What Experts Want You to Know About High-Carb Fueling

At the 2025 New York City Marathon, Joe Klecker consumed 175 grams of carbohydrate per hour on his way to a 10th-place finish. Weeks earlier, triathlete Solveig Lovseth took in 132 grams per hour during the marathon leg of her Ironman World Championship win. Nutrition brands, influencers, and a growing number of elite athletes have settled on a simple message: more carbs, more performance. The number 120 grams per hour has become the new floor.

The science points the other way.

No peer-reviewed study has demonstrated a performance benefit from consuming more than 90 grams of carbohydrate per hour during endurance exercise. Pushing past that ceiling may reduce performance in non-elite athletes and raise the odds of gastrointestinal trouble, according to physiologists who study fuel metabolism in sport.

The 90-gram ceiling rests on two decades of work on what researchers call multiple transportable carbohydrates. A 2017 study by Trommelen and colleagues in Nutrients found that combining glucose with fructose or sucrose raises exogenous carbohydrate oxidation rates above what glucose alone can deliver. The mechanism uses separate intestinal transporters for glucose and fructose, which lets the body absorb more total carbohydrate. Before this work, the ceiling sat at 30 to 60 grams per hour.

“There is not a single peer-reviewed study that shows performance benefits over 90 grams of carbs,” said Roxanne Vogel, PhD, director of research and development at GU Energy Labs. “It costs more, risks more, and has no proven performance benefit.”

Paul Booth, a PhD candidate who served as nutrition coach for both 2025 Ultra-Trail du Mont-Blanc champions, does not mince words. “There is a pretty big emphasis right now on 120 grams as a catch-all for anyone, but the data is not there,” Booth said. “No one has found any benefits over 100 grams, if not closer to 90. Our lab found that 90 was best and performance dropped off above that.”

The 120-gram idea traces to a 2020 study by Viribay and colleagues in Nutrients that compared 60, 90, and 120 grams per hour during a mountain marathon with 4,000 meters of cumulative elevation gain. Across 26 elite trail runners, the group taking 120 grams had less neuromuscular fatigue, lower internal exercise load, and faster recovery of high-intensity running capacity 24 hours after the race. Jump height, half-squat strength, and lactate clearance all held up better at the higher intake.

But the study measured recovery, not race performance. That distinction evaporated as the finding spread through social media into a blanket fueling prescription.

Even the 90-gram number is too broad. A 2014 review by Jeukendrup in Sports Medicine argued that carbohydrate intake during exercise should be individualized: oxidation rates shift with body size, exercise intensity, and individual gut physiology. The review gave elite coaches the evidence for personalized fueling protocols they now deploy.

“Body size is positively correlated with the ability to oxidize ingested carbs, suggesting that traditional guidelines are overfeeding some while underfueling others,” Vogel said. She recommends roughly 0.7 grams of carbohydrate per kilogram of body weight per hour, adjusted to the individual athlete.

Booth’s UTMB champions did not get near 120 grams per hour. Tom Evans averaged 94 grams during his winning run. Ruth Croft held at roughly 90 grams. Lab testing showed Evans could oxidize up to 1.7 grams of carbohydrate per minute, equal to 102 grams per hour, but Booth shifted intake with course demands rather than holding a fixed number. “We did not need to be at that level all the time because it would be overfueling,” he said. “We fed him more before sections where he would dig deeper, then backed down to give his gut an easy section.”

For amateurs, the downside of over-carbing shows up in the numbers. Oxidation efficiency drops from roughly 86 percent at 90 grams per hour to 75 percent at 120 grams. Unoxidized carbohydrate sits in the gut. In a long race, that can mean GI trouble or a DNF.

“There may be some utility for world-class athletes in multi-day events where recovery is tight and the chance to fuel is limited, but gut training is required,” Vogel said. “In all other cases, there is no point in taking in carbohydrates that are not metabolized.”

Endurance sport has gone through a real shift in fueling. Since Viribay’s 2020 paper, 14 of the 15 fastest female times and 12 of the 15 fastest male times at the Western States 100 have fallen. Better carb strategies played a role. But the evidence says most athletes should start at 80 to 90 grams per hour and, when possible, get individualized testing to dial it in. A decade ago, 60 grams per hour was the ceiling. Ninety grams, not 120, is where the research lands.

References

1. Viribay A, Urdampilleta A, Arribalzaga S, et al. Effects of 120 vs. 60 and 90 g/h carbohydrate intake during a trail marathon on neuromuscular function and high intensity run capacity recovery. Nutrients 12(7):2094. 2020. https://doi.org/10.3390/nu12072094
2. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine 44(Suppl 1):S25-S33. 2014. https://doi.org/10.1007/s40279-014-0148-z
3. Trommelen J, Fuchs C, Beelen M, et al. Fructose and sucrose intake increase exogenous carbohydrate oxidation during exercise. Nutrients 9(2):167. 2017. https://doi.org/10.3390/nu9020167