Caffeine has been shown in some studies to have adverse effects on the human body, while other studies show no significant or beneficial effects. Do your research on such Web sites as www.pubmed.gov, www.nih.gov, www.cdc.gov, www.webmd.com, and on Yahoo! and Google.
Caffeine is a stimulant of the central nervous system (CNS), having the effect of temporarily reducing drowsiness and restoring alertness. This is useful for some people while studying, working and driving. Beverages containing caffeine include coffee, tea, many soda drinks, and some energy drinks. They are very popular.
According to Wikipedia, caffeine is the most widely-used psychoactive substance, and it is legal. Caffeine is used to prevent lung disorders of newborns, and has been shown to reduce the risk of developing Parkinson's disease. Caffeine has been shown to increase blood pressure. See http://preventdisease.com/news/articles/062305_caffeine_bp.shtml and http://www.webmd.com/hypertension-high-blood-pressure/news/20020517/caffeines-effect-on-blood-pressure.
On the other hand, caffeine can be useful in treating patients with hypotension, as was shown in the medical journal article "Caffeinated Beverage Intake and the Risk of Heart Disease Mortality in the Elderly: a Prospective Analysis" by Greenberg J.A., Dunbar C.C., Schnoll R., Kokolis R., Kokolis S., and Kassotis J., published in the American Journal of Clinical Nutrition on February 2007, volume 85, number 2, pages 392to 398. Some studies have shown that caffeine boosts metabolism and so would be beneficial to take before a workout. You can try this for yourself.
Different foods have different effects on different people. However, you should not feel you need to use this or supplements to be healthy and fit and to sustain being healthy and fit.
Check out the Web site http://www.louizapatsis.com/! My book Pocket Guide to Fitness will be available at www.authorhouse.com (best buy), www.amazon.com andhttp://www.bn.com/ in about three weeks.