Coffee is ingrained in many people’s early morning routines because they rely on its mildly stimulating effect to get them going for the day. In addition to keeping people from crawling back to bed and eluding their responsibilities in the morning, the world’s favorite hot beverage can also help people power through long grueling workouts, according to new research published in the International Journal of Sport Nutrition and Exercise Metabolism.
Although endurance athletes commonly ingest caffeine in the form of powder or tablets as a means to enhance training intensity and competitive performance, conflicting evidence exists regarding the efficacy of coffee — a popular source of caffeine — in improving athletic performance. Unsure of coffee’s effect on performance, researchers conducted a meta-analysis review to evaluate how pre-exercise coffee impacts endurance performance.
Caffeine, the most popular psychoactive substance among people of all age groups and cultural backgrounds, is typically used to boost the central nervous system for cognitive or physical endeavors. The stimulant is produced by a variety of beans, leaves, and fruits, but is most commonly consumed in the form of coffee, according to the National Council on Strength and Fitness (NCSF).
For the study, Higgins and his colleagues reviewed nine randomized control trials that specifically used coffee to improve endurance. During the trials, study participants either cycled or ran after drinking coffee. They then exercised vigorously, and the results were measured.
Researchers from five studies found significant improvements in endurance performance. They found between 3 and 7 milligrams per kilogram of body weight of caffeine from coffee increased endurance performance by an average of 24 percent. The amount of caffeine in a cup of coffee can vary from 75 milligrams to more than 150, depending on the variety and how it’s roasted and brewed. Americans consume about 27 ounces of coffee each day, Medical News Today reported.
“This is helpful for athletes because coffee is a naturally occurring compound,” Higgins said. “There’s the potential that getting your caffeine by drinking coffee has similar endurance benefits as taking caffeine pills.”
Higgins said the endurance effects of coffee as a source of caffeine could be just as advantageous as taking caffeine in the form of powder or tablets.
“While there is a lack of high-quality research on coffee as a source of caffeine, there is an abundance of research on pure caffeine,” he said. “It’s surprising how little we know about caffeine from coffee when its endurance effects could be just as beneficial as pure caffeine.”
Nevertheless, before any recommendations can be given to athletes, more research will be needed to determine how different the effects of caffeine from coffee are against those from pure caffeine — especially since the amount of caffeine in a cup of coffee can vary.
Half of Americans start their day with coffee, and according to recent study, working out after downing a cup of java may offer a weight-loss advantage.
The Spanish study, published in the International Journal of Sport Nutrition and Exercise Metabolism, found that trained athletes who took in caffeine pre-exercise burned about 15% more calories for three hours, post-exercise, compared to those who ingested a placebo. The dose that triggered the effect was 4.5 mg of caffeine per kilogram of body weight. For 150-pound woman, that’s roughly 300 mg of caffeine, the amount in about 12 ounces of brewed coffee, a quantity you may already be sipping each morning.
If you’ve always thought of coffee as a vice—one you’re simply not willing to give up—you’ll be happy to know that it’s actually a secret superfood. And if you exercise, caffeine can offer even more functional benefits for your workouts.
The Skinny on Caffeine
“Caffeine is a stimulant that acts on the central nervous system, the heart, and possibly the ‘center’ that controls blood pressure,” all of which play a vital role in helping your mind and body push harder in a workout, says Heidi Skolnik, M.S., a sports nutritionist and owner of Nutrition Conditioning, Inc. “It can also increase the release of feel-good neurotransmitters like dopamine, which effects pain receptors and mood” while you’re working out.
In other words, you’ll actually enjoy getting sweaty and it will feel easier when you’re powering through those last few reps. Plus, researchers found that when people caffeinated before a workout, they ate 72 fewer calories later in the day and had an easier time keeping cravings in check. Not a bad deal.
Here are six more reasons to enjoy it as part of an active lifestyle:
(1) Improved Circulation
Recent Japanese research studied the effects of coffee on circulation in people who were not regular coffee drinkers. Each participant drank a 5-ounce cup of either regular or decaffeinated coffee. Afterward, scientists gauged finger blood flow, a measure of how well the body’s smaller blood vessels work.
Those who downed “regular” (caffeinated) coffee experienced a 30% increase in blood flow over a 75-minute period, compared to those who drank the “unleaded” (decaf) version. Better circulation, better workout—your muscles need oxygen!
(2) Less Pain
Scientists at the University of Illinois found that consuming the caffeine equivalent of two to three cups of coffee one hour before a 30-minute bout of high-intensity exercise reduced perceived muscle pain. The conclusion: caffeine may help you push just a little bit harder during strength-training workouts, resulting in better improvements in muscle strength and/or endurance.
(3) Cognitive Performance
A study of 68 Navy Seal trainees—published in the journal Psychopharmacology—found that caffeine, in a dose-dependent manner, mitigated many adverse effects of exposure to multiple stressors. Caffeine (200 and 300 mg) significantly improved visual vigilance, choice reaction time, repeated acquisition, self reported fatigue and sleepiness, with the greatest effects on tests of vigilance, reaction time, and alertness. Marksmanship, a task that requires fine motor coordination and steadiness, was not affected by caffeine. The greatest effects of caffeine were present 1 h post-administration, but significant effects persisted for 8 h.
Even in the most adverse circumstances, moderate doses of caffeine can improve cognitive function, including vigilance, learning, memory, and mood state. When cognitive performance is critical and must be maintained during exposure to severe stress, administration of caffeine may provide a significant advantage. A dose of 200 mg appears to be optimal under such conditions.
(4) Burn More Fat
Caffeine stimulates the nervous system, which sends direct signals to the fat cells to tell them to break down fat. Another thing that caffeine does is to increase our blood levels of the hormone Epinephrine, which is also known as Adrenaline. Epinephrine travels through the blood, to the fat tissues and send signals to break down fats and release them into the blood.
This is how caffeine helps to mobilize fat from the fat tissues, making it available for use as free fatty acids in the blood.
(5) Muscle Preservation
In an animal study, sports scientists at Coventry University found that caffeine helped offset the loss of muscle strength that occurs with aging. The protective effects were seen in both the diaphragm, the primary muscle used for breathing, as well as skeletal muscle. The results indicate that in moderation, caffeine may help preserve overall fitness and reduce the risk of age-related injuries.
(6) More Muscle Fuel
A recent study published in the Journal of Applied Physiology found that a little caffeine post-exercise may also be beneficial, particularly for endurance athletes who perform day after day.
The research found that compared to consuming carbohydrates alone, a caffeine/carb combo resulted in a 66% increase in muscle glycogen four hours after intense, glycogen-depleting exercise. Glycogen, the form of carbohydrate that gets stockpiled in muscle, serves as a vital energy “piggy bank” during exercise, to power strength moves, and fuel endurance.
Moral of the story: caffeine makes your workout more fun and helps you push a little bit harder during your workouts and burning more fat. Cheers to that!
Laboratory studies from the 1970’s suggested that caffeine enhanced endurance performance by increasing the release of adrenaline into the blood stimulating the release of free fatty acids from fat tissue and/or skeletal muscle. The working muscles use this extra fat early in exercise, reducing the need to use muscle carbohydrate (glycogen). The “sparing” of muscle glycogen made more available later in exercise to delay fatigue.
In the 1980’s, many studies found that caffeine did not alter exercise metabolism, and implied that it had no ergogenic effect, without actually measuring performance. A few reports did examine caffeine and performance during endurance exercise and generally found no ergogenic effect. By the end of the decade, it was suggested that caffeine did not alter metabolism during endurance exercise and may not be ergogenic.
Recent work reported that ingestion of 3-9 mg of caffeine per kilogram (kg) of body weight one hour prior to exercise increased endurance running and cycling performance in the laboratory. To put this into perspective, 3 mg per kg body weight equals approximately one mug or 2 regular size cups of drip-percolated coffee; and 9 mg/kg = approximately 3 mugs of 5-6 regular size cups of coffee. These studies employed well-trained, elite or serious, recreational athletes. Studies with untrained individuals cannot be performed due to their inability to reliably exercise to exhaustion.
The mechanism to explain these endurance improvements is unclear. Muscle glycogen is spared early during submaximal exercise following caffeine ingestion (5-9 mg/kg). It is unknown whether glycogen sparing occurs as a result of caffeine’s ability to increase fat availability for skeletal muscle use. Furthermore, there is no evidence supporting a metabolic component for enhancing performance at a low caffeine dose (3 mg/kg). Therefore, it appears that alterations in muscle metabolism alone cannot fully explain the ergogenic effect of caffeine during endurance exercise.
This experiment examined the effect of a moderate dose of caffeine on perceptions of leg-muscle pain during a bout of high-intensity cycling exercise and the role of anxiety sensitivity in the hypoalgesic effect of caffeine on muscle pain during exercise.
Methods: Sixteen college-age women ingested caffeine (5 mg/kg body weight) or a placebo and 1 hr later completed 30 min of cycling on an ergometer at 80% of peak aerobic capacity. The conditions were completed in a counterbalanced order, and perceptions of leg-muscle pain were recorded during the bouts of exercise.
Results: Caffeine resulted in a large reduction in leg-muscle pain-intensity ratings compared with placebo (d = −0.95), and the reduction in leg-muscle pain-intensity ratings was larger in those with lower anxiety-sensitivity scores than those with higher anxiety-sensitivity scores (d = −1.28 based on a difference in difference scores).
Conclusion: The results support that caffeine ingestion has a large effect on reducing leg-muscle pain during high-intensity exercise, and the effect is moderated by anxiety sensitivity.
Acute exercise is a natural stimulus that can transiently, safely, and reliably produce muscle pain (Cook, Jackson, O’Connor, & Dishman, 2004; Cook, O’Connor, Eubanks, Smith, & Lee, 1997; Cook, O’Connor, Oliver, & Lee, 1998; O’Connor & Cook, 1999, 2001) that might generalize to the pain experienced by patients with chronic pain (Cook, 2006). Moderate- to high-intensity exercise results in transient, naturally occurring pain in the activated muscles (Cook et al., 1997). The pain is described as exhausting, intense, sharp, burning, tiring, cramping, pulling, and rasping (Cook et al.); the same descriptors have been used for characterizing clinical pain conditions including menstrual pain, arthritic pain, cancer pain, chronic back pain, and fibromyalgia (Cook). We further note that pain ratings during exercise are nearly 1 SD above the mean scores associated with other laboratory methods for inducing pain (Cook et al., 1997). Acute exercise has been used as an experimental model for testing effects of caffeine ingestion on muscle pain (Motl, O’Connor, & Dishman, 2003; Motl, O’Connor, Tubandt, Puetz, & Ely, 2006; O’Connor, Motl, Broglio, & Ely, 2004).
Our first experiment indicated that ingesting a large dose of caffeine (10 mg/kg body weight) reduced quadriceps muscle pain intensity during moderate-intensity cycling in males (Motl et al., 2003). In a second experiment, ingesting a moderate (5 mg/kg body weight) and a large (10 mg/kg body weight) dose of caffeine dosedependently reduced quadriceps muscle pain intensity during moderate-intensity cycling in males (O’Connor et al., 2004). The third experiment demonstrated that ingesting a moderate (5 mg/kg body weight) and a large (10 mg/kg body weight) dose of caffeine similarly reduced quadriceps muscle pain intensity during moderate-intensity cycling in females (Motl et al., 2006). One limitation of previous research is that it only examined the effects of caffeine on muscle pain using moderate-intensity exercise (i.e., 60% of peak oxygen uptake [VO2peak]). The hypoalgesic effect of caffeine on muscle pain during exercise could be extended by testing its effects using high-intensity exercise (i.e., 80% VO2peak). This exercise stimulus would induce higher intensity pain in the activated muscles (Cook et al., 1997, 1998) and provide an additional test of caffeine-induced hypoalgesia during exercise. Another limitation is that previous research has not examined psychological factors that might account for individual variability in the effect of caffeine on muscle pain during exercise. Anxiety sensitivity is a personality trait characterized by the belief that the experience of anxiety or fear causes illness, embarrassment, or additional anxiety (Reiss, Peterson, Gursky, & McNally, 1986), and low anxiety sensitivity has been related to caffeine-induced hypoalgesia during a cold-pressor pain task in females (Keogh & Chaloner, 2002). Anxiety sensitivity might have a similar effect on caffeine-induced hypoalgesia during exercise. Accordingly, the current study examined the effect of caffeine on leg-muscle pain intensity during high-intensity (80% VO2peak) cycling exercise in women and the role of anxiety sensitivity in the hypoalgesic effect of caffeine on muscle pain during exercise.
This experiment involved an examination of the effect of caffeine on naturally occurring quadriceps muscle pain during high-intensity cycling exercise in women and investigated the role of anxiety sensitivity in moderating the hypoalgesic effect of caffeine during exercise. Compared with placebo, the moderate dose of caffeine (i.e., 5 mg/kg body weight) had a significant and large hypoalgesic effect during cycling exercise, and the effect was larger for those with lower anxiety sensitivity than those with higher anxiety sensitivity. Notably, there were no differences in the absolute and relative intensities of the two exercise bouts. Hence, the women experienced a reduction in pain-intensity ratings in the quadriceps during highintensity cycling after caffeine ingestion, and the effect was largest for those with lower anxiety sensitivity. The current findings replicate and extend previous work on caffeine and naturally occurring muscle pain during exercise. Three previous experiments have demonstrated that ingesting large and moderate doses of caffeine reduced quadriceps muscle pain-intensity ratings during moderate-intensity cycling exercise in males and females (Motl et al., 2003, 2006; O’Connor et al., 2004).
This study demonstrated that ingesting a moderate dose of caffeine reduced quadriceps muscle pain-intensity ratings during high-intensity cycling exercise in women. Of note, the high-intensity exercise did induce stronger quadriceps muscle pain-intensity ratings than reported in the placebo condition of previous studies that used moderateintensity exercise. One previous study with females reported a mean quadriceps muscle pain-intensity rating in the placebo condition of 2.4 ± 1.1 (Motl et al., 2003), whereas we reported a mean rating in the placebo condition of 3.8 ± 1.7. The difference of 1.4 units translates into a large difference in pain ratings between moderate- and high-intensity exercise (d = 0.96). An additional novel feature of this study was its examination of the influence of anxiety sensitivity on the hypoalgesic effect of caffeine during cycling exercise. This study demonstrated that those with lower anxiety sensitivity had a significantly larger reduction in quadriceps muscle pain-intensity ratings after caffeine ingestion compared with placebo than those with higher anxiety sensitivity. Of note, quadriceps muscle pain-intensity ratings were lower after caffeine ingestion than after placebo for both groups of individuals with lower and higher anxiety sensitivity. Our findings are partially consistent with previous research that examined the effect of anxiety sensitivity on caffeine-induced hypoalgesia during a cold-pressor pain task in females (Keogh & Chaloner, 2002).
That study indicated that low anxiety sensitivity was associated with an increase in pain threshold during a cold-pressor 112 Gliottoni and Motl pain task after caffeine ingestion compared with placebo; there were no effects of caffeine on pain threshold in those with medium or high anxiety sensitivity (Keogh & Chaloner). Therefore, the primary difference between our findings and those of previous research (Keogh & Chaloner) is that we observed an effect of caffeine on quadriceps muscle pain-intensity ratings in those with higher anxiety sensitivity, whereas previous research did not observe an effect of caffeine on pain threshold during a cold-pressor task in those with medium and high anxiety sensitivity. Another difference is that previous research did not report a moderating role of anxiety sensitivity on sensory and affective components of pain associated with the cold-pressor task after caffeine ingestion (Keogh & Chaloner). The difference in findings of the effect of anxiety sensitivity on caffeine-induced hypoalgesia might be linked with the nature of the pain stimulus or the measurement of pain. This is further illustrated by the apparent, although likely not statistically significant based on low power, 1-point difference in pain ratings between the lower and higher anxiety-sensitive groups in the placebo condition in the current study. The women with lower anxiety sensitivity reported higher pain-intensity ratings during exercise in the placebo, whereas other research has demonstrated that women with higher anxiety sensitivity reported greater sensory pain during a cold-pressor task (Keogh & Birkby, 1999).
The current study used exercise as a stimulus to produce pain and examined ratings of muscle pain intensity, whereas previous research used a cold-pressor pain task and measured pain threshold and tolerance, as well as the subjective experience of pain using the short form of the McGill Pain Questionnaire. Researchers might consider examining the precise conditions under which anxiety sensitivity moderates experimentally induced pain and the hypoalgesic effect of caffeine. Although anxiety sensitivity did influence the effect of caffeine on muscle pain during exercise, we did not examine the mechanism underlying this moderator effect. One possibility is that the effect is mediated by a negative interpretive bias (Keogh & Cochrane, 2002; Keogh, Hamid, Hamid, & Ellery, 2004). That is, anxiety sensitivity might be related to pain responses because of the tendency of individuals to interpret bodily sensations in a negative manner (Keogh & Cochrane; Keogh et al., 2004). Indeed, with a cold-pressor task, individuals with high anxiety sensitivity exhibited a greater interpretive bias and reported more negative pain experiences than those low in anxiety sensitivity, and the tendency to misinterpret innocuous bodily sensations mediated the associations between anxiety sensitivity and affective pain experiences (Keogh & Cochrane). Perhaps individuals with higher anxiety sensitivity misinterpret bodily sensations associated with caffeine ingestion, and this misinterpretation influences pain responses associated with exercise after caffeine ingestion. Another possibility is that individuals with higher anxiety sensitivity exhibit greater negative psychological response during caffeine ingestion (Telch, Silverman, & Schmidt, 1996), and this influences the effect of caffeine on muscle pain during exercise in those with higher anxiety sensitivity. The results of our study are consistent with experiments in humans that support caffeine’s influence on ischemic muscle-contraction pain (Myers, Shaikh, & Zullo, 1997), pain induced by a cold-pressor task (Keogh & Witt, 2001), and eccentricexercise-induced, delayed-onset muscle soreness (Maridakis, O’Connor, Dudley, & McCully, 2007).
Indeed, one previous study reported that ingestion of 200 mg of Caffeine, Pain, and Exercise 113 caffeine reduced pain-intensity ratings during artificially induced noxious muscle ischemia (Myers et al.). Another previous experiment reported that ingestion of 250 mg of caffeine increased pain threshold and pain tolerance during a cold-pressor task (Keogh & Witt). One final previous experiment reported that ingestion of 5 mg/kg body weight of caffeine produced a large reduction in pain-intensity ratings associated with eccentric-exercise-induced, delayed-onset muscle soreness (Maridakis et al.). Those findings combined with the consistent observation of caffeine-induced reductions in naturally occurring muscle pain during acute exercise suggest that caffeine has a hypoalgesic effect in humans under conditions of artificial and naturally occurring pain. This study is not without limitations. One limitation involves the dose of caffeine and the efficacy of the double-blinding. As might be expected, 14 of the 16 participants correctly guessed the order of caffeine administration, and this likely undermined our double-blinding of caffeine versus placebo administration. Another limitation is that of the median split. Although our primary analysis involved bivariate and partial correlations with anxiety sensitivity as a continuous variable, the tertiary analysis included a median split based on anxiety-sensitivity scores. The two groups differed in mean anxiety-sensitivity scores, but there are potential problems associated with small sample bias and people in the lower anxiety-sensitivity group actually being moderate in anxiety sensitivity and vice versa.
Nevertheless, the results of this study add to the body of evidence that caffeine ingestion influences muscle pain during exercise and provides novel evidence that anxiety sensitivity moderates the magnitude of the effect. Further research should address the mechanisms underlying caffeine-induced hypoalgesia during acute exercise, as well as test the effect of varying doses of caffeine on muscle pain in males, habitual users of caffeine, and individuals with a variety of personality traits including anxiety sensitivity. Such inquires will further highlight the effect of caffeine on naturally occurring muscle pain during acute exercise.
To view the study “Effect of Caffeine on Leg-Muscle Pain During Intense Cycling Exercise: Possible Role of Anxiety Sensitivity” published in International Journal of Sport Nutrition and Exercise Metabolism, click here.
When humans are acutely exposed to multiple stressors, cognitive performance is substantially degraded. Few practical strategies are available to sustain performance under such conditions.
Objective: This study examined whether moderate doses of caffeine would reduce adverse effects of sleep deprivation and exposure to severe environmental and operational stress on cognitive performance.
Methods: Volunteers were 68 U.S. Navy Sea-Air-Land (SEAL) trainees, randomly assigned to receive either 100, 200, or 300 mg caffeine or placebo in capsule form after 72-h of sleep deprivation and continuous exposure to other stressors. Cognitive tests administered included scanning visual vigilance, four choice visual reaction time, a matching-to-sample working memory task and a repeated acquisition test of motor learning and memory. Mood state, marksmanship, and saliva caffeine were also assessed. Testing was conducted 1 and 8 h after treatment.
Results: Sleep deprivation and environmental stress adversely affected performance and mood. Caffeine, in a dose-dependent manner, mitigated many adverse effects of exposure to multiple stressors. Caffeine (200 and 300 mg) significantly improved visual vigilance, choice reaction time, repeated acquisition, self reported fatigue and sleepiness with the greatest effects on tests of vigilance, reaction time, and alertness. Marksmanship, a task that requires fine motor coordination and steadiness, was not affected by caffeine. The greatest effects of caffeine were present 1 h post-administration, but significant effects persisted for 8 h.
Conclusions: Even in the most adverse circumstances, moderate doses of caffeine can improve cognitive function, including vigilance, learning, memory, and mood state. When cognitive performance is critical and must be maintained during exposure to severe stress, administration of caffeine may provide a significant advantage. A dose of 200 mg appears to be optimal under such conditions.
Caffeine is widely consumed throughout the world in beverages, foods, and as a drug for a variety of reasons, including its stimulant-like effects on mood and cognitive performance (for reviews see Fredholm et al. 1999; Lieberman 2001). Its positive effects on performance, notably sustained vigilance and related cognitive functions, are well documented when it is administered to rested volunteers in the doses found in single servings of foods (Amendola et al. 1998; Clubley et al. 1979; Fine et al. 1994; Lieberman et al. 1987a, 1987b; Smith et al. 1999a, 1999b). Caffeine, in moderate and high doses, also has been shown to have beneficial effects on cognitive performance when individuals are sleep-deprived (Patat et al. 2000; Penetar et al. 1993; Reyner and Horne 2000). However, few studies have examined the effects of caffeine during exposure to severe, multifactor stress to determine whether it can mitigate the adverse effects of simultaneous exposure to a combination of stressors. Furthermore, the optimal dose to employ under such conditions has not been determined (Akerstedt and Ficca 1997).
Military training environments can provide one of the few opportunities to examine the effects of severe, but controlled, multifactor stress on human performance. Therefore we examined the effects of caffeine on cognitive performance and mood during training of the United States Navy Sea-Air-Land Commandos. The SEALs are one of the most elite special warfare units in the U.S. Defense Department. Operational SEAL units conduct unconventional warfare and clandestine operations in maritime and riverine environments (Waller 1994). To become a SEAL member an individual must complete a four-part training program lasting about 7– 8 months at the Naval Special Warfare Training Center (NSWTC), Naval Amphibious Base, Coronado, California. The training is intense, difficult, and designed to identify individuals who can withstand the adverse effects of a variety of operational stressors, especially exposure to cold water and sustained high levels of intense physical activity, while maintaining high levels of physical and mental function. Due to the rigorous mental and physical challenges of training only about one in four individuals who attempt the course complete it (Waller 1994). One of the most acutely stressful periods of SEAL training is “Hell Week,” during which trainees undergo sustained sleep loss in combination with extensive environmental, physical and psychological stress. Most Hell Week activities are conducted on the beach, surf, or in small boats. These are environments in which SEAL members may work when they conduct operations.
During Hell Week trainees are under continuous supervision of trained SEAL instructors and engage in continuous 24-h activities. These include physical and mental challenges, environmental stress, especially cold stress, as well as constant psychological pressure to perform optimally as an individual and part of a small team (Waller 1994). The challenges of Hell Week include a variety of activities such as surf immersion, where students, arms linked, sit in a line so the surf strikes them in the face. This lasts for a period of 10–20 min depending upon water temperature. Boat push-ups are another frequent activity with trainees expected to raise inflatable boats over their heads, and then as a team push them up until their arms are fully extended. The boats contain life vests, paddles, and often a considerable amount of water. Other more traditional forms of physical training such as push-ups and sit-ups are frequently required of trainees by the instructors. Psychological stressors include verbal confrontations with instructors and activities with no-win outcomes (Smoak et al. 1988). During Hell Week trainees only have a few hours to sleep during irregular breaks in training and are often wet and cold. Since actual SEAL operations, including combat, can involve these challenges in combination with life-threatening danger, Hell Week provides an opportunity to determine which trainees have the physical and mental attributes to perform reliably under such conditions. Generally more than one-half the trainees who start Hell Week do not complete it and therefore cannot continue SEAL training. Most withdrawals from training are voluntarily initiated by the trainee, except for medical withdrawals. The training repeatedly pushes trainees to their physical and mental limits so that they will be prepared for the extraordinary challenge of serving in operational SEAL units.
Because caffeine may maintain cognitive performance under conditions of severe stress, we conducted a dose response study to evaluate its effects during Hell Week of SEAL training. We assessed a variety of behavioral functions, focusing on parameters sensitive to caffeine such as vigilance (Clubley 1979; Fine et al. 1994; Lieberman 1992) and mood (Amendola et al. 1998). We measured salivary caffeine and self-reported side effects. In addition, we utilized a simulated marksmanship task to provide information on a complex behavior that requires fine motor control and steadiness for optimal performance (Kruse et al. 1986; Zatsiorsky and Aktov 1990). Caffeine has been anecdotally reported to interfere with these psychomotor functions, although the literature suggests that caffeine does not adversely affect fine motor control (Lieberman et al. 1987b; Patat et al. 2000).
The position of The Society regarding caffeine supplementation and sport performance is summarized by the following seven points: 1.) Caffeine is effective for enhancing sport performance in trained athletes when consumed in low-to-moderate dosages (~3-6 mg/kg) and overall does not result in further enhancement in performance when consumed in higher dosages (≥ 9 mg/kg). 2.) Caffeine exerts a greater ergogenic effect when consumed in an anhydrous state as compared to coffee. 3.) It has been shown that caffeine can enhance vigilance during bouts of extended exhaustive exercise, as well as periods of sustained sleep deprivation. 4.) Caffeine is ergogenic for sustained maximal endurance exercise, and has been shown to be highly effective for time-trial performance. 5.) Caffeine supplementation is beneficial for high-intensity exercise, including team sports such as soccer and rugby, both of which are categorized by intermittent activity within a period of prolonged duration. 6.) The literature is equivocal when considering the effects of caffeine supplementation on strength-power performance, and additional research in this area is warranted. 7.) The scientific literature does not support caffeine-induced diuresis during exercise, or any harmful change in fluid balance that would negatively affect performance.
Research on the physiological effects of caffeine in relation to human sport performance is extensive. In fact, investigations continue to emerge that serve to delineate and expand existing science. Caffeine research in specific areas of interest, such as endurance, strength, team sport, recovery, and hydration is vast and at times, conflicting. Therefore, the intention of this position statement is to summarize and highlight the scientific literature, and effectively guide researchers, practitioners, coaches, and athletes on the most suitable and efficient means to apply caffeine supplementation to mode of exercise, intensity, and duration.
Caffeine and mechanism of action
To understand the effect of caffeine supplementation in its entirety it is necessary to discuss its chemical nature and how the compound is physiologically absorbed into the body. Caffeine is quickly absorbed through the gastrointestinal tract [1, 2, 3], and moves through cellular membranes with the same efficiency that it is absorbed and circulated to tissue [4, 5]. Caffeine (1,3,7-trimethylxanthine) is metabolized by the liver and through enzymatic action results in three metabolites: paraxanthine, theophylline, and theobromine [1, 6, 7, 8]. Elevated levels can appear in the bloodstream within 15-45 min of consumption, and peak concentrations are evident one hour post ingestion [1, 3, 9, 10]. Due to its lipid solubility, caffeine also crosses the blood-brain barrier without difficulty [5, 11]. Meanwhile, caffeine and its metabolites are excreted by the kidneys, with approximately 3-10% expelled from the body unaltered in urine [1, 7, 12]. Based on tissue uptake and urinary clearance circulating concentrations are decreased by 50-75% within 3-6 hours of consumption [3, 13]. Thus, clearance from the bloodstream is analogous to the rate at which caffeine is absorbed and metabolized.
Multiple mechanisms have been proposed to explain the effects of caffeine supplementation on sport performance. However, several extensive reviews have stated that the most significant mechanism is that caffeine acts to compete with adenosine at its receptor sites [5, 13, 14]. In fact, in an exhaustive review of caffeine and sport performance, it was stated that “because caffeine crosses the membranes of nerve and muscle cells, its effects may be more neural than muscular. Even if caffeine’s main effect is muscular, it may have more powerful effects at steps other than metabolism in the process of exciting and contracting the muscle ”.
Clearly, one of caffeine’s primary sites of action is the central nervous system (CNS). Moreover, theophylline and paraxanthine can also contribute to the pharmacological effect on the CNS through specific signaling pathways . However, as noted above, rarely is there a single mechanism that fully explains the physiological effects of any one nutritional supplement. Because caffeine easily crosses the blood brain barrier as well as cellular membranes of all tissues in the body , it is exceedingly difficult to determine in which system in particular (i.e. nervous or skeletal muscle) caffeine has the greatest effect .
In addition to its impact on the CNS, caffeine can affect substrate utilization during exercise. In particular, research findings suggest that during exercise caffeine acts to decrease reliance on glycogen utilization and increase dependence on free fatty acid mobilization [16, 17, 18, 19]. Essig and colleagues  reported a significant increase in intramuscular fat oxidation during leg ergometer cycling when subjects consumed caffeine at an approximate dose of 5 mg/kg. Additionally, Spriet et al.  demonstrated that following ingestion of a high dose of caffeine (9 mg/kg) net glycogenolysis was reduced at the beginning of exercise (cycling to exhaustion at 80% VO2max). Consequently, performance was significantly improved and results of this study  suggested an enhanced reliance on both intra- and extramuscular fat oxidation.
Another possible mechanism through which caffeine may improve endurance performance is by increasing the secretion of β-endorphins. Laurent et al.  demonstrated that when compared to the placebo group caffeine consumption (6 mg/kg) significantly increased plasma β-endorphin concentrations following two hours of cycling at 65% VO2peak and a subsequent bout of high intensity sprint activity. It has been established that plasma endorphin concentrations are enhanced during exercise and their analgesic properties may lead to a decrease in pain perception .
Research has also demonstrated that caffeine may result in alterations of neuromuscular function and/or skeletal muscular contraction [22, 23]. For example, Kalmar and Cafarelli  indicated a moderate dose of caffeine (6 mg/kg) significantly enhanced both isometric leg extension strength as well as the time to fatigue during a submaximal isometric leg extension.
Caffeine consumption also promotes a significant thermogenic response. In fact, caffeine consumption at a dose of 100 mg resulted in a significant thermogenic effect despite the fact that subjects in that particular investigation had a habitual caffeine intake of 100-200 mg per day . The increase in energy expenditure subsequent to caffeine ingestion had not returned to baseline 3 hours post-consumption.
Overall, the findings of research studies involving caffeine supplementation and physical performance indicate a combined effect on both the central and peripheral systems. Therefore, it is possible that caffeine acts on the central nervous system as an adenosine antagonist, but may also have an effect on substrate metabolism and neuromuscular function. Research in all areas of caffeine supplementation continues to emerge and it is necessary to understand that as a supplement, caffeine has wide ranging physiological effects on the body that may or may not result in an enhancement in performance. Caffeine supplementation can improve sport performance but this is dependent upon various factors including, but not limited to, the condition of the athlete, exercise (i.e. mode, intensity, duration) and dose of caffeine.
Caffeine and Cognitive Performance
Caffeine has been shown to enhance several different modes of exercise performance including endurance [8, 16, 25, 26, 27, 28], high-intensity team sport activity [29, 30, 31, 32, 33, 34], and strength-power performance [30, 35]. Additionally, the use of caffeine has also been studied for its contribution to special force operations, which routinely require military personnel to undergo periods of sustained vigilance and wakefulness. In a series of investigations, McLellan et al. [36, 37, 38] examined the effects of caffeine in special force military units who routinely undergo training and real life operations in sleep deprived conditions, where alertness and diligent observation are crucial to assignment.
In the McLellan et al. investigations [36, 37, 38], soldiers performed a series of tasks over several days, where opportunities for sleep were exceedingly diminished. Experimental challenges included a 4 or 6.3 km run, as well as tests for marksmanship, observation and reconnaissance, and psychomotor vigilance [36, 37,38]. During periods of sustained wakefulness, subjects were provided caffeine in the range of 600-800 mg, and in the form of chewing gum. The caffeine supplement was consumed in this manner as it has been shown to be more readily absorbed, than if it was provided within a pill based on the proximity to the buccal tissue . In all three studies [36, 37, 38], vigilance was either maintained or enhanced for caffeine conditions in comparison to placebo. Additionally, physical performance measures such as run times and completion of an obstacle course were also improved by the effects of caffeine consumption [36, 38].
Lieberman et al.  examined the effects of caffeine on cognitive performance during sleep deprivation in U.S. Navy Seals . However, in this investigation  the participants were randomly assigned varying doses of caffeine in capsule form delivering either 100, 200, or 300 mg. In a manner similar to previous investigations, participants received either the caffeine or placebo treatment and one hour post consumption performed a battery of assessments related to vigilance, reaction time, working memory, and motor learning and memory. In addition, the participants were evaluated at eight hours post consumption to assess duration of treatment effect in parallel to the half-life of caffeine, in a manner similar to a study conducted by Bell et al. .
As to be expected, caffeine had the most significant effect on tasks related to alertness . However, results were also significant for assessments related to vigilance and choice reaction time for those participants who received the caffeine treatment. Of particular importance are the post-hoc results for the 200 and 300 mg doses. Specifically, there was no statistical advantage for consuming 300, as opposed to 200 mg . In other words, those trainees who received the 300 mg (~4 mg/kg) dose did not perform significantly better than those participants who received 200 mg (~2.5 mg/kg). Meanwhile, a 200 mg dose did result in significant improvements in performance, as compared to 100 mg. In fact, it was evident from post-hoc results that 100 mg was at no point statistically different or more advantageous for performance than a placebo. These studies [36, 37, 38, 40] demonstrate the effects of caffeine on vigilance and reaction time in a sleep deprived state, in a distinct and highly trained population. These findings suggest that the general population may benefit from similar effects of caffeine, but at moderate dosages in somewhat similar conditions where sleep is limited.
An additional outcome of the Lieberman et al.  study is the fact that caffeine continued to enhance performance in terms of repeated acquisition (assessment of motor learning and short-term memory) and Profile of Mood States fatigue eight hours following consumption. These results are in agreement with Bell et al. , where aerobic capacity was assessed 1, 3, and 6 hours following caffeine consumption (6 mg/kg). Caffeine had a positive effect on performance for participants classified as users(≥ 300 mg/d) and nonusers (≤ 50 mg/d); however, nonusers had a treatment effect at 6 hours post-consumption, which was not the case for users – this group only had a significant increase in performance at 1 and 3 hours post- consumption. Taken together, results of these studies [40, 41] provide some indication, as well as application for the general consumer and athlete. Specifically, while caffeine is said to have a half-life of 2.5-10 hours , it is possible performance-enhancing effects may extend beyond that time point as individual response and habituation among consumers varies greatly.
Finally, it was suggested by Lieberman and colleagues  that the performance-enhancing effects of caffeine supplementation on motor learning and short-term memory may be related to an increased ability to sustain concentration, as opposed to an actual effect on working memory. Lieberman et al.  attributed the effects of caffeine to actions on the central nervous system, specifically the supplement’s ability to modulate inhibitory actions, especially those of adenosine. In fact, it was suggested that because caffeine has the ability to act as an antagonist to adenosine, alterations in arousal would explain the compound’s discriminatory effect on behaviors relating vigilance, fatigue and alertness .
Recently, it was also suggested that caffeine can positively affect both cognitive and endurance performance . Trained cyclists, who were moderate caffeine consumers (approximated at 170 mg/d) participated in three experimental trials consisting of 150 min of cycling at 60% VO2max followed by five minutes of rest and then a ride to exhaustion at 75% VO2max. On three separate days, subjects consumed a commercially available performance bar that contained either 44.9 g of carbohydrates and 100 mg of caffeine, non-caffeinated-carbohydrate and isocaloric, or flavored water. Results from a repeated series of cognitive function tests favored the caffeine treatment in that subjects performed significantly faster during both the Stroop and Rapid Visual Information Processing Task following 140 min of submaximal cycling as well as after a ride to exhaustion. In addition, participant time increased for the ride to exhaustion on the caffeine treatment, as compared to both the non-caffeinated bar and flavored water .
Overall, the literature examining the effects of caffeine on anaerobic exercise is equivocal, with some studies reporting a benefit [29, 30, 31, 32, 43, 44] and others suggesting that caffeine provides no significant advantage [45, 46]. As with all sports nutrition research, results can vary depending on the protocol used, and in particular, the training status of the athlete as well as intensity and duration of exercise. For example, Crowe et al.  examined the effects of caffeine at a dose of 6 mg/kg on cognitive parameters in recreationally active team sport individuals, who performed two maximal 60-second bouts of cycling on an air-braked cycle ergometer. In this investigation , untrained, moderately habituated (80-200 mg/d) participants completed three trials (caffeine, placebo, control) and underwent cognitive assessments prior to consumption of each treatment, post-ingestion at approximately 72-90 min, and immediately following exercise. Cognitive testing consisted of simple visual reaction time and number recall tests. Participants performed two 60-second maximal cycle tests interspersed by three min of passive rest. The results were in contrast to other studies that investigated cognitive parameters and the use of caffeine [25, 36, 37, 38, 40] in that caffeine had no significant impact on reaction time or number recall, and there was no additional benefit for measurements of power. In fact, in this study , the caffeine treatment resulted in significantly slower times to reach peak power in the second bout of maximal cycling.
Elsewhere, Foskett and colleagues  investigated the potential benefits of caffeine on cognitive parameters and intermittent sprint activity and determined that a moderate dose (6 mg/kg) of caffeine improved a soccer player’s ball passing accuracy and control, thereby attributing the increase in accuracy to an enhancement of fine motor skills.
Based on some of the research cited above, it appears that caffeine is an effective ergogenic aid for individuals either involved in special force military units or who may routinely undergo stress including, but not limited to, extended periods of sleep deprivation. Caffeine in these conditions has been shown to enhance cognitive parameters of concentration and alertness. It has been shown that caffeine may also benefit endurance athletes both physically and cognitively. However, the research is conflicting when extrapolating the benefits of caffeine to cognition and shorter bouts of high-intensity exercise. A discussion will follow examining the effects of caffeine and high-intensity exercise in trained and non-trained individuals, which may partially explain a difference in the literature as it pertains to short-term high-intensity exercise.