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Your Workout Fuel: The Truth About Sugars and Endurance

Glucose and sucrose and fructose, oh my! For fueling long runs and rides, which should you choose?

When you’re an endurance athlete, you know (and possibly even learned the hard way) that what you put into your body to keep it moving on race day is almost as important as the training you do leading up to it. From natural nutrition such as bananas, honey, and coconut water to commercially available specially concocted drinks and serums, the choice is often based on trial and gastrointestinal error, or a crapshoot based on what’s available on the course that day. But what if there’s a better way to choose fuel for maximum performance and minimal gut trouble? Researchers at the University of Bath in England may be honing in on an answer.

A quick science lesson
Our muscle cells (and, indeed, all our cells) run on glucose, a simple single-molecule sugar. To get it, our digestive systems break down more complex carbohydrates to isolate the glucose, and then either send it into the bloodstream to feed hardworking cells or store it in the muscles or liver as glycogen for use when there’s no easier-access blood glucose remaining. “Hitting the wall” during a race is when your body runs out of both types of fuel and must begin converting fat for fuel, a far less efficient process—and what you don’t want to happen when you’re gunning for that marathon PR. That’s why for endurance activities longer than an hour, it’s a good idea to have a refueling plan to get you through. Since endurance athletes need ready energy, the general thinking is to consume simple sugars, which don’t require much metabolizing before they can be circulated to the cells. Typically, this means glucose, fructose (fruit sugar), or sucrose (common table sugar, which is a combo of glucose and fructose).

The new research
But here’s where it gets a little dicier, as Javier Gonzalez, Ph.D., human physiology professor at the University of Bath explains, based on his team’s recent study: These sugars seem to be absorbed via different routes by the intestine. Glucose is absorbed through one route, which can be "maxed-out" when consuming more than 60 grams per hour (a potential problem during high-intensity endurance activities longer than 2.5 hours, where the recommendation is to consume up to 90 grams an hour). Not only that, the test subjects (cyclists) who ingested glucose complained of gastrointestinal discomfort. Further, the cyclists who ingested sucrose-based drinks reported that exercise felt easier than those who took in glucose drinks. The researchers didn’t test pure fructose because, “many people cannot absorb it very well when it is consumed alone, leading to stomach cramps and side effects, Gonzalez says. “There is evidence that consuming glucose with fructose enhances the intestinal absorption of fructose.”

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The takeaway
So what does this mean for your refueling needs? Sucrose—plain ole sugar!—may be be your best bet. Many commercially available drinks and gels rely heavily on glucose, which this latest research says isn’t ideal for very long, very intense endurance activities. Ones that add fructose to glucose can aid in absorption (after all, those are the molecules that make up sucrose). 

Because there are many synonyms for sugars that show up on packaging, we sent Gonzalez the ingredients of popular products from Gatorade, GU, PowerBar, Clif, and Powerade. Many contain combinations of glucose (a.k.a. dextrose and maltodextrin), sucrose (a.k.a. sugar, dried cane syrup), and/or high fructose corn syrup (chemically, a combo of fructose and glucose), all of which Gonzalez says would be fine choices for the purpose of mid-workout fueling. “The only one of these ingredients that is probably not suitable is isomaltulose,” he says. “It is very similar to sucrose, but the bond between the glucose and fructose molecules is broken down more slowly, which makes the absorption slower—not the best option when trying to maximise carbohydrate availability.”

Your RX
For endurance events less than 2.5 hours, the recommendation is to take in 30 to 60 grams of sugar per hour split into 15-minute increments, diluted with water. For longer events, go up to 90 grams per hour. No matter what, practice your nutrition before your race to be sure it will work for you.

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