Essentials of Nutrition for Sports

An athlete with 100% of normal muscle glycogen might be able
to exercise at 75% of VO

max for 80 minutes. With 75% of normal 2

muscle glycogen, exercise time might be reduced to 60 minutes. With glycogen loading to 125% of normal muscle glycogen, exercise time might increase to 100 minutes.

Glycogen Exhaustion

Repeated bouts of moderate- or high-intensity endurance work

can quickly exhaust glycogen reserves.

Figure 4

shows what happens.

Figure 4. Progressive depletion of glycogen stores after three days of two-hour bouts of heavy endurance training with either 40% or 70% carbohydrate diet. Adapted from Costill & Miller, Int J Sports Med, 1980.

A daily program of two hours of activity leads to reduced

glycogen levels. Glycogen levels are

maintained in proportion to the

amount of carbohydrate ingested. Glycogen exhaustion occurs quickly unless a high-carbohydrate diet is maintained. On the morning after three days of heavy endurance training, an athlete consuming a 70% carbohydrate diet still has about 75% of normal glycogen levels. An athlete consuming a 40% carbohydrate diet has less than 15% of normal levels.

Glycogen Loading

Increased glycogen stores can be created through what is called

glycogen loading. This involves a period of (1) glycogen use or exhaustion with heavy exercise,

followed by (2) reduced activity

accompanied by a high-carbohydrate diet. (Athletes also used to consume a high-fat diet in the first period, but further studies have shown that there is no need to incorporate this strategy to successfully load glycogen.)

A Lot of Carbohydrate Is Needed

Do some very rough arithmetic. A 132-pound (60 kilogram)

bicycle racer might use 1,800 calories in basal metabolism and 2,200 in training or racing, for a total of 4,000 calories.

Of those 2,200 calories, 400 might come from fat and 1,800 from

glycogen for high-energy use. All th

at glycogen needs to be replaced

in order for the bicycle racer to work as hard the next day.

Carbohydrate calories are needed

elsewhere. For example, the

brain works only on glucose—it cannot burn fat or protein.

No metabolic processes are 100% efficient. Let us assume that we need about 2,400 carbohydrate calories to

replace the 1,800 lost glycogen calories, to fuel the brain, and to account for inefficiencies.

Therefore, we need 2,400 out of a total 4,000 calories to be

carbohydrate, or 60%.

Nutrition for Sports, Essentials of 49