Guarana, also known as Paullinia Cupana, is a climbing plant of the maple family. It is a rain forest vine that was domesticated in the Amazon for its fruits which are rich in caffeine. Guarana is also especially common in Brazil. The plant features large leaves and clusters of flowers. Guaraná has long been used as a tonic to treat various disorders in Brazil, and became a national soda in Brazil about a century ago. 
Guarana is known as an effective energy booster. It contains about about twice the amount of caffeine found in coffee beans.  Over the last two decades, guarana has gained popularity as an energy supplement, and is now widely used in various sports and energy drinks, and as well as in a wide variety of fat burners and diet pills. Guarana drinks such as "Guarana Antarctica" are very popular in Brazil.
Because Guarana is rich in caffeine, it has been reported to have beneficial effects for cognitive functions. It works as an energy enhancer, and also has weight loss uses. Some studies have also found Guarana to have antioxidant and antibacterial benefits.
Bawls Guarana Energy Drinks & Guarana Antarctica
The Effects of Guarana
The positive effects of Guarana on cognitive functions and mental fatigue is well documented in various studies involving lab animals, and human subjects. These studies have found Guarana to help improve cognitive performance, while reducing mental fatigue. It has also been found to help memory performance, and improve mood and alertness.
In a UK study conducted at Northumbria University in Newcastle upon Tyne, 129 healthy young adults aged 18 to 24 years old participated in a guarana extract study. The double-blind, randomised, placebo-controlled, parallel groups study assessed the acute effects of a vitamin, mineral, and guaraná supplement. In this study, the participants completed a set of cognitive tasks to measure mental performance. It was found that the group consuming the Guarana supplement had improved task performance compared to the placebo group. The study found that the Guarana supplement attenuated the mental fatigue, and thus demonstrates Guarana's cognition enhancing properties, and is able to reduce mental fatigue associated with sustained mental effort. 
In another UK study performed also at Northumbria University, a double-blind, counterbalanced, placebo-controlled study assessed the acute mood and cognitive effects throughout the day of four different doses of a standardized guaraná extract. The four doses were 37.5mg, 75mg, 150mg and 300 mg. The 26 human subjects performed various mental and cognitive exercises. The study found that Guarana increased the alertness and improved the mood ratings of the participants. It also improved the secondary memory performance of the subjects.  The research found that the two lower doses (37.5mg and 75mg) produced more positive cognitive effects than the two higher doses (150mg and 300mg).
While I found many studies with overwhelming evidence of Guarana's beneficial effect on cognition and mental alertness, I also did come across one study where the effects of Guarana was not apparent in a group of elderly volunteers. This was a Brazilian study assessing the effects of long term administration of guarana on cognition in 45 normal, healthy, elderly volunteers. However the results of this double blind study could not determine any significant cognitive alterations in these volunteers. 
Guarana and Weight Loss Benefits
Aside from its popular use as an energy drink / supplement, Guarana is also widely used in numerous fat burners and diet pills. Guarana contains caffeine, which is a stimulant that's known to have weight loss benefits. Because of this, it is a common ingredient you will find in weight loss supplements. Studies performed on common weight loss ingredient mixtures including Yerba Mate, Guarana, Damiana, and green tea have consistently found a positive correlation between using these ingredients and achieving weight loss.
In another 12 week study, 105 subjects were giving either a placebo or a weight loss formula consisting of Asparagus, Green tea, Black tea, Guarana, Mate and Kidney beans. The results of this study found a significant change in the body composition improvement in the weight loss formula group compared to the placebo group. There was greater body fat loss and a significant change in body composition. However, the study also notes that there was not a significant change in weight or BMI. 
Side Effects of Guarana Extract
In the United States, guarana has the status of being generally recognized as safe (GRAS).  However, despite this, there are some potential side effects of Guarana because it contains the stimulant caffeine. Some possible Guarana side effects include:
jitteriness and anxiety
trouble falling asleep
increased blood pressure
shortness of breath
increased urination frequency
These Guarana side effects will vary from individual to individual. Some individuals who tolerate stimulants well may not notice any side effects, while others will experience these side effects at varying degrees. In a study performed at the Nova Southeastern University in Florida, a study was undertaking to assess the safety issues associated with commercially available energy drinks. Their research involved Red Bull, caffeine, glucose, ginseng, guarana, taurine, and bitter orange. Based on their study, most energy drinks contain natural products such as guarana, ginseng, and taurine with as much as 80 to 300mg of caffeine per average 8 oz serving.
Based on their research results, they stated that "no reports were identified of negative effects associated with taurine, ginseng, and guarana used in the amounts found in most energy drinks." However, "the commonly reported adverse effects seen with caffeine in the quantities present in most energy drinks are insomnia, nervousness, and headache."  Based on their study results, they concluded that the amounts of guarana, taurine, and ginseng contained in various popular energy drinks are much below the required amounts to deliver either therapeutic benefits or adverse side effects.
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