Are all Carbohydrates created Equal

Energy stores during exercise

Most people who carry out frequent exercise are aware of the importance of their glycogen stores as an energy reservoir. High intensity exercise can decrease blood glucose (sugar) levels as the body uses glucose for fuel. When this occurs the carbohydrate polymers stored as glycogen in liver and muscles are needed. As blood glucose decreases, the body mobilises the glucose units in glycogen in order to yield more energy for movement. During the course of the exercise, these glycogen stores can become depleted leading to “bonking” when the athlete experiences extreme fatigue or may even collapse. This is most frequently seen at marathons.

How to maximise glycogen stores

The average, well nourished adult can store 400g of carbohydrate as glycogen (the equivalent of 2000kcal in energy) but an athlete can store double this amount. However, inadequate carbohydrate in the diet can result in not enough glycogen being stored. To ensure there is enough glycogen in the body, people who do moderate exercise should obtain 50% of their dietary energy from carbohydrate, before a competition athletes should obtain 70% of their dietary energy from carbohydrate rich foods such as pasta, rice, bread and potatoes.

How to optimise blood glucose levels for exercise

The type of carbohydrate consumed before a meal affects blood glucose levels, hence altering the amount of glucose readily available energy for exercise. The amount that a carbohydrate increases blood glucose levels is called the Glycaemic Index (GI). High GI foods cause a fast spike in blood glucose levels and low GI foods cause a more steady increase and decrease. Therefore the foods that are lower in GI should be consumed before exercise to raise and maintain blood glucose levels at a good level. Consumption of high GI foods (such as sweets, chocolate or cake) before exercise should be avoided as the rapid reduction in the blood glucose levels can cause dizziness, exhaustion and blurred vision – none of which are ideal when exercising!

Why different carbohydrates have different Glycaemic Index values

The GI value of the carbohydrate is affected by the length of the carbohydrate chain. All carbohydrates are digested into glucose molecules that enter the blood and increase the blood glucose levels. The longer carbohydrate molecule is, the slower it is digested so it causes a slower raise in blood sugar levels. If the chain is relatively short (such as in the carbohydrates in sweets), it is digested quickly causing a sharp increase in blood glucose followed by a crash.

Great “low GI” meals for before exercise