Caffeine can be found in some plant leafs, nuts, and seeds, such as the coffee bean, tea leafs (for example, green, black, oolong), kola nut, cacao seed, and certain herbal extracts (such as yerbe maté, guarana) shrubs. An average cup of coffee may contain 50 to 150 milligrams of caffeine, while a cup of tea may contain 50 milligrams. A 12-ounce (355 milliliters) can of soda can contain about 35 milligrams. Although chocolate contains some caffeine, most of its caffeine-like potency comes from a similar substance called theobromine, while tea contains more theophylline.
Caffeine can promote wakefulness and alertness, which in turn can promote more activity and more calories burned. Caffeine seems to do this by competing with the neurotransmitter adenosine in the brain. Adeno-sine seems to be more of a relaxing substance, as it appears to decrease the activity of the brain. However, to counter the effects of caffeine competition, the brain adapts by producing more and more receptors for adeno-sine. So adenosine can overcome the presence of caffeine. This means that we will begin to need to ingest more caffeine to feel the same stimulating effects. This also explains why we feel especially groggy and "washed out" when we do not have the usual morning coffee.
Caffeine also can have cardiovascular stimulatory effects and possibly increase the use of fat as an energy source in the body. In fat cells caffeine promotes the breakdown of fat from stores and the release of fat into the blood. If muscle and the liver are using fat as a principal fuel source, such as in-between meals and during exercise, this can help optimize fat utilization during those times.
In general the research performed on caffeine suggests that it can play a supportive role in weight loss efforts by increasing energy expenditure (thermogenic) slightly above normal. Over the long term this could lead to additional weight loss in conjunction with a caloric imbalance favoring weight loss.
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