Properties and Characteristics of Caffeine
Properties and Characteristics of Caffeine
Caffeine is classified as a central nervous stimulant and an ergogenic compound. The active ingredient in coffee is 1,3,7-trimethylxanthine, and is metabolized primarily in the liver, with assistance from the kidneys and brain. Worldwide, the average daily consumption of caffeine is 70 mg, or the equivalent of one small cup of round coffee per person. Besides coffee, caffeine is also present in chocolate, kola nut, soft drinks, caffeinated teas, and various prescription and over-the-counter drugs (eg Anacin, Vanquish, Midal, Excedrin). The amount of caffeine in a cup of coffee or tea varies with the strength of the beverage and the volume of the cup. The caffeine contents of various commonly consumed beverages are listed in Table 1 below.
Beverage Caffeine Content/Serving Size
Instant Coffee 66-74 mg/150 mL
Brewed Coffee 146 mg/150 mL
Decaffeinated brewed coffee 3-9 mg/150 mL
Weak tea (1-min brew) 19-28 mg/140 mL
Strong tea (5-min brew) 38-70 mg/mL
Green tea 15 mg/140 mL
Iced tea 22-36 mg/240 mL
Cola soft drink 47 mg/360 mL
Cocoa 0-16 mg/217 mL
The principal effects of acute ingestion of caffeine are to stimulate the central nervous system (heart rate/blood pressure), increase urine output from the kidneys, stimulate cardiac muscle, decrease peripheral resistance in blood vessels, increase gastric secretions, and relax smooth (e.g. bronchial) muscle. (4) Caffeine is readily absorbed and easily diffusible, or absorbed, thus an oral dose is a reliable predictor of levels in the body when taken on an empty stomach. This makes caffeine an effective supplement when taken, but also a possible dangerous compound if proper guidelines for safe use are not correctly followed.
Caffeine in Sports & Exercise
Exercise physiologists have thoroughly studied and examined caffeine for its effects on sports performance and exercise metabolism. With regards to endurance performance, the literature suggests that a dose of 9 mg caffeine/kg bodyweight, taken 1 hour prior to exercise, enhances performance at intensities of about 80-85% VO2max. (4) The specific metabolic mechanisms of this ergogenic effect are unkown, but plasma epinephrine, a known CNS stimulant, rises after caffeine is ingested. This may effect substrate, or fuel utilization in the body causing a preservation of energy. (2) The research does support the effects of caffeine, however, the types and duration of the exercise is a significant factor. The majority of studies demonstrate that caffeine does not enhance performance during incremental exercise lasting 8 to 22 minutes, or during sprints lasting less than 90 sec (4). Several research groups although have demonstrated that caffeine enhances short-term exercise lasting about 5 minutes, as well as maximal muscular power.
The effects of caffeine are influenced by several factors. The ingestion of a meal prior to supplementation, and caffeine tolerance appear to be the predominant factors. Ingesting caffeine with a meal causes lower levels in plasma and urine, suggesting an effect on digestion and absorption. Additionally, with regular consumption of caffeine, tolerance develops to many of its effects, in as little as 4 or 5 days, in most adults. When this occurs, a typical physiological response requires a greater amount of caffeine (4). In an important study, researchers determined that the average effective dose of caffeine was 0.48 mg/kg bodyweight when subjects abstained from caffeine for 2 months. After a tolerance was induced, the minimum effective dose increased to 1.12 mg/kg bodyweight.
Safety Concerns & Guidelines
Due to the metabolic and cardiovascular effects of caffeine, its use or misuse can result in particularly detrimental acute consequences. Large doses of caffeine induce what are called stress-like neuroendocrine responses that are characterized by increased serum corticosterone and B-endorphin, and decreased serum growth hormone and thyroid-stimulating hormone. These clinical findings show the possible detrimental effects of chronic intakes of large doses of caffeine. However, these effects would not occur in adult individuals consuming the doses of caffeine related to approximately 1 to 2 cups of coffee a day. It is with the large doses of caffeine, which can easily be reached with improper use, that these effects are apparent.
The use of caffeine in athletics should only occur with caution, and preferably under the supervision of a professional in nutrition or physiology. Presently, both the National Collegiate Athletic Association (NCAA) and the International Olympic Committee (IOC) classify caffeine as a banned substance, due to its ergogenic properties. Because the ergogenic effects of caffeine can be achieved with as little as 3 mg/kg bodyweight, athletes should abstain from its use 48-72 hrs prior to competition. The NCAA and IOC define concentrations of 15 to 12 ug caffeine/mL urine as “positive tests”, respectively. It is therefore the responsibility of coaches and other professionals who are responsible for the health and welfare of college athletes to properly educate them on the dangers of misusing caffeine, or other ergogenic aides.
1) Dodd, Strephen L, Herb, Robert A, Powers, Scott K. Caffeine and exercise performance. Sports Medicine. 15(1): 14-23, 1993.
2) Graham, Terry E, Rush, James WE, Soeren, Mary H van. Caffeine and Exercise: Metabolism and Performance. Canadian Society for Exercise Physiology. 19(1): 111-138, 1994.
3) Astrup, A, Toubre, S. Thermogenic, metabolic, and cardiovascular response to ephedrine and caffeine in man. International Journal of Obesity. 17 (S-1): S41-S43, 1993.
4) Armstrong, Lawrence A. Caffeine, body fluid-electrolyte balance, and exercise performance. International Journal of Sport Nutrition and Exercise Metabolism. 12: 189-206, 2002.