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Dr. Tom McLellan - Protective Clothing Expert


Energy drinks (EDs) contain caffeine and are a new, popular category of beverage. It has been suggested that EDs enhance physical and cognitive performance; however, it is unclear whether the claimed benefits are attributable to components other than caffeine. A typical 235 mL ED provides between 40 and 250 mg of caffeine, equating to doses that improve cognitive and, at the higher levels, physical performance. EDs often contain taurine, guaraná, ginseng, glucuronolactone, B-vitamins, and other compounds. A literature search using PubMed, Psych Info, and Google Scholar identified 32 articles that examined the effects of ED ingredients alone and/or in combination with caffeine on physical or cognitive performance. A systematic evaluation of the evidence-based findings in these articleswas then conducted.With the exception of some weak evidence for glucose and guaraná extract, there is an overwhelming lack of evidence to substantiate claims that components of EDs, other than caffeine, contribute to the enhancement of physical or cognitive performance. Additional well-designed, randomized, placebo-controlled studies replicated across laboratories are needed in order to assess claims made for these products.


Energy drinks (EDs) represented a $6.7 billion industry in 2010,1 with more than 50% of the consumer market consisting of adolescents and young adults under the age of 35 years.2 In the United States, most ED manufacturers package and market their products so as to ensure the EDs are classified as dietary supplements. By doing so, manufacturers are not required to disclose the quantities of active ingredients in their products.3 In addition, unlike manufacturers of soft drinks, which, in the United States, are classified as a food product and regulated by the Food and Drug Administration, manufacturers of EDs are not limited to a maximum dose of caffeine in a given volume or serving.3 Several case reports4-10 have associated the consumption of high volumes of EDs with manic, seizure, or cardiac episodes that resolved following cessation of the product's ingestion. It has also become increasingly common for younger adults to mix EDs and alcohol11-13; while this reportedly leads to high-risk behavior,13 the association does not necessarily imply a cause-and-effect relationship since caffeine and EDs have been shown to antagonize some, but not all, of the depressant effects of alcohol.14-17 Several reviews are available that address the potential adverse risks associated with EDs.18-25

By its classic physiologic definition, energy represents work performed per unit of time and is a purely physical concept. More recently, however, the concept of mental energy has been introduced to define a uniquely cognitive domain of energy that refers to cognitive performance and mood.26-30 Energy drinks are marketed to improve physical or cognitive performance as well as to promote weight loss through increased energy expenditure. Table 1 provides a summary of the ingredients found in several of the more common EDs. It is apparent that these drinks contain many common ingredients, such as caffeine, taurine, glucuronolactone, glucose, vitamins, and herbal supplements such as ginseng, guaraná, and yerba maté. However, the exact amounts of these ingredients are often not disclosed.

Many randomized, placebo-controlled, crossover studies have documented the effectiveness of EDs as thermogenic, ergogenic, or cognitive aids.31,32 However, the predominant experimental design that has been used to establish the evidence-based support for EDs has involved a drink and placebo comparison. Given the varied composition of individual EDs (Table 1), however, this design does not make it possible to ascribe any positive effects to a single ingredient or to an interactive effect between ingredients. Moreover, many of these studies were sponsored by the manufacturer of the product being evaluated. One could assume it is probably in the manufacturer's best interest to determine whether their product is efficacious, but not necessarily to determine which ingredients, when given alone, are effective. However, it is certainly of scientific and regulatory interest to review the evidence indicating that the effects of EDs containing a variety of ingredients are greater than the effects of caffeine alone.

It is well documented that caffeine can act as a thermogenic, ergogenic, and cognitive aid,33-36 effects which are consistent with the claims of EDmanufacturers. It has been suggested that caffeine is the active component of EDs and is therefore responsible for their physiological and behavioral effects.37 However, a key question is whether these purported effects of EDs are any greater than would be expected given the dose of caffeine the drinks contain.28,34,38-40


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Tom M. McLellan, Phd, is an Exercise and Environmental Physiologist whose expertise is in understanding the limitations of working in Protective Clothing, as well as developing strategies to Optimize Physical and Cognitive Performance During Sustained Operations.

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