Does a Bigger Carb Dose During a Marathon Actually Beat the Old 60-Gram Rule?

For years the ceiling on race-day fueling was roughly 60 grams of carbohydrate per hour, the point where your gut's glucose transporters were thought to saturate. That number came from single-sugar studies, and it held up until researchers started mixing glucose with fructose. Glucose rides in on one intestinal transporter (SGLT1) and fructose on a separate one (GLUT5), so combining them opens a second lane on the highway. Trials using a 2:1 glucose-to-fructose blend showed athletes could oxidize closer to 90 grams per hour with less of the sloshing, cramping misery that comes from overwhelming a single pathway (see [1]). That is the science behind why every gel on the expo floor now brags about its glucose-fructose ratio.

The newer, more interesting question is whether you can push past 90. A 2023 field study on trail ultramarathoners tested intakes around 120 grams per hour and found that the highest-fueling runners better maintained muscle function and reported less muscle damage over the race, without a proportional spike in gut distress (see [2]). And in a controlled cycling protocol, Podlogar and colleagues showed that when you train the gut to handle 120 grams per hour, exogenous carbohydrate oxidation keeps climbing rather than plateauing at the old ceiling (see [3]). The gut, it turns out, is trainable tissue, not a fixed pipe.

Here is where I temper the hype. High intakes only pay off over long durations, think 2.5-hour-plus efforts, and only if you have rehearsed them. The runners in these studies practiced their fueling for weeks; they did not chug 120 grams cold on race morning and hope. Gut tolerance adapts to the carbohydrate load you repeatedly ask it to absorb, which is why "nothing new on race day" remains the single most reliable rule in this sport. If your longest runs have been fueled on 45 grams an hour, your marathon is not the place to audition 90. Log every long-run fueling trial in /nutrition so you can see what your stomach actually tolerated at mile 20, not what you remember.

So the 60-gram rule is not wrong, it is just outgrown. For most 5K-to-half runners it is irrelevant. For marathoners and ultra folks willing to train the gut through the Build block, the honest answer is that more carbohydrate, delivered as a glucose-fructose blend and practiced ahead of time, genuinely helps. Train smart. Race honest.