Carbo-loading for runners

In normal metabolic function, the body draws its energy by and large from carbohydrate and fat stores. At the onset of exercise, the energy dependency moves toward carbohydrate and moves along the continuum as intensity increases. At maximal exercise, running energy is almost solely derived from carbohydrate. This is because the body's preferred source of fuel is carbohydrate. Carbohydrate is quite a simple source of fuel for energy release. Simple, that is, in comparison to fat.

Without going into the chemical reactions of energy release, it is important to note that fat firstly 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; around two hours of continued steady state exercise. Any continued exercise after this usually proves difficult.

At this point, it is hard for the body to break down and burn its other energy source; fat. From this we can understand why the runner faces difficulty to continue when your body’s carbohydrate stores are exhausted. This is commonly known in running circles as "hitting the wall."

In the mid-1960s, the Scandinavian physiologist Astrand led the way in exploring ways of overcoming carbohydrate depletion. It was here that the classic "bleed run" was devised. The practice was as follows:

A week prior to a race, the marathoner completed a running session for around two hours or twenty miles of running. This was to take the body to a state of carbohydrate depletion, thereby greatly reducing muscle glycogen stores. After this, for the next three to four days the athlete avoided eating foods with high levels of carbohydrate, reducing their intake to around 60 – 100g per day. Instead their diet constituted mostly fats and proteins. Water intake was maintained as normal.

In conjunction, they tapered their running to moderate activity to prevent further muscle glycogen depletion. This regime was maintained up to and at least three days before competition. By denying the body its preferred source of fuel, glycogen synthase activity (the natural resynthesis of glycogen) is accelerated. Following this, the runner then greatly increased their carbohydrate intake ten 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.

However, with such a regime there were problems. During the days of carbohydrate denial, athletes reported feeling irritable, restless, disorientated, muscle weakness, and difficulties in performing mental tasks. Therefore, other less harsh methods have since been proposed.
Wilmore and Costill (1994), suggest that in the week prior to competition, runners should merely reduce their mileage, whilst having a 55% carbohydrate intake. For the final three days, they should take on even greater carbohydrate quantities, and reduce activity to a ten to 15 minute warm-up. Findings show that muscle glycogen levels in this instance were the same or similar to Astrand’s.

Interestingly, recent research expresses a gender difference in such practices. Tarnopolsky et al (1995), reported that women runners who carbo-loaded showed no increase in muscle glycogen storage. That was when total carbohydrate intake was increased from 60 to 75% in the days prior to competition. The concept of sex-specificity to this procedure should be noted, although further investigation is required before a solid conclusion can be made.