The need for additional energy during exercise depends on the length and intensity of the activity. Carbohydrate, fat and protein are all used to fuel working muscles.
The muscles choice of fuel depends on the length and intensity of an activity. At a lower intensity more fat will be used. As the intensity increases so does the proportion of carbohydrate used. Only a very small amount of protein is used for energy by a working muscle. For strenuous prolonged (endurance) exercise stores of carbohydrate in muscle and liver will be depleted after 90-120min. It is necessary in these circumstances to consume foods or fluids containing carbohydrate to replenish the stores if planning to continue to exercise at the same intensity. Carbohydrate is also the brain and central nervous systems fuel of choice.
What to choose?
What food you choose to use will depend on a number of factors:
- How long will you be exercising?
- Does your choice agree with your stomach?
- Is it easy to carry?
- Is it in a packet that is easy to open?
- Is it easy to swallow?
- Do you like the flavour?
A recent review of published studies examined the effects on exercise performance with the addition of carbohydrate compared to a placebo. Carbohydrate intake was from fluids containing not more than 8% carbohydrate and intake was between 30-88g of carbohydrate per hour.
The review concluded that the intake of carbohydrate improved performance significantly in activities lasting at least 1 hour. It also recommended looking at the sources of carbohydrate when consuming over 80g of carbohydrate per hour.
Practicing the intake of food and fluid while training is necessary if you are planning to compete in an endurance event.
- There seems to be a limitation by the gastrointestinal system to comfortably and continually absorb more than ~60g of glucose in 1 hour. Many of the original sports products and foods used in exercise provide only glucose.
- The use of multiple transport carbohydrates will allow for the absorption of more carbohydrate, via different intestinal transporters, and the provision of more energy to working muscles.
- Foods or sports products containing glucose and fructose are considered to contain multiple transport carbohydrates.
- What carbohydrates are in what foods? Read the labels and ingredients.
- When looking for multiple transport carboydrates look at the ingredients or look up the ingredients in that food or product.
- Lactose is Glucose:Galactose
- Sucrose is Glucose:fructose
- Maltose is Glucose:Glucose
- Maltodextrin is a long chain of Glucose
- Starch/maltodextrin is an even longer chain of Glucose
The table below outlines very simply when to include carbohydrates, what to choose and how much you should practice the strategy.
1) Temesi, J. et.al., , Carbohydrate ingestion during endurance exercise improves performance in adults. J. Nutr. May 1, 2011 vol. 141 no. 5 890-897
Rebecca Hay – Accredited Practising Dietitian, Accredited Sports Dietitian & Nutrition Consultant