Fuelled or Fooled?

For endurance athletes, carbohydrate fuelling has become one of the most visible and increasingly debated topics in sport science.

Scroll through social media and you’ll see professional triathletes targeting 120 grams of carbohydrate per hour, while others talk about “training the gut” as though it is the missing ingredient to breakthrough performances.

Some have even called it a carbohydrate revolution.

And in many ways, that label fits. Modern fuelling strategies have reshaped endurance sport. Compared to decades ago, athletes today are better fuelled and far less likely to implode from running out of energy midway through a race.

But according to Dr. Dan Plews, the conversation around carbohydrate intake has also been increasingly misunderstood.

“People are starting to assume that if some carbohydrates are good, then more must always be better,” Plews explained. “But the evidence does not really support that conclusion.”

This is the central question behind a paper Plews and colleagues recently published: are endurance athletes truly becoming better fuelled, or are some being fooled into believing that ultra-high carbohydrate intake is automatically superior?

Evidence Shows Benefit – Up to a Point

As reviewed in depth in Plews’ paper, the evidence is well-established for carbohydrate intakes in the range of 60 to 90 grams per hour.

Within this range, performance benefits are clear. Where the picture becomes less clear is with the “ultra-high” intakes now being reported by many elite athletes.

At these levels, the evidence becomes both limited and inconsistent. Some studies suggest that beyond a certain threshold, increasing carbohydrate intake may offer no additional benefit, and in some cases may even impair performance.

This is precisely where researchers like Plews begin to question the prevailing narrative.

If ultra-high carbohydrate strategies are becoming more common among elite athletes, yet the evidence does not consistently support a performance benefit, something does not fully add up.

That gap between what is observed in practice and what is explained by current science is where the conversation becomes interesting – and where new mechanisms need to be considered.

*Important Note: The mechanisms presented below are theoretical and remain unproven.*

Potential Mechanism 1: Fuel Efficiency and Oxygen Cost

The first potential mechanism considers something endurance athletes rarely think about directly: not just how much energy a fuel provides, but how efficiently that energy can be produced.

As many triathletes know, there are two primary substrates that power endurance performance: carbohydrates and fats.

Historically, long-course racing has emphasized maximizing fat oxidation and metabolic flexibility, particularly in Ironman, where events were viewed as fundamentally limited by energy availability. In simple terms, the goal was to preserve finite glycogen stores by relying more heavily on fat, an effectively unlimited fuel source. Graphically, this has often been represented as “shifting the curve to the right.”

Fuelling with large amounts of carbohydrates was not always seen as beneficial in this context, because increased carbohydrate utilization is known to suppress fat oxidation.

But this traditional framework may be missing a piece of the puzzle: the metabolic cost of using each fuel. Fat oxidation is more “expensive,” requiring more oxygen to produce the same amount of usable energy compared to carbohydrate metabolism.

At lower intensities, this difference would not matter, as oxygen availability is not limiting performance. But as the sport continues to evolve, and professional athletes race Ironman at increasingly high intensities, this difference in oxygen cost could theoretically become relevant.

As Lionel Sanders has noted, Kristian Blummenfelt has set the standard such that you can no longer “survive” an Ironman – you must race it.

At those intensities, even small differences in efficiency could become meaningful.

All of this represents a shift in thinking. Rather than viewing ultra-high carbohydrate intakes as a way to further preserve glycogen stores, a mechanism not consistently supported in the literature, this perspective reframes carbohydrate as a more efficient fuel under high-performance conditions.

Graphically, this may still represent a shift in the curve – but in the opposite direction, and for a fundamentally different reason.

Potential Mechanism 2: Lactate as a Reusable Aerobic Fuel

The second potential mechanism builds on our evolving understanding of lactate metabolism. Rather than being viewed solely as a marker of fatigue, lactate is now recognized as a dynamic intermediary that can be recycled and used as fuel.

For athletes consuming glucose-fructose mixtures, fructose is rapidly converted to glucose and lactate, increasing the circulating pool available for oxidation. Elite endurance athletes also demonstrate an enhanced ability to metabolize lactate, characterized by elevated monocarboxylate transporter (MCT1 and MCT4) expression and greater mitochondrial volume.

This represents another theoretical explanation for the reported benefits of ultra-high carbohydrate intakes: an enhanced capacity for energy production through lactate metabolism. This pathway could theoretically provide a greater advantage to elite athletes who have developed these metabolic adaptations beyond what is typically seen in age-group populations.

Potential Mechanism 3: The Brain

The final mechanism proposed in Plews’ paper shifts the focus away from metabolism and toward something different but still influential: the brain.

(It is also important to note that these mechanisms are not necessarily mutually exclusive. In practice, they could be additive or even synergistic, rather than one replacing another.)

The brain has long been shown to play a central role in regulating pacing, perception of effort, and how much we feel we have “left” in endurance performance. Concepts such as the central governor theory suggest that performance is not determined solely by physical limits, but also by how the brain interprets and responds to physiological signals.

Studies have also shown that even carbohydrate mouth rinsing (without ingestion) can improve performance. Simply sensing carbohydrate in the mouth appears to signal energy availability to the brain, reducing perceived effort and allowing athletes to sustain higher outputs.

Building on this, it is plausible that higher carbohydrate intake during long-course racing could influence performance through similar central mechanisms. Again, this remains unproven, but represents a potential pathway worth further investigation.

Bringing It All Together

We’ve looked at three potential mechanisms that may, individually or collectively, contribute to the purported benefits of ultra-high fuelling strategies used by today’s professional athletes.

Importantly, all three remain theoretical at this stage and require further study.

So what should the broader age-group population take from the current evidence? Does this mean we should all be consuming more, mirroring what we see on social media?

Based on the current evidence, no, we should not.

Beyond the absence of proven benefits, there are risks associated with ever-increasing carbohydrate intake. Evidence supporting performance benefits above ~90 grams per hour remains limited and inconsistent, with some studies even reporting impaired performance. At the same time, the risk of GI distress increases as intake rises.

There are also broader health considerations for much of the age-group population. Consistently consuming ultra-high carbohydrate doses in both training (to “train the gut”) and racing may carry its own form of metabolic risk. One study, for example, reported increased pre-diabetic markers – an effect that reversed when carbohydrate intake was subsequently reduced.

As a result, when balancing the current evidence with broader health considerations, the best guidance for most athletes remains carbohydrate intakes in the range of 60 to 90 grams per hour.

In the meantime, we can continue to watch our favourite pros, like Kristian Blummenfelt, push the limits of fuelling – sometimes visibly so – while also redefining what is possible on race day, even as the science works to keep pace.

Read the full paper in Sports Medicine: Fuelled or Fooled? Examining the Evidence and Mechanisms Behind Ultra-High Carbohydrate Intake in Endurance Athletes