Energy drinks are a category of beverages that typically contain caffeine. Products in this category include Red Bull, Redline, Monster, Full Throttle, as well as many others (Table). From increasing trends in sales over the previous years, the energy drink category will soon become a multibillion-dollar industry. The purpose of this review is to examine the current scientific evidence regarding the “energy drink” category.
REVIEW OF LITERATURE
In many acute clinical human trials, some over-the-counter energy drinks have shown a positive stimulation of resting energy expenditure (REE) (9,12,29). With an increase in energy expenditure at rest, more energy is being expended as long as the chemicals are within the system. A greater amount of calories being expended at rest translates into weight loss over a period. Furthermore, several research studies have demonstrated that short-term thermogenic drink (TD) ingestion does not result in adverse health effects, making it safe for healthy individuals (2,9,12).
Recently, Dalbo et al. examined the acute effects of ingesting a commercial TD on changes in energy expenditure and markers of lipolysis in 60 healthy, college-aged (men: 23.2 ± 4.0 years, women: 23.4 ± 3.1 years), nonhabitual, caffeine users. The subjects were given either a 336 mL of TD (in this study, the drink was Celsius, Delray Beach, FL) or a non-caloric, non-caffeinated placebo. Preingestion measurements of height, weight, heart rate, blood pressure; REE; respiratory exchange ratio (RER); and glycerol and free fatty acid (FFA) concentrations were recorded. The results showed that when compared with the placebo, the TD significantly increased REE at 60, 120, and 180 minutes (p < 0.05). Free fatty acid concentrations were significantly greater in TD compared with the placebo at 30, 60, 120, and 180 minutes after consumption (p < 0.05). The investigators found no between-group differences in RER, and no adverse side effects were noticed. However, the results of this study suggest that the proprietary blend (1,810 mg: taurine, guarana extract, green tea leaf extract, Epigallocatechin gallate (EGCG), caffeine anhydrous [200 mg], glucuronolactone, and ginger extract) contained in one serving of Celsius may be an effective stimulus to promote weight loss independent of modifications in diet or exercise (9).
A follow-up to the previous Celsius study has been completed to investigate efficacy and safety of a popular TD (Celsius) after 28 days of ingestion. The investigators tested this TD to evaluate the acute thermogenic and lipolytic effects. Body composition and clinical chemistry safety markers were also evaluated. Results showed that percent body fat and fat mass decreased in the TD group compared with the placebo (PLA) group after 28 days. On day 28, the FFA area under the curve (AUC) values was greater in the TD group compared with the PLA group. There was no significant difference in day 28 REE AUC values or glycerol AUC values. However, there was a significant increase in REE values in the PLA group. There were no differences between groups concerning blood and clinical safety markers (25). Tying back to the previous Celsius study, Dalbo did note that from a biochemical perspective, changes in serum concentrations of glycerol were not as outstanding as FFA concentrations (9). This has also been shown by Van Soeren et al. in 1996 that when 6 subjects with impaired epinephrine responses were tested at rest to investigate the effects of a 6 mg/kg caffeine liquid supplement, there were significant increases found in both serum FFAs and glycerol (30).
The results from this study however did show that data they obtained are consistent with prior research on the subject of REE. Results also have been similar in finding that FFA concentrations are significantly elevated up to 180 minutes post ingestion, while glycerol values were only significantly elevated 60 minutes post ingestion (9,11,25). However, investigators hypothesized that following caffeine ingestion, the increased glycerol concentrations are most likely a function of catecholamine secretion and/or adenosine receptor antagonism, which results in glycerol release from adipose tissue causing an increase in fatty acid oxidation (4,11). This has yet to be concluded, and further research is needed.
A few recent studies have also compared the effects of a coffee beverage with additional caffeine to regular coffee. Depending on serving size and how it is prepared, regular drip coffee can contain anywhere from 65 to 200 mg caffeine (29). JavaFit Energy Extreme (Javalution Coffee Co., Fort Lauderdale, FL) is an energy drink that tastes and smells like coffee but with an additional dose of caffeine. Results from these JavaFit studies concluded that in male and female habitual coffee drinkers, there was a significant effect for REE (p < 0.01), systolic blood pressure (p < 0.01), RER (p < 0.01), and o2 (p < 0.01) compared with regular caffeinated coffee. Furthermore, there were no differences between trials on resting heart rate and diastolic blood pressure. The JavaFit beverage showed a significant overall mean increase in REE of 14.4% (men = 12.1%, women = 17.9%) over the 4-hour testing period (p < 0.05), while the regular coffee group produced an overall decrease in REE of 5.7% (29). For anaerobic and aerobic purposes, JavaFit did not appear to affect maximal aerobic or anaerobic parameters; however, results from the study suggest that JavaFit does cause a short-term increase in excess post-exercise oxygen consumption following maximal aerobic exercise. The results indicated that JavaFit significantly increased o2 at 3 minutes post exercise when compared with BASELINE (p = 0.04) and DECAF (p = 0.02) values (26). These data might possibly be beneficial in enhancing post-exercise fat metabolism. While JavaFit does elevate heart rate due to its sympathomimetic (substances that mimic the effects of the catecholamines, epinephrine (adrenaline), norepinephrine, and/or dopamine) ingredients, it does not appear to adversely affect hemodynamic markers. Nor does JavaFit show any negative side effects at rest or during maximal exercise bouts for healthy adult male and female habitual coffee drinkers (19,26,29).
From the previous studies, it can be concluded that caffeine increases resting energy metabolism. This increase is caused through stimulation of beta-adrenergic receptors and adenyl cyclase. This stimulation causes activation of cyclic adenosine monophosphate (cAMP), which will increase the circulation of FFAs (24) and epinephrine (12,17). Caffeine might also boost lipolysis by inhibiting nucleotide phosphodiesterase. This possible inhibition caused by caffeine in the blood stream will then be able to increase tissue concentrations of cAMP and activation of hormone-sensitive lipase (9). A study in 2004 in 8 healthy young men concluded that caffeine ingested during normal fasting conditions and also during β-adrenoceptor blockade (beta-blockers) showed a few differences in variables between the lipolytic effect and triacylglycerol and FFA levels. Furthermore, the investigators tested to see if the effects of caffeine are mediated via the sympathetic nervous system for these nonhabitual caffeine users. During steady-state conditions, 240 minutes after ingestion of caffeine (10 mg/kg), lipid turnover increased 100%, thermic effect was increased 13.3 ± 2.2%, and oxidative FFA disposal increased 44%. During postabsorptive conditions (post exercise with caffeine taken prior to activity), 34% of lipids were oxidized and 66% were recycled. For the third test, beta-adrenoceptor blockade decreased and variables from the previous results were not inhibited when caffeine was ingested during beta-adrenoceptor blockade (1).
In 2007, Forbes et al. conducted the first Red Bull study that examined the immediate effects of the energy drink Red Bull on repeated Wingate cycle performance and bench press muscular endurance. The results show that Red Bull energy drink significantly increased total bench press repetitions over 3 sets (Red Bull = 34 ± 9 versus placebo = 32 ± 8, p < 0.05) in young physically active men and women who were nonhabitual caffeine users. Clearly, Red Bull significantly does increase upper-body muscle endurance even with the trivial amount of caffeine that it contains. However, the drink showed to have little or no effect on anaerobic peak or average power during repeated Wingate cycling tests (Red Bull = 701 ± 124 W versus placebo = 700 ± 132 W, Red Bull = 479 ± 74 W versus placebo = 471 ± 74 W, respectively) (13). No side effects were seen in either the control group or the PLA group.
In addition to the previous study that showed Red Bull to have little or no effect on anaerobic peak power, another study in 2007 found that a 6 mg/kg caffeine supplement had little effect on a 1-repetition maximum on bench press and leg press in young men (4). Furthermore, results from another study in 2008 indicated that Redline Extreme (Vital Pharmaceuticals, Inc., Davie, FL) energy drink (120 mL) caused a significant increase in reaction performance (Makoto Testing Device, Makoto USA, Centennial, CO) during exercise, but with no effect on 20-second Wingate trials measuring anaerobic power performance (18). Recently, research has shown that energy drinks increase exercise performance at moderate intensity levels but also help combat fatigue and drowsiness as well as increase alertness (3). This is important to persons engaging in longer activities. Ingestion of Redline significantly improved subjective feelings of focus and energy as well as increased levels of alertness and decreased fatigue (20). Also, an energy drink can significantly reduce sleep-related driving incidents and is beneficial in reducing sleepiness for drivers (20,27). It remains unclear as to why a caffeine supplement shows greater ergogenic properties to endurance type of training but yet has little effect in intense near maximal anaerobic activity (7).
A study in 1994 investigated the effects of the taurine-enriched drink “Red Bull” on performance in endurance athletes. The subjects received 500 mL of a test drink after 30 minutes of submaximal cycling: “Red Bull” without taurine, without glucuronolactone (U1); “Red Bull” without taurine, without glucuronolactone, without caffeine (U2); and “Red Bull” original drink containing taurine, glucuronolactone, and caffeine (U3). The results showed that heart rate level was significantly lower in U3 (p = 0.0031) 15 minutes after application. The plasma catecholamines increased slightly from beginning of exercise to 15 minutes after application of the drinks in all trials but remained on a significantly lower level in U3 (epinephrine [p = 0.0011] and norepinephrine [p = 0.0003]). Endurance time was significantly longer with “Red Bull” original in U3 (p = 0.015) (16). The authors concluded from the data that there is a positive effect of a taurine-containing drink on hormonal responses, which lead to a higher endurance performance. This study did not differentiate if taurine or caffeine alone caused the increase in exercise capacity.
MECHANISM OF ACTION
Most supplemental energy drinks have a long list of ingredients in addition to caffeine and vitamins. Most of these ingredients make up the proprietary blend. A proprietary blend is a complex list of ingredients that make up a smaller portion of the supplement. Some of the latest ingredients in these blends have good scientific backing to help support the claims being made. Beta-Phenylethylamine HCl is a chemical related to amphetamines and raises blood pressure and blood glucose levels. It is believed to work by making the brain release beta-endorphin (21,28). Evodiamine is an extract from the evodia fruit that has been shown to induce heat loss and heat production at the same time and dissipate food energy, preventing the accumulation of visceral fat and the body weight increase in rats, but it has not yet been clinically tested in humans (22).
Taurine is another common ergogenic supplement that the body produces naturally. However, this amino acid is not a derivative of caffeine nor are they similar in properties. It is produced by the metabolism of methionine and cysteine. And it is important in multiple metabolic processes, like osmoregulation to antioxidation to glycolysis (5). Furthermore, taurine has been researched for decades to examine its effects for performance purposes, and the results are consistent throughout various testing conditions. Both ingredients, taurine and caffeine, elicit similar effects to endurance performance, making these chemicals highly potent. Taurine, in dosages of 1,000 mg per serving, has shown to significantly increase stroke volume over a placebo without taurine and caffeine mainly because of a reduced end-systolic diameter and volume (6). With the increase in stroke volume, the heart is able to pump more oxygenated blood each beat to the rest of the body and thus causing a greater increase in performance in the aerobic energy system.
Yohimbine HCL is now making its way into the energy drink arena. This ingredient is starting to be used in drinks, and from what research is speculating, it could possibly be another powerful component for weight loss as well as for muscular hypertrophy. Yohimbine is an herbal preparation from the bark of the Pausinystalia and Corynanthe yohimbe trees that supposedly decreases body fat and enhances plasma testosterone levels and promotes skeletal muscle hypertrophy and sexual vigor (14,23). HCL is the hydrochloride version of yohimbine. Furthermore, yohimbine HCL stimulates the sympathetic nervous system by being an alpha2 adrenergic receptor antagonist. This allows norepinephrine to act on adipocytes by deactivating the alpha2 adrenoceptors that would typically block mobilization of FFAs out of the cells. Yohimbine appears to affect the nervous system by decreasing the activity of the sympathetic nervous system and increasing the activity of the parasympathetic nervous system. It also increases blood flow to adipose tissue, which provides support for the active metabolism. However, this blood flow is dependent on body weight and nutritional state (8,14). In animal testing, the drug increased O2 consumption and CO2 and heat production 30 minutes after its administration. The ergogenic effect continued over the length of the experimental period; however, the respiratory quotient remained unchanged. Yohimbine did cause temporary increase in plasma insulin level and increased heart rate and blood pressure at rest (15). Investigators can only hypothesize and speculate that this increase may occur because yohimbine has alpha 2-antagonist properties. So, this supplement could be used to increase the rate of fat mobilization and promote weight loss, but further human research is needed. Keep in mind that with the little research on human subjects, there have been no long-term side effects that have been seen with yohimbine supplementation in animals or in humans, so only recommendations can be made as to how and when and who should use a yohimbine supplementation. Acute side effects such as feelings of panic, clumsiness, confusion, chills, nausea, tremors, and anxiety have been seen in healthy and obese human subjects simply because of yohimbine's reaction with alpha2-receptors present in a number of tissues including heart, arteries, lung, or adipose tissue (10,14,31).
It has been shown that in both habitual and nonhabitual caffeine users consuming one serving of a caffeinated beverage that contains at least 2 mg/kg caffeine, REE is increased. Over time, this elevation will lead to a small increase in weight loss and this can certainly be important for persons or athletes who have difficulty in weight loss because of scheduling restraints or hormonal indifferences. There is no evidence that the consumption of energy drinks in recommended serving amounts poses any harmful effects in otherwise healthy individuals.
Many energy drinks contain sugar. Be advised that sugar is a carbohydrate and can add empty calories to the drink. For fat loss purposes, the amount of calories that are within the drink outweighs the benefits of the purpose of the drink if the intention for consuming is solely weight loss. Long-term speaking for habitual energy drink consumers, the increased sugar consumption can cause an unhealthy increase in insulin levels as well as adding on unwanted weight. For increased athletic performance, it is most beneficial to ingest an energy drink 30 to 60 minutes prior to activity to allow caffeine and other ingredients to be fully absorbed in the body, leaving the body in a more alert and energetic state. It is recommended that the upper limit of caffeine for those who are not caffeine sensitive is 6 mg/kg. However, research has shown that in dosages of both 2 and 6 mg/kg caffeine, similar ergogenic effects have been seen in both amounts during moderate activity (3,4,7,13,18). When using energy drinks for their ergogenic properties for fat loss and/or athletic performance, it is highly recommended to follow the labeled instructions on the bottle as most drinks are contraindicated for many individuals. If you are still unsure, please consult your physician.
It should be noted that excessive caffeine consumption or caffeine sensitivity can cause a wide variety of nonthreatening effects such as anxiety, nervousness, irritability, restlessness, headaches, and diarrhea (17,25,29). So, it is important to establish the appropriate amount needed.
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