Physical Activity

How does the body use carbohydrate?

Carbohydrate is the unique fuel source for the brain and central nervous system. To use carbohydrate (which consists of sugars, starches, and fibre) for energy, it must be converted to glucose, the body’s preferred fuel. Glucose is the only form of carbohydrate that is used directly by the muscles for energy. Each gram of carbohydrate (sugars and starches) provides 4 calories of energy. Fibre also provides energy at 2 Calories per gram. Glucose can be used to provide immediate energy to the body’s cells or be stored in small quantities for future use. Excess glucose is stored as glycogen in the liver as well as in muscle cells. Glycogen can be converted back to glucose when the body needs energy and blood glucose levels are low. If glycogen reserves are full, excess glucose may be stored as fat, a secondary fuel source.

When a sugar-containing food is eaten, the body cannot tell whether the sugar (sucrose) in the food came from a fruit or vegetable or whether it was spooned from a bowl. Regardless of its source, sugar (sucrose) is broken down in the same way.

To learn more, see our resource Clips on Sugars - Fuel for Your Activity.

Should athletes eat a lot of carbohydrate, including sugar?

People who are very active (e.g., athletes) have particularly high carbohydrate requirements. Athletes should consume at least 60% of their calories from carbohydrate-containing foods. During low intensity exercise, the body gets most of its energy from body fat. As the intensity of exercise increases, so does the percentage of energy coming from carbohydrate. Carbohydrate is stored in muscles in the form of glycogen, and the more glycogen people have stored in their muscles, the longer they can exercise before feeling tired. This can be accomplished by eating carbohydrates before, during and after high intensity exercise. Restricting carbohydrate intake has consistently been shown to be detrimental to exercise performance. 

A recreational athlete who regularly eats a carbohydrate-rich diet will probably have enough carbohydrate stored to fuel activity. Canada’s Food Guide is designed to meet the nutrient and energy needs for the majority of Canadians. Active Canadians may require higher intakes of energy and certain nutrients, such as carbohydrate, than recommended by the Guide. The DRI report recommends that all Canadians consume 45 to 65% of their total calories from carbohydrate. This range ensures sufficient intakes of essential nutrients and is based on evidence that suggests a role for carbohydrates in the prevention of chronic disease.

What is carbohydrate loading?

Traditionally, “carbohydrate loading” was used by athletes as a dietary training strategy over the 3 days prior to an event, which was designed to maximize muscle glycogen stores. Glucose, the body’s preferred fuel, is stored as glycogen in muscles and in the liver. When muscles are exercised, they use both fats and carbohydrates as fuel. As the intensity of the workout increases, muscles depend more and more on carbohydrates, from glycogen and glucose in the blood. For most people, glycogen stores are enough to keep them going during exercise. But if an activity lasts longer than an hour, glycogen stores may get used up, tiring muscles.

This classic method used by endurance and/or elite athletes exhausted glycogen stores through intense exercise coupled with a low-carbohydrate diet. When glycogen stores were depleted, a high carbohydrate diet would then be consumed (>90% of total Calories). Athletes often experienced low blood glucose, irritability, and chronic fatigue by adhering to this regime. Today, evidence suggests that similar results, without the negative side effects, can be achieved by gradually decreasing the amount of exercise during the six days prior to competition while progressively increasing total carbohydrate intake up to 70% of total calories in the last 72 hours before the event.

To learn more about sugars and physical activity, see our resources Carbohydrates and Physical Activity infographic and Clips on Sugars - Balancing Food and Activity for Healthy Weights.