A study by Alford et al (2001) found that the intake of energy drinks is correlated with improved aerobic endurance and anaerobic performance, as well as significant improvements in choice reaction time, concentration, and memory.¹ Horne and Reyner (2001) found that energy drinks significantly improved the reaction time and reduced lane drifting of sleepy individuals when compared to a placebo, with the greatest improvement lasting an hour after ingestion.² Scholey and Kennedy (2004) determined that the combination of caffeine and sugar found in most energy drinks have a possibly synergistic relationship which results in improved secondary memory and speed of attention.³

Since it has been demonstrated that energy drinks do have a physiological effect on the body, it’s important to investigate which ingredients are contributing to this effect.  The most common ingredients found in energy drinks are: sugar, caffeine, taurine, guarana, and ginseng.  Studies on taurine’s impact on the body are predominantly inconclusive (Triebel et al 2006), although it has been noted that this free amino acid may play an important role in osmoregulation, immunomodulation, and bile salt formation (Bouckenooghe et al 2006), and consuming energy drinks with taurine increases taurine levels in the cerebellum (Roser et al 1998). ^4 5 6  A separate study by Geiß et al (1994) found that energy drinks with taurine led to lower sustained levels of the hormones epinephrine and norepinephrine, which led to significantly longer endurance times in cyclists.⁷  Guarana and ginseng extract are believed to be stimulants that affect cognition (Kennedy et al 2004). ⁸  A study by Haskell et al 2007 determined that Guarana improves memory and general attentiveness,⁹ and Sorensen and Sonne (1996) reported that ginseng improved simple reaction performance as well as abstract thinking.  Other studies have found ginseng supplementation has no effect on physical performance, psychomotor performance and cognitive function, immunmodulation, diabetes mellitus and herpes simplex type-II infection (Engels and Wirth 1997) (Vogler et al 1999) (Persson et al 2003). ^{10 11 12 13}

Despite all of the alleged beneficial properties of including the aforementioned ingredients in energy drinks, a study by Clauson et al (2008) found that popular energy drinks do not have high enough levels of Guarana, Taurine, or Ginseng to have any beneficial physiological or psychological effects, and that the only ingredients in high enough concentrations to have effects are caffeine and sugar (Clauson et al 2008). ¹⁴  Caffeine levels in energy drinks can range from 0mg per serving to 141.1mg which is almost a three fold increase over Coke which contains ~44.5mg per serving (McCusker et al 2006).¹⁵  Additionally, Candow et al 2009 found no improvement in physical endurance as a result of sugar free energy drinks.¹⁶

While the research around caffeine and sugar intakes will require articles of their own, some studies have been done regarding the risks of energy drinks themselves.  Aside from the high level of sugar in energy drinks which could contribute to diabetes, studies have shown that people who have had prior psychiatric issues should avoid energy drinks as it may cause a relapse (Chelben et al 2008) and that the drinks have significant erosive effects on dental enamel (Kitchens et al 2007).^17 18  The effects of energy drinks and hypertension has had conflicting research and is as of this writing, unresolved (Hirata et al 2003) (Winkelmayer et al 2005).^19 20

1. The effects of Red Bull Energy Drink on human performance and mood
2. Beneficial effects of an “energy drink” given to sleepy drivers
3. Cognitive and physiological effects of an “energy drink”: an evaluation of the whole drink and of glucose, caffeine and herbal flavouring fractions
4. Is taurine a functional nutrient?
5. Rapid analysis of taurine in energy drinks using amino acid analyzer and Fourier transform infrared (FTIR) spectroscopy as basis for toxicological evaluation
6. Dangerous Increase of Taurine in the Human Brain after Consumption of an ‘Energy Drink’?
7. The effect of a taurine-containing drink on performance in 10 endurance-athletes
8. Improved cognitive performance in human volunteers following administration of guarana (Paullinia cupana) extract: comparison and interaction with Panax ginseng
9. A double-blind, placebo-controlled, multi-dose evaluation of the acute behavioural effects of guaraná in humans
10. No ergogenic effects of ginseng (Panax ginseng C.A. Meyer) during graded maximal aerobic exercise
11. A double-masked study of the effects of ginseng on cognitive functions
12. The memory-enhancing effects of Ginseng and Ginkgo biloba in healthy volunteers
13. The efficacy of ginseng. A systematic review of randomised clinical trials
14. Safety issues associated with commercially available energy drinks
15. TECHNICAL NOTE: Caffeine Content of Energy Drinks, Carbonated Sodas, and Other Beverages
16. Effect of Sugar-Free Red Bull Energy Drink on High-Intensity Run Time-to-Exhaustion in Young Adults
17. Effects of amino acid energy drinks leading to hospitalization in individuals with mental illness
18. Effect of Carbonated Beverages, Coffee, Sports and High Energy Drinks, and Bottled Water on the in vitro Erosion Characteristics of Dental Enamel
19. Caffeine increases aortic stiffness in hypertensive patients
20. Habitual Caffeine Intake and the Risk of Hypertension in Women