Carbohydrates provide most of the calories for normal daily activities, becoming even more important as a fuel source during exercise. They contain 4 calories per gram, and provide between 40 and 60% of the calories in a normal diet.
The basic building blocks of all carbohydrates are single sugar molecules (monosaccharides or simple sugars) made up of 6 carbon units. The linking of two monosaccharides results in a disaccharide, while long chains of sugar molecules (made up of multiples of the 6 carbon units) are referred to as complex carbohydrates or polysaccharides. During digestion, these complex carbohydrates are broken down into simple sugars, absorbed, and transported to the cells in the blood. These sugar molecules are either metabolised immediately to provide energy for the cell or stored in liver and muscle cells as glycogen (polysaccharide) to be used for future energy needs.
The digestive system handles all carbohydrates in much the same way - it breaks them down (or tries to break them down) into single sugar molecules, since only these are small enough to cross into the bloodstream. It also converts most digestible carbohydrates into glucose (also known as blood sugar), because cells are designed to use this as a universal energy source. Most dietary carbohydrates are in the form of the two monosaccharides - sucrose (found in table or cane sugar, apples, bananas, oranges) and lactose (milk sugar found in dairy products), or complex carbohydrates (starches) which are primarily supplied by grains. Before they can be absorbed from the intestinal tract, all disaccharides and complex carbohydrates must first be digested and converted back to a monosaccharide sugar form.
Monosaccharides, the single sugar molecules, deliver energy to the body quickly as they do not need to be broken down (digested) into smaller pieces before absorption takes place. Glucose and fructose are the two most common monosaccharides in our diet.
Carbohydrates can be rated by their glycaemic index (GI) which is the rate at which oral carbohydrates are absorbed into the blood stream (and thus are available as an energy source for exercising muscle). The higher the glycaemic index, the more rapidly the blood sugar will respond to what is eaten. Simple (or single molecule) sugars are generally the most quickly absorbed, but some complex (multiple molecule) carbohydrates can elevate the blood sugar almost as quickly.
It has been found that a carbohydrate-rich diet consumed for 3 days prior to endurance exercise (cycling, swimming or running), accompanied by a decrease in training intensity, results in increased muscle glycogen concentrations of the same magnitude as those achieved with the traditional carbohydrate loading procedure. In contrast, multiple-sprint sports (such as football, hockey, tennis, basketball) that involve a mixture of brief periods of exercise of maximum intensity followed by recovery periods of rest or light activity, and last up to 90 minutes are not be improved by carbohydrate loading. Low glycaemic index carbohydrate (lentils) improve endurance capacity to a greater extent than the high glycemic index food (potatoes and glucose).