Caffeine can help you exercise longer. Your muscles get much of their energy from sugar that they store inside their fibers. When a muscle runs out of its stored sugar, it hurts and cannot contract as effectively. Caffeine raises blood levels of a fat called triglycerides, causes the muscles to burn more fat for energy and thus spares the sugar inside muscles to prolong endurance. This means that drinks containing caffeine can help increase endurance during exercise.

When you take caffeinated drinks such as coffee, tea, or cola and do not exercise, the caffeine can act as a diuretic and drain fluid from your body. However, during exercise, caffeine does not increase urination, so it does not take fluid from your body. A study from the University of Connecticut showed that caffeinated drinks are safe for most people when they exercise (International Journal of Sport Nutrition and Exercise Metabolism, Volume 12, Issue 2, 2002).

However, too much caffeine may cause problems for some people. Researchers at Queens University in Kingston, Ontario reported that caffeine may raises blood sugar levels in healthy people and diabetics, which cannot be reversed by exercise or weight loss (Diabetes Care, March 2005). Another study from the Netherlands showed that drinking coffee can raise blood pressure (Journal of Hypertension, May 2005). Moderation is always advisable.