Energy Sources at Various
Exercise Levels
Energy for exercising muscle comes from carbohydrate, fat, and
protein.
Carbohydrate may come from blood sugar (from the liver by
way of stored glycogen or metabolized amino acids or from the intestine by the absorption of car
bohydrate) or from glycogen stores
in muscle. Typically, athletes with normal stores have about 2,000 carbohydrate calories stored as glycogen: about 1,500 calories of intramuscular glycogen are stored in muscle cells, and about 500 calories in the liver.
Fat energy may come from the bloodstream by way of adipose
tissue or the intestine, or from fat stores in muscle. About 2,500 calories of intramuscular fat (triglyceride) energy are stored in muscle cells. Depending upon percent body fat, about 50,000 calories are stored elsewhere as fat.
Protein supplies the least amount of energy and is usually
omitted from consideration. If fat and carbohydrate sources are plentiful, protein supplies about 5% of energy sources. Protein contribution to energy production increases after several hours, when intramuscular carbohydrate and fat stores are depleted. As much as 15% of energy sources may derive from protein if muscle fat and glycogen are depleted.
Figure 2
depicts the contribution of carbohydrate and fat to
energy production in a cyclist when energy stores are plentiful. Low activity corresponds approximately to a heart rate of 65% of maximum, moderate activity to a heart rate of 75% of maximum, and high activity to a heart rate of 90% of maximum.
800 600 400 200 0
1000
Low
Medium
High
Activity Level
Blood Fats
Muscle Fats
Blood Sugar
Muscle Glycogen
Calories per Hour
Carbohydrates Fats
Figure 2. Contribution of energy sources at selected exercise intensities. Adapted from Romijn
et al. J Appl Physiol. 1993.
Traditionally the amount of carbohydrate energy used was
believed to be higher than in
Figure 2
. Carbohydrate metabolism was
originally calculated based on respiratory exchange ratios—determined from the relative con
centrations of carbon dioxide and
oxygen expired. Newer techniques have suggested that fat contribution is greater than was previously determined.
The most recent studies show that at low levels of exercise
intensity, about 85% of calories are supplied by fats; at medium levels, about half. At high levels of
exercise intensity, 70% of energy
needs are derived from carbohydrate.
Cyclist and runners use their legs, rather than their arms as do
kayakers. At mild-moderate exer
cise levels, the percentage
contribution from carbs and fat is the same whether the legs or the
Nutrition for Sports, Essentials of 43