Caffeine is a crystal-like substance found in many beverages (coffee, tea, and cola), medications, and foods. Caffeine is quickly absorbed into the bloodstream and attains maximum values within 15 to 120 minutes of consumption. Through the blood, caffeine is spread to many tissues in the body, including the brain. Enzymes in the liver breakdown caffeine, leaving very little to be cleared in urine.
Caffeine has both central and peripheral effects in the body. In the central nervous system caffeine affects parts of the brain and the spinal cord, while in the periphery it affects organs and tissues. At low doses (2-10 mg/kg), it can cause increased alertness, less fatigue, reduced reaction time, increased ventilation, and poorer performance of some fine motor skills. At higher doses (>15 mg/kg), caffeine can cause nervousness, insomnia, headaches, and unsteadiness. Caffeine also has inconsistent effects on the cardiovascular system. Depending on where it acts in the body, caffeine can either raise or lower heart rate and cause blood vessels to constrict or dilate. Caffeine can cause a mild increase in urine output from the kidneys and dilation of the bronchi. Caffeine causes the release of epinephrine from the adrenal glands, which causes lipolysis (break down of fat) in muscle and adipose tissues. This increased mobilization of free fatty acids can lead to glycogen sparing early in the exercise period because the body uses relatively more free fatty acids for energy. Caffeine also directly acts on muscle cells by enhancing the release of calcium from the sarcoplasmic reticulum in muscles cells, which improves muscle contraction.
There are a variety of possible mechanisms by which caffeine is proposed to enhance athletic performance, many of which are mentioned above. The most researched and widely acknowledged mechanism is by glycogen sparing via increased fat utilization, thus allowing the athlete to exercise longer at a given submaximal intensity. Other mechanisms include increased catecholamine release, increased calcium release from muscle cells, and reduced perception of effort. On a cellular level, caffeine inhibits adenosine, causing increased urine excretion, stimulation of the central nervous system, increased lipolysis in fat cells (adipocytes) and increased gastric secretion. Increased lipolysis in fat cells occurs because caffeine reduces the inhibitory influence of adenosine on lipolysis, thus increasing lipolysis. These methods of enhanced athletic performance and other possible mechanisms are examined further throughout the course.
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