The body uses carbohydrates to create energy, breaking them down into glucose that enters the circulatory system. Leftover glucose is converted into fat for long-term storage. The body can only store one or two days' worth of carbohydrates at a time, so they are used for energy before fat.
One important kind of carbohydrate is fiber. The body cannot digest fiber but instead uses it to gather up waste material in the intestinal tract and help it pass through the body.
When carbohydrates are not in adequate supply, the body starts breaking down fat tissues for energy. Some diets that deliberately provoke this state are known as "ketogenic diets," named for the production of ketones that the body supplies to fuel certain cells that cannot survive on fat. When the body is producing ketones, the liver and kidneys are usually able to convert glucose out of protein. However, without adequate protein sources, the body starts breaking down muscle cells as well.
Carbohydrates are one of three important macronutrients that the body needs to survive. The other two are fat and protein. Normally, fats provide over half of the body's daily energy needs. Unneeded fatty acids are packaged into triglycerides and stored in fat cells for later usage. Fat cells have an unlimited capacity. Proteins are broken down into amino acids and built into new proteins with specific and varied functions.