You're fuelling. So why aren't you performing?
Part 1: Why carbohydrate absorption matters
Most endurance athletes know that carbohydrates are important. They eat before training and carry a gel, bar or sports drink during longer sessions. Yet some still struggle to maintain their performance as the hours pass.
Sometimes the problem is simply not consuming enough carbohydrate. But another important consideration is how much the body can absorb and use.
Why carbohydrates matter
During endurance exercise, your muscles require a continuous supply of energy. This comes from a combination of carbohydrate and fat, with reliance on carbohydrate generally increasing as exercise intensity rises.
Carbohydrate is stored in the body as glycogen, primarily in the muscles and liver. Depending on body size, muscle mass, training status and diet, a well-fuelled endurance athlete might begin an event with approximately 500–700 g of stored glycogen following effective carbohydrate loading.
That sounds substantial, but the demand can also be substantial.
During prolonged, relatively high-intensity exercise, such as a marathon, an athlete might use roughly 120–200 g of carbohydrate per hour. The precise rate varies considerably according to pace, fitness and physiology, but it illustrates the potential imbalance: over a three- or four-hour event, total carbohydrate would likely approach or exceed the body's initial glycogen reserves.
Not every gram of stored glycogen is freely available to every working muscle and liver glycogen must also help maintain blood glucose. As glycogen availability declines, maintaining the same intensity can become increasingly difficult.
Consuming carbohydrate during exercise provides another source of energy, helps maintain blood glucose and may reduce the rate at which limited glycogen stores are depleted. It does not need to replace every gram being used, but it can help narrow the gap between carbohydrate demand and supply.
Consuming is only the first step
It seems logical that consuming more carbohydrate should provide more energy. However, carbohydrate from a gel, drink or bar must first be digested and absorbed before the muscles can use it.
The amount consumed and the amount absorbed are not necessarily the same.
If intake exceeds what the digestive system can comfortably process, it may increase the likelihood of bloating, nausea or digestive discomfort. The goal is therefore not simply to consume as much as possible, but to find an intake that supports the effort while remaining absorbable and well tolerated.
Tolerance may also be trainable. Regularly practising a planned fuelling strategy may improve the body's ability to absorb and tolerate carbohydrate. This is known as gut training, which we'll revisit later in the series.
This leads to a more useful question:
How can athletes absorb and use more of the carbohydrate they consume?
In Part 2, we'll explore the 2:1 carbohydrate ratio and why combining glucose and fructose may increase carbohydrate availability.
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