Are Runners 'Overfueling'?
A balanced take on the high-carb revolution.
Elite marathoners, pro cyclists, triathletes, and ultrarunners are pushing carbohydrate intake higher than what most sports nutrition guidelines have traditionally recommended. The old 30 to 60 grams per hour advice gave way to 90 grams per hour. And now the cutting edge of endurance fueling has moved even further to 100, 110, 120 grams per hour… sometimes more (though I find some claims by athletes to be somewhat dubious).
This has become one of the most interesting nutrition shifts in endurance sport. I’ve felt it personally, too. A few years ago, I would have considered even 40 grams per hour a solid fueling plan for a long run or marathon workout. Now, for long runs and race-specific sessions, I’m often thinking in the 70 to 90 gram range, and I know many runners who are experimenting above that.
But are athletes actually performing better because they are taking in ultra-high amounts of carbohydrate, or have we started confusing “more advanced” with “more”?
That’s the question asked in a new commentary article, titled “Fuelled or Fooled? Examining the Evidence and Mechanisms Behind Ultra-High Carbohydrate Intake in Endurance Athletes.” It takes a critical look at this new carbohydrate revolution.
And the authors’ argument is not anti-carb. Far from it. The case for carbohydrates during prolonged endurance exercise is strong. The question is whether pushing beyond 90 grams per hour is broadly supported by the evidence.
Their answer: not yet.
The authors define ultra-high carbohydrate intake as anything above 90 grams per hour during endurance exercise. That’s important because 90 grams per hour has long been treated as something like the upper evidence-based range, especially when using multiple transportable carbohydrates like glucose and fructose.
The logic is straightforward. Glucose and fructose use different intestinal transporters, so combining them allows athletes to absorb and oxidize more carbohydrate than glucose alone. This is why modern gels and drink mixes often use glucose-to-fructose ratios such as 2:1, 1:0.8, or 1:1. The goal is to get more carbohydrate from the gut into the blood and then into working muscle. That part is well supported.
Where things get murkier is the jump from “carbs help performance” to “more carbs always help.”
The authors point out that ultra-high carb fueling has gained momentum largely because of elite athlete practice, anecdote, and impressive race performances. We see athletes breaking records while reportedly consuming huge amounts of carbohydrate, and it is tempting to connect the dots: more carbs must be one of the secrets.
But controlled studies have not consistently shown that taking in more than 90 grams per hour produces additional performance benefits. In fact, much of the research suggests that carbohydrate ingestion improves performance compared with little or no carbohydrate, but the dose-response curve flattens out somewhere below the ultra-high range.
More carbs do not seem to spare more muscle glycogen
One of the classic arguments for fueling during endurance exercise is that taking in carbohydrate “spares” your stored carbohydrate, especially muscle glycogen. Since glycogen depletion is one of the major contributors to fatigue during long races, the idea makes intuitive sense: eat more carbs, burn more external fuel, preserve more internal fuel, run stronger late. But the evidence does not cleanly support that idea at ultra-high intakes.
The authors highlight studies showing that increasing intake from 90 to 120 grams per hour can increase exogenous carbohydrate oxidation, meaning you burn more of the carbohydrate you consume.
However, that extra oxidation does not necessarily reduce endogenous carbohydrate oxidation, meaning it does not reliably spare muscle glycogen.
If an athlete takes in 120 grams per hour and oxidizes more external carbohydrate, but their muscles keep burning stored carbohydrate at the same rate, then the main benefit is not glycogen sparing. The extra carbohydrate may simply be replacing some fat oxidation rather than preserving precious muscle glycogen.
This matters because a lot of the practical messaging around high-carb fueling still rests on the idea that higher intake protects your internal carbohydrate stores. According to this commentary, that explanation is probably too simple, and possibly even wrong at the highest intake levels.
The authors also point out that oxidation efficiency drops as intake rises.
In one cited study, oxidation efficiency fell from about 86% at 90 grams per hour to about 76% at 120 grams per hour. As you push intake higher, a larger fraction of what you consume may not be immediately burned for energy.
That unoxidized carbohydrate has to go somewhere. Some may remain in the gut. Some may be retained in the liver. Some may contribute to better recovery after the race or workout. But it may not be directly powering your legs in the moment.
And for runners, the gut piece is not trivial. It is one thing to tolerate 120 grams per hour on the bike or in a controlled lab session. It is another thing to do it at marathon pace, late in a race, with heat, jostling, dehydration, and rising sympathetic stress. The paper notes that laboratory trials may be too short to capture the point at which gastrointestinal distress often appears in long events. Anyone who has raced a marathon knows that the stomach can be perfectly fine at mile 10 and not so fine by mile 22.
The performance evidence doesn’t match the hype
The authors argue that, while carbohydrate ingestion reliably improves endurance performance compared with placebo or no fuel, the evidence for a clear dose-response benefit above 90 grams per hour is weak.
