Energy in food comes from three sources: fat, protein, and carbohydrates. A typical energy bar weighs between 45 and 80 g and is likely to supply about 200–300 Cal (840–1,300 kJ), 3–9 g of fat, 7–15 g of protein, and 20–40 g of carbohydrates. In order to provide energy quickly, most of the carbohydrates are various types of sugars like fructose, glucose, maltodextrin and others in various ratios, combined with complex carbohydrate sources like oats and barley. Proteins come mostly in the form of fast digesting whey protein. Energy bars generally don't contain sugar alcohols, since these bars, due to type of carbohydrate content, don't require low calorie sweeteners to improve their taste. Fats in energy bars are kept to minimum and their main sources are often cocoa butter and dark chocolate.
Energy bars are used as energy source during athletic events like marathon, triathlon and other events and outdoor activities, where energy expenditure is high, for longer period of time.