In normal daily function, the body draws its energy largely from carbohydrate and fat stores. At the start of exercise, the energy dependency moves toward carbohydrate, and increasingly so as the intensity of exercise increases. At maximal effort exercise, running energy is almost solely derived from carbohydrate. This is because carbohydrate is quite a simple source of fuel for energy release in comparison to fat.

Fat relies on the combustion of carbohydrate as a catalyst for it to be broken down from a complex form called triglyceride. It then releases free fatty acids (FFAs), which are used for energy. Unlike fat, carbohydrate has a finite capacity — after around two hours of continued steady exercise, the body will be spent and continued exercise will prove difficult.

It is at this point, where the body has used up its carbohydrate stores, that it finds it difficult to break down and burn fat. This is why the runner faces difficulty after running for around two hours. When the body's cabohydrate stores are exhausted, the runner may expereince what is known as "hitting the wall."

In the mid-1960s, Scandinavian physiologist Astrand started to explore ways of overcoming carbohydrate depletion. His research focused on the "bleed run" practice. This consisted of:

• One week prior to a race, the marathoner completed a running session of around two hours or 20 miles (32.2km) of running — the intention being to get the body to a state of carbohydrate depletion, with minimal muscle glycogen stores.

• For the next three to four days the athlete avoided eating foods with high levels of carbohydrate, with intake reduced to around 60 to 100g per day. The diet instead consisted of mostly fats and proteins, and water intake was maintained as previously. Running was tapered to moderate activity to prevent further muscle glycogen depletion. By denying the body its preferred source of fuel, glycogen synthase activity (the natural resynthesis of glycogen) is accelerated.
• Around 3 days prior to the race, the runner increases their carbohydrate intake 10 fold (400 to 600g), which along with greater resynthesis, led to an increased glycogen uptake by the muscles in comparison to pre-bleed-run levels. In such an instance, it followed on that the distance runner maintained performance for longer.

Problems with the bleed run technique

Denying athletes carbohydrate for three to four days after the bleed run produced side effects with athletes reporting feeling irritable, restless, disorientated, suffering muscle weakness, and even facing difficulties in performing mental tasks. Other researchers therefore experimented with the process to find less harsh methods of getting carbo-loading right for races.

Wilmore and Costill (1994), suggest the following:

• In the week prior to competition, runners should merely reduce their mileage, whilst having a 55 per cent carbohydrate intake.
• For the final three days, they should take on even greater carbohydrate quantities, and reduce activity to a 10 to 15 minute warm-up. Findings show that muscle glycogen levels in this instance were the same or similar to Astrand’s.

Gender differences in carbo-loading

Research has also suggested gender differences in carbo-loading practices. Tarnopolsky et al (1995), reported that female runners who carbo-loaded (increasing their carb intake from 60 to 75 per cent in the days prior to comeptition) showed no increase in muscle glycogen storage. However, more research is required before being able to firmly assert that glycogen loading does not occur in the same manner for males and females.