Carbohydrates are broken down into simple sugars that are used by the body for energy. As carbohydrates are eaten, the digestive tract breaks them down into monosaccharide units, or glucose. The glucose enters the bloodstream and travels first to the brain, which runs entirely on energy from glucose.
After the glucose needs of the brain have been met, the remaining glucose travels to the other tissues of the body. Once all of the immediate glucose needs of the body have been met, the remaining glucose converts to glycogen, a polysaccharide storage form of glucose. Glycogen is mainly found in the liver and muscles of the human body. If there is more glycogen available than room to store it, the body converts the excess to fat. The fats are stored in the adipose tissue. Once glucose is changed into glycogen and then fat it cannot be changed back into glucose, but it can still be used for fuel. Parts of the body that can only run on glucose for energy, such as the brain, cannot use fats for fuel. Carbohydrates exist in almost every type of food that humans ingest. Simple carbohydrates are found in foods like white bread, pasta and fruit. Complex carbohydrates are found in items such as whole grains.