Essentials of Nutrition for Sports

Fat requires relatively more oxygen to burn than carbohydrate.
Although fat contributes to energy production during exercise, carbohydrate is the key to high efforts. Once your heart rate climbs over 75% of your maximum, more than 50% of your energy is coming from carbohydrate.

This subject is discussed in more depth

Energy Sources at

Various Exercise Levels

, on page

43

.

Heart Rate % of Max

Carbohydrate Energy %

65

15

75

50

85

60

90

70

Table 5. Percent contribution of carbohydrate to energy metabolism at selected percentages of maximum heart rate.

Glycogen Is Crucial for High-End Energy

When not depleted, the body has about 2,000 carbohydrate

calories stored in the form of glycogen. About 500 calories of glycogen are found in the liver and about 1,500 calories are found in muscle.

A one-hour time trial uses up almost all the glycogen stored in

muscle. It is easy for a high-performance athlete to burn up almost all stored glycogen with a day’s workout. We need glycogen replacement for repeated day-after-day training. Glycogen exhaustion limits or eliminates the

role of stored carbohydrate in

long-distance endurance activities that take most of a day or longer to complete.

Performance Time Related to Glycogen

There is a direct relationship between the amount of time a fit

individual can perform threshold level work and the amount of glycogen present. The more glycogen initially present, the longer an individual can maintain an anaerobic-threshold level of effort.

The graph in

Figure 3

demonstrates this linear relationship at

75% of VO

max. 2

Figure 3. Endurance at threshold is directly correlated with glycogen stores in muscle prior to exercise. Adapted from Bergström et al., Acta Physiol Scand, 1967.

A well-rested, recovered athlete has 100% of normal muscle

glycogen. Prior exercise or not replacing carbohydrate results in lower levels. It is possible for musc

le to have more than a “normal”

amount of glycogen through a process known as glycogen loading, described below.

Nutrition for Sports, Essentials of 48