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The role of glucose during exercise

By Mitch Kemmis

Glucose, which is stored as glycogen in the muscles and liver, is vital for physical activity.

During exercise, the liver breaks down it’s glycogen stores and releases glucose into the bloodstream. Our muscles use this as well as their own glycogen stores to fuel their work.

The greater the muscle glycogen stores, the longer the glycogen will last during exercise, which ultimately influences exercise performance. Once your glycogen stores have been depleted, your muscles will start to fatigue.

Glycogen storage is influenced by how much carbohydrates are consumed. Therefore, in order to fill glycogen stores, you need to eat plenty of carbohydrate rich foods, such as rice, oats, potatoes and bread.

How long your glycogen stores last during exercise is not only dependent on your diet, but also the intensity of the exercise being performed, duration of activity and level of training.

Moderate intensity activities such as jogging, which have steady breathing, use glycogen slowly. High intensity activities, where you are out of breath, use glycogen quickly.

Within the first 20 minutes of moderate intensity activity, the body uses mostly glycogen for fuel. As the muscles burn all their own glycogen stores, the liver begins to release all its glycogen stores to provide the muscles with more glucose. After about 20 minutes, if you continue to exercise moderately, your body starts to use less and less glycogen and more and more fat for fuel.

Your level of training also impacts how much glycogen muscles will store. Muscle cells that repeatedly deplete their glycogen through hard work adapt to store greater amounts of glycogen to support the workload. Conditioned muscles also rely less on glycogen and more on fat for energy, so glycogen breakdown and glucose use are slower in trained individuals as opposed to untrained individuals at a similar work intensity.

The majority of an individual’s glucose is provided by eating carbohydrate rich foods throughout the day. In addition, consuming glucose within a few hours of exercise helps top up stores, providing the greatest possible glucose supply to support exercise performance.

In addition to their own glycogen stores, muscles can obtain glucose from food and beverages consumed during exercise. Consuming carbohydrates during exercise is especially useful for exhausting endurance activities (greater than 60 minutes) or team sports such as soccer or rugby. Endurance athletes regularly run short of glucose by the end of events. Sports nutrition experts recommend consuming 30-60g per hour during prolonged events to ensure glucose levels are sustained.

Muscles that have been depleted of their glycogen stores have greater insulin sensitivity, which increases glucose uptake and promotes glycogen synthesis. Therefore, eating a good source of carbohydrates after exercise also enlarges glycogen stores. To optimise the rate of glycogen storage, it’s recommended to consume a high carbohydrate meal within two hours of completing exercise. After two hours, the rate of glycogen storages decreases by almost half.

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