Simple carbohydrates

Simple carbohydrates (sugars, sweets, chocolate etc) called monosaccharides are absorbed from the digestive system very quickly. The result of this rapid ingestion is that these simple sugars can cause 'Hyperglycaemia' meaning that the blood sugar level is higher than normal. Insulin is then released by the body to help move the glucose (sugar) from the blood into the energy producing cells. As a result of this, the body's energy producing systems become overloaded with fuel (glucose) and converts this excess into 'FAT'. So excess amounts of carbohydrate will be converted to fat by the body under certain situations.
This can be controlled to a certain degree by limiting the amount of carbohydrates that you ingest in one sitting. By eating a smaller amount (50g or 200 Kcals) more regularly i.e. every 2-3 hours will allow the body to store the glycogen more efficiently.

Examples of Simple Carbohydrates

Complex carbohydrates

Complex carbohydrates (starches) require much more time for ingestion by the body and therefore produce a much slower rise in blood sugar level so are less likely to be converted to and stored as 'FAT'. Complex carbohydrates are also good sources of essential vitamins, minerals and dietary fibre.
The fat conversion from carbohydrates is also less likely to occur in exercising individuals, as their muscle glycogen stores should always need replenishing and exercised muscles can store greater amounts carbohydrates as glycogen than sedentary muscles.

Examples of Complex Carbohydrates

Due to the manufacturing processes available today there are foods that contain a mixture of both simple and complex carbohydrates. They are usually made with flour (complex) and sugars (simple) and these can be found in the following products.

Further Reading : -

Simple and complex carbohydrates....whats the difference?
Carbohydrates and exercise, why is it important ?
A more indepth look at carbohydrates.