Why Carbohydrate Intake and Carbohydrate Use Are Not the Same Thing

Why carbohydrate intake matters in endurance events

During exercise lasting longer than around 90 minutes, carbohydrate availability becomes a key limiter of performance.

Muscle glycogen stores are finite, and liver glycogen plays a crucial role in maintaining blood glucose. As these stores become depleted, the body is forced to rely more heavily on fat as a fuel, which cannot support the same exercise intensities. The result is a progressive drop in pace, power output, and coordination, what many athletes recognise as “hitting the wall”.

This is why carbohydrate intake during long-duration exercise is consistently associated with improved performance, particularly in events such as marathons, long-distance cycling, triathlon, and ultra-endurance races.

But intake alone does not tell the full story.

Intake is not the same as utilisation

Consuming carbohydrate during exercise does not guarantee that it will be absorbed, transported, and oxidised (burned) by working muscles.

For carbohydrate from gels, drinks, or chews to contribute to performance, it must successfully pass through several steps:

• Leave the stomach
• Be absorbed across the intestinal wall
• Enter the bloodstream
• Be taken up and oxidised by skeletal muscle

Limitations at any stage of this process reduce the amount of carbohydrate that actually contributes to energy production during exercise.

This distinction is critical. Two athletes can consume the same amount of carbohydrate during the same race, yet derive very different performance benefits from it.

Athlete calibre and carbohydrate use

On average, well-trained endurance athletes are able to oxidise more exogenous (ingested) carbohydrate during exercise than recreationally trained athletes. Higher training volumes, repeated exposure to fuelling during exercise, and sustained race intensities all contribute to adaptations that support carbohydrate absorption and utilisation.

However, averages hide an important reality.

Even among athletes of a similar calibre – training similar hours, competing at similar levels, and following similar fuelling strategies – there can be large differences in how much ingested carbohydrate is actually used.

In practice, this means that two competitive marathon runners taking in identical gel strategies may experience different outcomes: one maintains pace and feels strong late in the race, while the other fades or develops gut issues.

Training status matters, but it does not eliminate individual variability.

Too little carbohydrate: running out of fuel

When carbohydrate intake and utilisation are insufficient, the consequences are well known.

As muscle and liver glycogen availability decline, blood glucose becomes harder to maintain. The body shifts toward a greater reliance on fat oxidation, which reduces the maximum sustainable intensity. Perceived effort rises sharply, pace drops, and decision-making and coordination can suffer.

This is the classic endurance “bonk”.

At higher intensities, it can occur surprisingly quickly.

Too much carbohydrate: when the gut becomes the limiter

At the other end of the spectrum, consuming more carbohydrate than can be absorbed and utilised does not lead to better performance.

Instead, carbohydrate accumulates in the gastrointestinal tract, increasing fluid retention and mechanical stress within the gut. This commonly results in bloating, cramping, nausea, or an urgent need to slow down or stop.

Importantly, this is not always a sign of poor discipline or poor fuelling choices. It reflects a mismatch between carbohydrate intake and an individual’s capacity to absorb and oxidise it during exercise.

More fuel only helps if it can be used.

Why generic fuelling advice often falls short

Standard fuelling guidelines are useful starting points, but they cannot account for the wide variation between individuals.

Differences in carbohydrate utilisation can be influenced by:

• Intestinal transporter capacity
• Gastric emptying rates
• Habitual carbohydrate intake
• Training history and intensity
• Race pace and environmental conditions
• Carbohydrate type and formulation

As a result, one-size-fits-all fuelling recommendations inevitably work better for some athletes than others.

Measuring what actually matters

The missing piece in many fuelling strategies is understanding how much of the carbohydrate you consume is actually contributing to energy production during exercise.

Exogenous carbohydrate oxidation testing allows this to be measured directly. Rather than estimating needs based on body mass or race duration, this approach assesses how much ingested carbohydrate an athlete can absorb and oxidise under realistic exercise conditions.

This provides practical insight into:

• Whether carbohydrate intake is limiting performance
• Whether gastrointestinal issues are driven by excess intake
• What intake range best matches an athlete’s individual physiology

Armed with this information, fuelling strategies can be adjusted with far greater precision.

Fuelling as a performance variable

Carbohydrate fuelling is not just a checklist to be followed on race day. It is a trainable, testable component of endurance performance.

Understanding the difference between carbohydrate intake and carbohydrate use allows athletes to move beyond generic recommendations and towards strategies that genuinely support their performance, without unnecessary risk of running out of fuel or overwhelming the gut.

In endurance sport, what matters most is not how much carbohydrate you consume, but how much you can actually use.

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