Essentials of Nutrition for Sports

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