Carbohydrates are the body's main source of energy, providing 4 calories of energy per gram. When carbohydrates are broken down, the sugar glucose is produced. Glucose is critical to help maintain tissue protein, metabolize fat, and fuel the body.

Glucose is absorbed into the bloodstream through the intestinal wall. Some of this glucose goes straight to work in our brain cells and red blood cells, while the remainder most make its way to the liver and muscles, where it is stored as glycogen, and to fat cells and stored as fat. Glycogen is the body's auxiliary energy source, tapped and converted back into glucose when we need more energy. Although stored fat can also serve as a backup source of energy, it is never converted into glucose. Broken down carbohydrates fructose and galactose go straight to the liver to be converted into glucose.

Complex carbohydrates, have a high molecular complexity requires our bodies to break them down into a simpler form to obtain the much-needed fuel, glucose. Our bodies digest and absorb complex carbohydrates at a rate that helps maintain the healthful levels of glucose already in the blood. These Carbohydrates, starch foods, include whole-grain breads and cereals, pasta, corn, beans, peas, and potatoes.

Simple sugars, naturally occurring in fruits, milk products; and honey, maple sugar, and sugar cane, require little digestion and are quickly absorbed. The body's rapid absorption of simple sugars elevates the levels of glucose in the blood, which triggers the release of the hormone insulin. When Glucose levels may fall after eating foods high in simple sugars, the body responds by releasing chemicals known as anti-insulin hormones. This surge in chemicals can leave a person feeling irritable and nervous.

Carbohydrates, primarly complex, contain indigestible dietary fibers that play a vital role in our health. This dietary fiber, found only in plants, is classified as soluble or insoluble.
Soluble fiber, in oats, barley, beans, peas, apples, strawberries, and citrus fruits, mixes with food in the stomach and prevents or reduces the absorption by the small intestine of potentially dangerous substances from food. Soluble fiber also binds dietary cholesterol and carries it out of the body preventing it from entering the bloodstream where it can accumulate in the inner walls of arteries.
Insoluble fiber, in vegetables, whole-grain products, and bran, provides roughage that speeds the elimination of waste, which decreases the time that the body is exposed to harmful substances, possibly reducing the risk of colon cancer. Experts believe with a proper diet, it is one of the most preventable types of cancer.

Many processed foods contain high levels of added simple sugars, and tend to be high in fat and lacking in the vitamins and minerals found naturally in complex carbohydrates. Nutritionists often refer to such processed foods as junk foods and say that they provide only empty calories, lacking the essential nutrients our bodies need.

Nutritionists recommend foods rich in complex carbohydrates, which provide vitamins, minerals, some protein, and dietary fiber and are an abundant energy source, should make up roughly 50 percent of our daily calories.