About aerobic activity

Fuel for aerobic exercise

Fat represents one of the two main sources of fuel that support cell function in your body. The other main fuel used is glucose. Fat is stored as adipose tissue around the body, and glucose is stored as glycogen in the liver and the muscle cells. Both fat and glucose are also present in the blood as products of digestion.

Many bodily functions that take place when you are at rest, eg, brain activity, the pumping of your heart and the functions of your internal organs, use glucose as a source of energy. Even short bursts of high energy muscular activity lasting around a minute or so will use glucose as the main fuel.

Will I burn glucose or fat?

Exercise intensity and duration and diet are the main things that affect whether or not your body uses fat or glucose for fuel during exercise. Once the activity of energy-hungry muscle cells increases beyond ‘at rest’ levels for more than about one minute, the body uses aerobic processes that combine oxygen (supplied by breathing in more air) with either glucose or fat to generate energy to sustain the increased activity.

The type of task that causes you to breathe more deeply, ‘aerobic exercise’, is the type of exercise that has the potential to burn fat. Examples include brisk walking, jogging, swimming, cycling, gardening, cross-country skiing and roller-blading. You’ll notice these activities all use the large muscles of your body, those in your arms, legs and back, continuously.

‘Burning fat’ means using stored fat as a fuel to support body function, whereas reducing total body fat, which is what most people want when they say that they want to lose weight, involves burning more kilojoules, whether from stored fat or stored glucose, than are replaced by kilojoules consumed as food and drink. Weight loss is achieved by meeting this goal, often with the assistance of exercises that burn fat and exercises that build muscle.

Low to moderate intensity aerobic exercise

Low to moderate intensity aerobic exercise tends to burn fat, whereas high intensity aerobic exercise tends to burn glucose. People who are new to regular exercise, or who are returning to exercise after a break, can work towards doing low to moderate intensity aerobic exercise for at least 20 to 30 minutes on 4 to 5 days each week. This is a practical and safe way (low risk of injury) to burn body fat.

Exercising for longer at a lower intensity is better than only managing a short time at a higher intensity. This approach to exercise also has significant additional health benefits such as reducing the risk of heart disease and type 2 diabetes. If combined with healthy eating that aims for a slight energy deficit, rather than an energy surplus, low to moderate intensity, moderate duration aerobic exercise can be an effective tool in weight reduction and weight management.

The role of high intensity aerobic exercise

For people who already have an established level of physical fitness, a higher intensity or longer duration of aerobic exercise may be needed in order to burn fat.

Even though high intensity exercise tends to burn the body’s stores of glucose rather than its stores of fat, in high intensity aerobic exercise which lasts say 30 minutes, the total kilojoules burned, irrespective of the source, will be higher than the kilojoules burned in 30 minutes of moderate intensity aerobic exercise. That is, the harder you exercise in your given amount of time, the more you will burn and that includes after you’ve finished and when you’re recovering.

So, if you have moved beyond a beginner exerciser’s level of fitness, aiming to do regular aerobic exercise at high intensity (‘as hard as you can’), may be a more useful guideline than simply continuing to exercise at moderate intensity. Before starting high intensity aerobic exercise, seek individual advice from your doctor, and be aware of the pitfalls of over-exercising, including an increased risk of injury.