FOOD ENERGY CONTENT

Some foods provide more energy per gram than others. Not only does the fibre content of foods vary, but the energy contained in equal weights of the basic ingredients - carbohydrate, fat, and protein - is not equivalent.

In most literature, the energy content of foods is expressed in Calories (note the capital "C") as opposed to the use of calories or kilojoules (kj) in scientific literature. The energy contained in one nutritional Calorie is the equivalent of a kilocalorie (1000 calories, lower case "c") or 4.18 kilojoules. Carbohydrates and protein each contain a little more than 4 Calories of energy per gram while a gram of fat has more than double the energy value at 9 Calories per gram.

MUSCLE ENERGY SUPPLY

Although carbohydrates supply the majority of the energy for muscles during vigorous activity, fats can be a major contributor for less strenuous activities. Carbohydrate is stored as glycogen in muscle and liver cells. On a normal diet there is enough glycogen to support 2 hours of aerobic exercise before the bonk occurs. These internal stores can be extended by using oral carbohydrate supplements for events expected to last more than 2 hours. It is best to begin the carbohydrates at the start of the event as they are much less effective after the bonk has occurred.

In addition to extending the time to fatigue in longer, moderate activity events, several studies have also suggested that maximal performance in a 1 hour, high intensity event can be improved with oral carbohydrate supplementation. Drinking a total of 1 litre of a 7% carbohydrate solution at the beginning and during the event improved times by 2%.

Skeletal muscle oxidizes carbohydrate in the form of glucose, and other sugars must be converted to glucose by the liver before they can be used as fuel by the muscle. Studies have demonstrated no additional benefit for glucose polymers, fructose, or sucrose (common table sugar) for carbohydrate replacement. In large amounts, fructose can cause diarrhea.

Although carbohydrates are superior to fats in supporting maximal performance, there is some controversy over the relative benefits of simple vs complex carbohydrates as the ideal supplement to be used during prolonged exercise. Examples of complex carbohydrates are rice, spaghetti, and baked potatoes.

Fats provide over 50% of the Calories expended during moderate exercise even when adequate carbohydrates (glycogen) are available. As the level of exercise increases, the proportion of the total energy expenditures replaced by fats diminishes. And in maximum performance events, where metabolism becomes anaerobic, fat metabolism ceases and only carbohydrates are available as an energy source. Although there has been speculation that using fats in a dietary program both during training and as supplements during competitive events might improve athletic performance, the only hard evidence to date suggests that it may help endurance athletes involved in long events.

Protein is a maintenance material being used to repair muscle (and other) cell injuries - including the microtrauma that occurs with exercise. It is NOT used by the body as an energy source except in very malnourished states. Even in endurance activities such as the Tour De france, protein needs of 1.5 gms protein/kg body wt/day were easily met by a normal (unsupplemented) diet that replaced the total Calories expended. A review of literature fails to demonstrate any advantage to protein supplements over carbohydrate supplements alone. One study actually demonstrated a DECREASE in overall performance.

TOTAL ENERGY STORES

In the well fed and rested state, the human body contains approximately 1500 carbohydrate Calories (stored as glycogen) in the liver and muscle tissue, and over 100,000 Calories of energy stored as fat. This is adequate carbohydrate for several hours of brisk cycling, and enough fat to continue to support cycling at a reduced speed for days.