Caffeine

Do you know kola nuts and cocoa beans are also included among the most common caffeine sources? And do you know how much caffeine content can vary from food to food? Turns out it's quite a lot actually, depending on the type and serving size of a food or beverage and how it's prepared.Caffeine content can range from as much as 160 milligrams in some energy drinks to as little as 4 milligrams in a 1-ounce serving of chocolate-flavored syrup. Even decaffeinated coffee isn't completely free of caffeine. Caffeine is also present in some over-the-counter pain relievers, cold medications, and diet pills. These products can contain as little as 16 milligrams or as much as 200 milligrams of caffeine. In fact, caffeine itself is a mild painkiller and increases the effectiveness of other pain relievers.Caffeine, a purine alkaloid, is an odourless, slightly bitter compound, which in pure form is an extremely toxic white powder. In natural, brewed-coffee form, caffeine is a common stimulant that, once ingested, rapidly affects the central nervous system and equally rapidly leaves the body. Its effects vary from person to person. It can increase your metabolism and give you a feeling of reduced fatigue, but it can also increase nervousness. Depending on your gender, weight, genetic heritage and medical history, caffeine can be a positive pick-me-up or cause levels or discomfort, so it is important to have an awareness of how it makes you feel and the affect on your health.

Sources: http://en.wikipedia.org/wiki/Caffeine
http://www.dailymail.co.uk/sciencetech/article-2927430/Why-coffee-REALLY-gives-buzz-Protein-caffeine-mimics-effect-morphine-claims-study.html
http://www.webmd.com/balance/caffeine-myths-and-facts