Many studies have examined the metabolic and performance effects of caffeine ingestion. Caffeine can affect many tissues including the central nervous system, heart and vascular system, adipose tissue and others.

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 In addition to direct effects on these tissues, caffeine increases sympathetic activity (by increasing epinephrine secretion and sympathetic nervous system activity) resulting in multiple metabolic effects (increased fat burning and thermogenesis) that could have beneficial effect on weight loss and performance. But perhaps the most consistent effect of caffeine is its ability to alter the perception of pain. Researchers at Kent State specifically addressed this effect in an experiment designed to show the effects of caffeine on perceived pain during a task that elicited discomfort. They had healthy college students perform a grip task that consisted of holding on to a metal block with their arm extended and resist dropping the weight. During one trial they chewed gum containing caffeine and during another identical trial they were provided gum with no caffeine.  Subjects were able to hold on to the metal for an average of about 100 seconds. During this time they reported their level of pain on a scale from 0 to 10 where 0 = no pain, 5 = moderate pain, 10 = worst possible pain. During this grip to exhaustion test, the pain was perceived to be significantly less during the caffeine trial (3.5) versus the placebo (4.8). Similar to other studies this study supports the use of caffeine as an analgesic to reduce the perception of pain during strenuous physical exertion.

    . Bellar D, Kamimori GH, Glickman EL. The effects of low-dose caffeine on perceived pain during a grip to exhaustion task. J Strength Cond Res. 2011 May;25(5):1225-8.