Recent research has indicated that energy drinks are the most popular supplement besides multivitamins in the American adolescent and young adult population (17,28). More than 30% of all American male and female adolescents use these supplements on a regular basis. Energy drinks are also reported to be the most popular supplement (41.7% of the 403 athletes surveyed) among young (17.7 ± 2.0 years) elite British athletes (40). The primary reason for their use is thought to be related to a desire for reducing or controlling body fat (6,13,17,28). However, many competitive athletes also use these energy drinks for their potential ergogenic effect. The basic active ingredient in these energy drinks is caffeine, and although ergogenic benefits have been seen with caffeine supplementation in doses ranging from 3 to 9 mg·kg−1 (equivalent to approximately 1.5-3.5 cups of automatic drip coffee in a 70-kg person), there does appear to be a difference in the ergogenic potential when caffeine is ingested in a food source (coffee or sports drink) compared with its anhydrous form. Although both forms have been shown to provide an ergogenic effect, the magnitude of performance improvements appears to be greater when caffeine is ingested in tablet form (21). To maximize the effectiveness of caffeine in an energy drink, supplement companies will often add several additional ingredients to exacerbate the stimulatory potential of caffeine. This brief review will examine the efficacy of these energy drinks regarding performance improvements and metabolic enhancement. In addition, issues relating to the safety of energy drink consumption will also be discussed.
ENERGY DRINKS AND WEIGHT LOSS
Caffeine alone has been demonstrated to be effective in enhancing lipolysis, fat oxidation, and reducing glycogen breakdown (1,15); however, when it is combined with other thermogenic agents, its effectiveness appears to be magnified (12,27). Caffeine, in combination with ephedra, has been shown to be an effective supplement for increasing metabolic rate and stimulating fat loss (8,23). However, as a result of the Federal Drug Administration's ban on ephedrine alkaloids in 2004, the use of alternative therapeutic means to combat obesity has also been examined. Synephrine is a mild stimulant and is thought to contribute to appetite suppression, increased metabolic rate, and lipolysis (18). To maximize its effectiveness as an effective weight loss supplement, it appears that synephrine may need to be combined with other herbal products (26). Some of these products may include yohimbine, yerba mate extract, hordenine, phenylethylamine, and methyl tetradecylthioacetic acid. All of which have been shown to play a role in enhancing lipolysis and increasing energy expenditure (3,5,19). However, many of these additive ingredients are used for dual purposes. For instance, phenylethylamine is an endogenous neuroamine that is often included in weight loss supplements to enhance mood. Several studies have shown that phenylethylamine can relieve depression and improve mood in clinical populations (24,46). Whether these ingredients can enhance mood in an apparently healthy population is not well established. A recent examination of an energy drink containing several ingredients including phenylethylamine was unable to demonstrate any significant effect on mood state (32).
A recent study examined the thermogenic effect of an acute ingestion of an energy drink marketed as Redline Princess (Vital Pharmaceuticals, Inc., Davie, FL) (32). The supplement contained a host of ingredients that included caffeine, beta-alanine, vitamin C, beta-phenylethylene, hordenine HCL, evodiamine, N-methyl tyramine, 5-hydroxytryptophan, potassium citrate, vinpocetine, yohimbine HCL, and St. John's wort extract. Results of the study indicated that acute ingestion of the supplement enhanced fat utilization, as reflected by a greater caloric expenditure from fat (see Figure 1). Other studies have shown that an acute ingestion of a supplement containing caffeine, citrus aurantium, garcinia cambogia, and chromium polynicotinate can significantly increase caloric expenditure (30,49). These multi-ingredient formulations do appear to be effective in stimulating metabolic changes. The role that each ingredient contributes though is not well understood. Part of the problem in understanding individual contribution of these ingredients is related to the proprietary nature of the ingredient panel. To maintain a competitive advantage, sport supplement companies are not apt to disclose specific ingredient concentrations and tend to group various ingredients as specific “matrixes.”
The effect of prolonged consumption of energy drinks on weight loss has shown some promising results. In a clinical examination, Boozer et al. (8) have reported significant decreases in body mass and body fat, with positive alterations to lipid profiles following 6 months of using an ephedrine and caffeine supplement. Providing further support, an additional study examining a combination of ephedra, caffeine, omega-3 fatty acids, and several vitamins for 9 months in women showed significant decreases in body mass, body fat, and improvements in various metabolic indices such as insulin sensitivity and lipid profiles (25). The beneficial effect of energy drinks in short-duration studies and without the use of ephedra compounds has also been reported. Roberts and et al. (44) demonstrated that following 28 days of consuming the energy drink Celsius (containing 200 mg caffeine, guarana extract, green tea leaf extract, glucuronolactone, ginger extract, and taurine), a significant decrease in body fat and body mass in healthy college-aged athletes was seen. These beneficial effects observed for the multi-ingredient energy formulations have been demonstrated without concomitant alterations to diet or exercise habits. Whether the addition of energy drink consumption to a dietary and exercise intervention can exacerbate weight and body fat loss is not well understood. In consideration of the large obesity epidemic within the United States (48), additional research appears warranted to determine whether energy drinks can play a significant role, in combination with dietary and exercise intervention, in the treatment options associated with weight loss.
ENERGY DRINKS AND ATHLETIC PERFORMANCE
Energy drinks are often used by athletes as a pre-exercise or pre-game supplement to either enhance the quality of their workout or improve athletic performance. Although caffeine has been used as an ergogenic aid for many years, consistent benefits have only been seen during endurance activities, in which time to exhaustion is often reported to increase (14,20,21,31). This delay in fatigue is thought to be related to caffeine's ability to alter exercise metabolism by enhancing fat oxidation, thereby preserving muscle glycogen content (47). Although caffeine has also been suggested to augment strength and power performance by enhancing muscle contraction efficiency through accelerated mobilization of intracellular calcium ions from the sarcoplasmic reticulum (35) and/or by enhancing glycolytic regulatory enzyme kinetics (47), evidence demonstrating its ergogenic benefit during anaerobic performance has been limited.
To increase the ergogenic potential of energy drinks, caffeine is often combined with other ingredients to provide a synergistic effect and increase the probability of a performance response. Several recent studies have demonstrated that a pre-exercise energy supplement can delay fatigue and improve the quality of a resistance training workout (31,33,41). The combination of 450 mg of caffeine, 1,200 mg of garcinia cambogia (50% hydroxycitric acid), 360 mg of citrus aurantium extract (6%), and 225 μg of chromium polynicotinate in an enriched coffee drink was shown to significantly enhance time to exhaustion during cycle ergometer exercise by 29% compared with subjects consuming decaffeinated coffee (31). However, within the same study, no difference in anaerobic power performance was noted between the consumption of the supplement and placebo (decaffeinated coffee). These results were similar to another study that showed that an energy supplement (Red Bull energy drink; Red Bull GmbH, Fuschl am See, Austria) increased upper-body muscle endurance but had no effect on power performance during repeat Wingate anaerobic power tests (16). Improvements in the volume of training (defined as total number of sets × repetitions in a workout) from an energy drink were also confirmed by a recent study by Hoffman et al. (33). They showed that an energy drink containing 110 mg of caffeine, 1,500 mg of l-taurine, 350 mg of glucuronolactone, and 5.2 g of branched-chain amino acids consumed 10 minutes before a resistance exercise session enhanced acute exercise performance by increasing the number of repetitions and the total volume of exercise performed during that training session. The greater volume of training was also shown to augment both the growth hormone and the insulin response to exercise, indicating that consumption of this pre-exercise energy supplement enhanced the anabolic response to the training session.
Energy drinks improve endurance performance and the quality of a resistance exercise workout; however, many athletes use energy drinks primarily for its stimulatory effect, specifically to enhance focus, alertness, and reaction time. The data to support this effect are limited but they do provide evidence to support many of these empirical claims made by athletes. The popular energy drink Red Bull has been shown to enhance cognitive performance through improved choice reaction time, concentration, and memory, which reflected an improved alertness (2). Recently, Hoffman et al. (29) examining an energy drink containing caffeine and a number of additional herbal and botanical compounds that included evodiamine, N-acetyl-l-tyrosine, hordenine, 5-hydroxytryptophan, potassium citrate, N-methyl tyramine, sulbutiamine, vinpocetine, yohimbine HCL, and St. John's wort extract (marketed as Redline Extreme; Vital Pharmaceuticals, Inc., Davie, FL) reported significant increases in focus, alertness, and reaction time. Improvements in reaction time (see Figure 2) were assessed through both visual and auditory stimuli. Interestingly, despite a significant improvement in reaction ability, no significant improvements were noted in anaerobic power performance as assessed by repeated Wingate anaerobic power tests.
SAFETY ISSUES RELATED TO ENERGY DRINK CONSUMPTION
As mentioned earlier, caffeine is a mild stimulant and is commonly found in coffee, tea, chocolate, and soft drinks. The concentration of caffeine in these products has been reported to range from 40 to 150 mg (37). In contrast, the top selling energy drinks have caffeine levels that range from 75 to 174 mg per serving, while in some of the higher caffeine energy drinks, levels may exceed 500 mg per serving (42). The adverse effects seen with caffeine in these doses include insomnia, nervousness, headache, and tachycardia (9). However, changes in the blood pressure response have been inconclusive. Several studies have reported significant elevations in systolic blood pressure (30,49), while others have shown no change (2,10,32,45). Differences between the studies are not clear, but they are likely related to differences within the combination of ingredients that are generally associated with these energy drinks. These studies have consistently shown no alterations in diastolic blood pressure. Interestingly, a recent discussion of the safety issues associated with energy drinks suggested that the products that are generally added to these supplements such as guarana, ginseng, and tuarine are in concentrations that are far below the amounts associated with adverse events (9). However, in energy drinks that contain ephedra alkaloids or other β-agonists such as citrus aurantium (e.g., synephrine), a higher risk for an exaggerated sympathetic response may be present.
Several case reports have indicated that energy drinks may increase the risk for ventricular tachycardia (38) or myocardial ischemia (7). However, a European study has reported no association between caffeine consumption and cardiac conduction abnormalities (36). A health concern though has been raised if energy drinks are consumed with alcohol. A recent study has reported a blunted cardiac autonomic control in healthy subjects consuming energy drinks mixed with alcohol (50). Although no significant arrhythmias were reported, the authors suggested that individuals who were predisposed to arrhythmia may be at an increased risk for a significant adverse event if they combine alcohol with an energy drink.
A concern that energy drinks can increase the risk for dehydration was raised based on evidence that caffeine can induce diuresis and natriuresis (43). However, in several well-designed studies, caffeine consumption has not been shown to impair hydration, exacerbate dehydration, or impair thermoregulation (11,22). In a recent review of the literature, Armstrong et al. (4) have surmised that caffeine does not reduce exercise-heat tolerance or increase the risk for hyperthermia. However, it should be acknowledged that energy drinks that contain ephedra or other β-agonistic compounds may not have the same benign risk. Several well-documented heat deaths of professional athletes who were using ephedra had resulted in part to the banning of that herbal ingredient in 2004. It may be prudent to advise against the use of energy drinks that contain these ingredients in individuals who are poorly conditioned, overweight, and exercising in the heat.
Additional concerns recently raised about energy drinks surround the issues relating to dependence, withdrawal, and tolerance (42). Although many of these issues have been studied with caffeine, direct studies with energy drink consumption are limited. There is considerable debate whether caffeine can produce a dependence syndrome that is similar to that associated with a narcotic. A few studies have suggested that habitual caffeine users may fulfill diagnostic criteria for substance dependence (34,39); however, there is no evidence to suggest such behaviors in individuals consuming energy drinks. The issue of withdrawal has been noted in more than 60 studies published in the medical literature in the past 10 years (42). Symptoms such as headache, tiredness/fatigue, sleepiness, and irritability are associated with caffeine withdrawal. How this is affected by energy drink consumption is also not well understood. The issue of tolerance is a major concern for athletes who use energy drinks on a regular basis during their competitive season. In athletes who play on a daily basis such as in baseball, the issue of tolerance may have important implications as the season progresses. Although high caffeine ingestion has been associated with tolerance (42), there are no studies to date that have examined the issue of tolerance in energy drinks.
Energy drinks are the most popular supplement being used by young adult and athletic populations today. Although caffeine is the primary active ingredient in these drinks, the combination of various other ingredients including herbal and botanical compounds makes it quite difficult to provide a general statement of efficiency for energy drinks. The answer regarding the efficacy and risk associated with an energy drink is specific to the ingredients within each respective supplement. Based on a number of studies examining various energy drink formulations, the efficacy of these supplements has been established regarding improvements in exercise endurance (e.g., delaying fatigue). However, ingestion of energy drinks does not appear to provide any significant effect on strength and power performance but does appear to enhance the quality of a resistance training workout. In addition, many strength/power athletes use energy drinks for their stimulatory potential. Ingestion of these drinks has been shown to enhance alertness, focus, and reaction time to various stimuli.
Energy drinks have also been shown to increase energy expenditure and enhance fat utilization. Several studies of various durations have provided interesting evidence, suggesting that energy drinks may have a role in weight loss regimens and may provide a potential therapeutic effect when combined with dietary and exercise interventions. However, additional research is still warranted in this area.
The majority of energy drinks appear to be well tolerated with minimal risks associated with their use. However, energy drinks containing ephedra alkaloids or other β-agonists may increase the sympathetic response (i.e., elevations in heart rate and blood pressure) that can potentially exacerbate an underlying cardiovascular problem. Interestingly, there are a number of well-controlled clinical studies that showed that a low-dose ephedra/caffeine combination can significantly alter body composition. With appropriate medical supervision, the use of such energy drink combinations may prove to be beneficial. Regardless, prudence should be used in the use of these supplements for individuals who are untrained, over fat, and exercising in the heat.
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