Energy drinks can serve as a refreshing beverage choice. Sport drinks, which aid in rehydration, differ slightly from energy drinks, which can have a "pick-me-up" quality. These drinks usually do not emphasize energy derived from the calories they contain, but rather through a combination of caffeine, vitamins, and herbal supplements contained in the drink.
Several studies have shown that energy drinks may enhance mental performance and alertness, due to caffeine and other supplements added to the drink. The caffeine content of energy drinks varies according to the brand. As a comparison, energy drinks typically contain half the caffeine found in regular coffeehouse coffee. Generally, energy drinks include methylxanthines, vitamin B and herbs. Other common ingredients are guarana, acai, and taurine, plus various forms of ginseng, maltodextrin, carbonated water, inositol, carnitine, creatine, glucuronolactone, and ginkgo biloba. Some contain high levels of sugar, while other brands offer artificially sweetened 'diet' versions.
Due to their ingredients, energy drinks have a variety of physiological and psychological effects, improving mental and cognitive performances as well as an increase in subjective alertness.