Carbohydrates are the main source of energy for the body. The glucose from the carbohydrates is used as fuel for the body. There are 2 types of carbohydrates: fast and slow. Slow carbs take more time to digest and release energy over a longer period of time, whereas fast carbs are quickly digested and cause a quick rise in blood sugar yet also a rapid drop in blood sugar which can cause a "sugar crash".
The glycaemic index (GI) is a way of ranking carbohydrate-containing foods based on how slowly or quickly they are digested and increase blood glucose levels over a period of time. Fast carbs, such as jelly babies, cake and white bread usually have a GI of 55 of higher. Yet, carbohydrates with a GI lower than 55, for example: whole-grain breads, pasta, oats and brown rice are more slowing digested and cause a slower rise in blood glucose.
The main difference with slow release carbohydrates is that they are digested in the small intestine instead of the stomach. This makes them digest more slowly and keep blood sugar levels stable. Since blood sugar is balanced, the body doesn’t pump out as much insulin.
So what type of carbohydrate is the best for performance?
Slow carbohydrates are the best for sports that last for a longer period of time and have limited opportunities for breaks to eat or drink. Therefore, in a football match fast carbs are neccessary. The glucose ingested before the match will start to appear in the circulation 5 minutes later and over the next 45-90 minutes the availability will increase. After 45 minutes there is another opportunity to take carbohydrate in the break period. Therefore, for slow carbs to be effective the sport would need to last over 45 minutes- with no access to food or drink.
Therefore, fast-acting carbs are necessary for sports where speed is needed and it does not last a long time- for example a 100m sprint race as when the athlete is performing they need a quick peak in blood glucose and they will not feel the effect of a sugar crash. However, for example for a marathon the most effective type of carbohydrate would be slow carbs as they release energy over a longer period of time. So, the effectiveness of slow or fast carbs depends on the duration and intensity of the sport.