The human body is incredibly complex. Every second, millions of physiological and chemical reactions take place in order to maintain good health. The body both creates things (eg. heat, muscle, proteins, RNA, hair, nails, enzymes, fat tissue, bones) and breaks things down (food, stored fat, etc.). These anabolic and catabolic processes, along with all organs and systems, need fuel to enable them to function. The fuel or energy that the body uses, comes from the food and drink that we consume in our diet. In a nutshell, food is human gasoline.
Carbohydrate is one of the three macronutrients, the other two being protein and fat. Carbohydrates provide essential energy for the brain, red blood cells and a growing fetus. Carbohydrate is also the principal fuel source for strenuous muscular activity. The carb food group includes many foods that are packed with vitamins, minerals and phytochemicals (disease-protective plant chemicals). A healthy diet must include carbohydrates.
The brain's favorite fuel is carbohydrate. If absolutely necessary, the brain will utilize fat (in the form of ketones), but it performs less efficiently on this type of energy. Current evidence demonstrates a clear association between intellectual performance and the presence of carbs. For example, better performance was observed in all types of age and social groups after a carbohydrate meal. Although research is ongoing, it seems that for optimum intellectual activity, we need a regular intake of carbs.
In terms of chemical structure, the two main types are Simple Carbohydrate (or "simple sugars"), like Monosaccharides and Disaccharides; and Complex Carbohydrates (or "complex sugars"), like Oligosaccharides and Polysaccharides. Carbohydrate comes mainly from plant-food, such as vegetables, fruits, beans, and cereal grains, although the simple sugar lactose is found in milk and milk products.
Simple carbs include: glucose, fructose and lactose. Common sources include table sugar (sucrose), boiled sweets, syrups and honey.
Complex carbs comprise starches or fiber. Good sources of starches include, bread, pasta, rice, beans and some vegetables. Good sources of dietary fiber include: vegetables, fruits, beans, along with the indigestible part of the grain, like wheat bran and oatbran.
Our food fuel comprises the protein, carbohydrate, fat and alcohol we eat. But the exact mixture our body uses typically varies according to circumstances (eg. our physical activity, our last meal etc.). There is an "order of priority" that dictates which fuels are burned first. Alcohol calories are burned first. This is because we cannot store alcohol energy. Next, we burn protein, then carbohydrates, then fat. In practice, however, we typically burn a "mixture" of carbs and fat, with the ratio being dependent on meals. Just after meals we burn mainly carbohydrate, while between meals we burn more fat.