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Guarana is a native species of South America and has stimulating properties when taken by mouth. Guarana is also used to enhance athletic performance and to reduce fatigue. It has been used in the past as an aphrodisiac, diuretic, astringent, and to prevent malaria and dysentery, diarrhea, fever, headache, and rheumatism.
The active ingredient in guarana was formerly called guaranine (tetramethylxanthine), but was later found to be caffeine. Guarana has one of the highest caffeine contents of all plants (up to 7%), and has been used by manufacturers for its caffeine content (e.g., Dark Dog Lemon®, Guts®, and Josta®).
Although there is no scientific evidence that guarana itself increases mental alertness, its relationship to caffeine makes it probable that it would possess the same effects. It is proposed that the stimulatory effect of guarana is more gradual and sustained than caffeine due to the caffeine-tannin complex. Guarana is generally regarded as safe when not combined with other stimulatory agents, such as ephedra.
Guarana has not been shown to alter cognitive function or arousal in preliminary studies. Caffeine found in guarana may improve simple reaction time, but may not improve immediate memory. Additional study is needed in this area.
Caffeine may have positive effects on mood, and may increase alertness and feelings of well-being. Limited research has been conducted on guarana in this area, and more study is needed.
Caffeine has been used as a weight loss agent due to its thermogenic effects (the process of fat or calorie burning caused by increasing heat output). In available studies, guarana has been studied with other herbs making it difficult to draw a conclusion based on the effects of guarana alone. Additional study is needed in this area.