathon.

“Low glycemic carbs are released slower into your bloodstream, almost like how a log would burn slowly over a long period of time,” explains Cristina Sutter, MHSC, RD, a sports dietitian from North Vancouver, Canada. “Whereas a high GI food would be your kindling that catches fire quickly—it releases a lot of energy quickly, and then it dies out.”

With a growing sports nutrition product market, we now have a variety of options, some of which are like fast-burning kindling, while others burn slow and steady like firewood. So which is your best bet? Well, there are a few factors to consider.

How Slow-Release Carbs Affect Your Energy Levels

Our bodies absorb simple carbs super fast. When you take a gel filled with glucose and fructose, you’ll get a boost within five to 15 minutes. “You can almost think of it as jet fuel,” says Kristy Baumann, RD, a Minnesota-based dietitian who specializes in working with runners. How long that energy boost lasts depends on several factors, including the intensity of your run—the harder the effort, the faster you’ll burn through those carbs.

This quick spike, however, can come with a downside. If you don’t take these gels often enough (an all-too-common mistake among runners), you could end up with a blood sugar crash when they wear off.

On the other hand, slow-release carbs meter out more measured energy for a longer period of time. For instance, UCAN gels and drink mixes—which are made with a hydrolyzed cornstarch that’s a low GI, complex carbohydrate—promise a slow, steady burn that lasts 75 or more minutes. Certain gels made by Muir Energy add in some fat and protein along with their mix of carbs, which also slows down the release of sugar into the bloodstream. The fat and protein can make these gels harder to digest at higher intensities, so they’re mainly used by ultrarunners.

Baumann tells clients to think of slow-release carbs as “baseline carbohydrates” because they provide a steady drip but  won’t give you that helpful bump, which can be problematic when you need that extra pep to maintain your race pace.

“We don’t want to operate under the assumption that slower is better, because if that delays our ability to actually get the fuel from the product, then we can wind up bonking,” says Winnipeg-based dietitian and running coach Stephanie Hnatiuk, RD, CPT. Because these carbs take longer to get to work, “you have to adjust your strategy so that you are fueling sooner so that you are getting in that steady stream of energy to your working muscles,” says Hnatiuk.

Switching from simple to slow-release carbs can apply to both what you eat during your run, and beforehand. In both cases, you’ll need to think about eating earlier. For a pre-run meal, Sarah Cuff, RHN, a Vancouver-based holistic sports nutritionist and run coach, recommends oatmeal, which is low GI but also not too high in fiber. “Eat it early—three to four hours ahead of time,” she says. “Then just top up with little bites as you get closer.” This strategy falls right in line with the findings of a 2006 study in the International Journal of Sport Nutrition and Exercise Metabolism which found that runners who ate a low GI meal three hours before running had more endurance than those who had a high GI meal.

The Digestion Difference

Sometimes, slow-release carbs can be easier on certain runners’ stomachs. “Some of the lower glycemic index carbs might work really well for some people who have sensitive guts to some of the more rapidly-absorbing sugars,” Hnatiuk says. She says that some people have a hard time digesting those sugars, which end up pulling water into the GI tract and causing symptoms like cramping, bloating, and the dreaded runner’s trots. Yet other runners find carbs that take longer to digest are what cause those very same problems. “We have to just try different things and find what works for us,” she says.

Hnatiuk will suggest options like UCAN to runners who’ve found traditional sports products problematic, or who have been avoiding mid-run fuel altogether because they have a digestive disorder like irritable bowel syndrome. Baumann also recommends slow-release carbs for anyone concerned about their blood sugar levels, since they won’t cause the spike and crash you’ll get from sugar.

But if their traditional gels and chews have been working for them, “I wouldn’t necessarily change what they’re using just for the sake of changing it,” Hnatiuk says.

The Bottom Line

The type of carb you take matters less than how much you take. “Regardless of where your carbs come from, the biggest determinant on whether or not you’ll bonk is how much carbohydrates you fuel with per hour when you’re running—and that’s something that people underestimate a lot,” Sutter says. Guidelines range from 30 to 90 grams per hour, depending on body size, fitness level, and the intensity of your run.

But when you’re not running, slow-release carbs are definitely the way to go on a day-to-day basis. “A low glycemic index diet as a whole is a fantastic strategy for runners,” says Hnatiuk. “This is going to include your higher fiber, whole foods, whole grains, starchy vegetables, plant proteins like beans and lentils and chickpeas.”

A 2018 study published in the journal Nutrients found that endurance runners who followed a low GI diet for three weeks increased their time to exhaustion in a cycling test and were able to run further on a 12-minute run. And a 2024 trial published in Sports Medicine – Open also found a 10-week low GI diet showed endurance benefits. So with this research in mind, many experts, like Cuff, suggest that low glycemic carbs before and after running are ideal for runners—though there’s still much more to study on the topic.

Whether or not these carbs are something to test out while running, well, that’s up to you and your stomach.