During a marathon, many athletes encounter a physiological phenomenon known as the “30-km wall”: a sharp drop in performance at about the 30th kilometre. Mostly affecting less experienced runners, it occurs when the body switches energy sources to support the physical exertion. But there’s no reason to panic! With training and a good nutritional strategy, it is not that difficult to break through the wall.

Glycogen, the body’s first source of energy

At the start of a race, the body uses glycogen, a form of carbohydrate stored in the muscles and liver. “These are medium-sized molecules that provide energy very quickly,” explains Caroline Nicol, a research professor at the Étienne-Jules Marey science of movement institute (ISM)1. “But its reserves are finite, and are depleted faster as the effort intensifies.” Like super unleaded petrol for vehicles, glycogen is an effective fuel for maintaining a high speed – but is only available in limited quantities.

An amateur racer’s glycogen reserve will be nearly exhausted around the 30th kilometre. The body begins to limit its use for a potential emergency. It must then find an additional source of energy to complete all 42.195 kilometres of the marathon.

The metabolic transition to fat

The body draws this energy from fat. However, metabolising it is a more complex process than for glycogen, and this switch changes the body’s response to exertion. “Lipids are a more abundant and sustainable energy source, but they take more time to activate, providing a slower energy flow,” notes Arnaud Hays, a researcher at the HIPE Human Lab2. “When you’re running, it feels as though you can no longer maintain the same intensity, forcing you to slow down.”

Liver cells (hepatocytes) in which glucose is stored as glycogen.

Liver cells (hepatocytes) in which glucose is stored as glycogen.

JosLuis/ Stock.adobe.com

JosLuis/ Stock.adobe.com

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For less experienced runners, this change in metabolism results in a sudden drop in performance – the famous “30 km wall”. The body is switching from its usual high-performance fuel to “diesel mode”.

Cramps and cognitive decline

The effects of this transition are not limited to a slower pace. They can comprise a cascade of physical symptoms. The legs and the impact of the feet feel heavier. Running, which had been relatively energy-efficient at first, suddenly requires much more power. Some athletes may have cramps induced by the loss of water and mineral salts through perspiration. There may also be gastrointestinal discomfort, as blood is diverted away from the digestive tract in favour of the muscles3.

These disruptions can also be cognitive. The brain, which consumes a great deal of glucose, reacts to the drop in blood sugar, along with dehydration and hyperthermia. The 30 km barrier is therefore both mental and physical. As Nicol points out, “At this stage of the race, runners don’t have the same level of energy or of concentration. As a result, they are more liable to make errors or even injure themselves.”

Training to optimise performance

With all of these factors in play, training is essential. According to Hays, “Athletes who are well prepared in endurance begin using fat earlier in the period of exertion.” Called the “sparing effect”, this mechanism preserves glycogen reserves until the end of the effort.

Practise can even increase the body’s capacity to store glycogen in the muscles and liver. “An untrained person can store a total of 300 to 400 grams of glycogen, compared with 600 grams for an experienced runner,” Hays adds.

Women have a higher capacity for endurance

Women generally have more fat reserves than men, including in the muscles. They also have a larger surface area of so-called “slow” muscle fibres, which are especially well-adapted to endurance.

“For this reason, they can mobilise more fat reserves than men, giving them an advantage in a prolonged effort,” Nicol explains. This characteristic suggests that female physiology is naturally more suited to long-duration exercise.

Good nutrition for a good run

In a marathon, the challenge is twofold: to cover a distance of just over 42 kilometres and to optimise the use of energy reserves. Before the race, certain nutritional strategies play a decisive role. “The most recent methods are based on 36- to 48-hour nutrition techniques, in particular a high-carbohydrate diet to temporarily increase the body’s storage capacity,” Hays specifies. “It’s also important not to neglect hydration.”

Refreshment stands help marathon runners withstand the stress of running more than 42 kilometres.

Refreshment stands help marathon runners withstand the stress of running more than 42 kilometres.

Kara /Stock.adobe.com

Kara /Stock.adobe.com

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During the event, refreshment stands offer sweet and savoury foods as well as drinks. Many runners also consume gels containing glucose and fructose, which deliver fast-assimilating carbohydrates, reducing the burden on the body’s glycogen reserves. However, they must be consumed at the right time…

“Any such foods must be taken before hitting the wall,” Nicol says. “Once fatigue sets in, it’s already too late.” In addition, moderation is the rule. At high doses, gels and other sugary mixtures can trigger gastrointestinal distress, which is bad news in the middle of a race.

Now that you know how to get through a marathon, all you have to do is slip on your running shoes and tighten your laces – the finishing line is only 42.195 kilometres away!

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