Given the inconclusive evidence surrounding the use of caffeine as a performance-enhancing supplement and the significant metabolic consequences on glucose disposal in a sedentary state, athletes should use caution and consume caffeine in moderation.
Researchers continue to study caffeine, which is allowed by the NCAA and US Olympic Committee, to determine its enhancement effects on athletic performance in training and competition. Caffeine is rapidly absorbed by the body within five to fifteen minutes of ingestion. Peak levels in the blood occur between forty and eighty minutes, making it ideal for immediate training benefit (Spriet, 2014). With a half-life of three to five hours, caffeine’s effects can last for the better part of a day.
Low doses of caffeine, 3mg/kg body weight or less, improve vigilance, alertness, mood, and cognitive abilities without negative side effects (Spriet, 2014). Higher doses often result in gastrointestinal upset, dizziness, nervousness, insomnia, confusion, tachycardia (rapid heart rate), and the inability to focus (Spriet, 2014).
The first dose-response study, performed in 1995, involved a cycling time trial performance test. Cyclists ingested 3, 6, and 9mg/kg of caffeine sixty minutes before the time trial (Spriet, 2014). The cyclists who took the 3 and 6mg/kg doses showed a 22% increase in time trial performance while the high-dose group demonstrated only an 11% non-significant increase (Spriet, 2014).
Another study, where well-trained cyclists ingested low-dose caffeine late in an endurance race, showed that both 1.5 and 3mg/kg were ergogenic when ingested late in an exhaustive ride (Spriet, 2014). Caffeine intake pre-workout showed 4.2% and 2.9% improvement in cycling performance when 3 and 6mg/kg were consumed, respectively, indicating a decrease in dose-response efficacy similar to the first two studies mentioned (Spriet, 2014).
In running performance, the evidence is a bit less consistent. Some researchers found that 150-200mg of caffeine improved 1500m performance by 4.2s in well-trained males, whereas another study involving a longer distance event (3 x 18km in 8 days) showed no performance effect of low-dose (90mg or ~1.3mg/kg) (Spriet, 2014). In an 8K time trial involving well-trained male runners, a 24-second, or 1.8%, improvement was observed with 3mg/kg caffeine ingested sixty minutes before the event (Spriet, 2014). An average of 3.6% performance improvement across multiple endurance sports was collected from studies with ingestion amounts ranging from a 2-5mg/kg (mean = 3% enhancement) and >5mg/kg loading dose (mean = 7% enhancement) (Shearer & Graham, 2014).
In power-based sports requiring short, anaerobic bursts of activity, the evidence of caffeine’s ergogenic effect on performance is conflicting. An increasing number of studies have been published involving HIIT training, resistance training, and force-production activity. Studies observed improvements in peak power (Wingate test) and absolute strength when consuming 5 and 7mg/kg body mass, respectively.
Few studies exist on the effect of low-dose supplementation (Spriet, 2014). One study by Lorino, Lloyd, Crixell, and Walker (2006) examined caffeine’s effect on agility performance in the Proagility run and 30-second Wingate test. Sixteen recreationally active males, who were in a two-hour fasted state, received a dose of 3mg/kg of body weight an hour before testing (Lorino et al., 2006). Researchers based the dosage on the midpoint of the commonly tested range of 3-9mg/kg bodyweight (Lorino et al., 2006). There was no significant change in peak power, mean power, percent power decrease, and proagility performance (Lorino et al., 2006). The study concluded that caffeine ingested at this dosage did not enhance performance in recreationally active males, but that the results could not be extrapolated to anaerobically trained athletes (Lorino et al., 2006).
Popular theory states that caffeine produces positive effects on fatty acid metabolism and carbohydrate utilization in the tissue, but these metabolic changes are unlikely to occur in exercise lasting less than thirty to forty minutes (Shearer & Graham, 2014). The mechanism by which caffeine affects skeletal muscle metabolism involves its interaction with ryanodine calcium receptors. Specifically, caffeine augments the release of intracellular calcium for increased force production and the shortening of muscle fiber (Shearer & Graham, 2014). Of course, the positive effects are extremely time- and temperature-sensitive and largely dependent on fiber type due to the differences in calcium kinetics, with a greater benefit in slow-twitch than fast-twitch fibers (Shearer & Graham, 2014).
Caffeine’s use as a performance-enhancing supplement should be carefully restricted to athletes. The consumption of caffeine and caffeinated beverages has significant metabolic consequences on glucose disposal in a sedentary state (Shearer & Graham, 2014). Administering caffeine before a glucose tolerance test or an insulin clamp (the gold standard for measuring insulin resistance) resulted in a 30% disposal rate in both tests, creating a hyperinsulinemic and hyperlipidemic state of metabolism (Shearer & Graham, 2014). This means that less than one-third of the glucose is taken up into the cells, and even less makes it to skeletal muscle for glycogen storage (Shearer & Graham, 2014).
Given the half-life of caffeine, its effects on insulin resistance may last through several meals of the day. The consequences of this have implications in the development and progression of chronic diseases, even in previously healthy individuals (Shearer & Graham, 2014). An analysis of healthy subjects showed that caffeine impairs glucose uptake by 26% (Shearer & Graham, 2014). Importantly, a decrease in insulin sensitivity under similar testing conditions was not improved with exercise in another experimental study (Shearer & Graham, 2014). Because caffeine’s benefits are not conclusively supported, from the standpoint of both performance and metabolic physiology, athletes should take caution and supplement with caffeine in moderation.
Spiret, L. L. (2014). “Exercise and sport performance with low doses of caffeine.” Sports Medicine. 44(Suppl 2) (2014): S175-S184.
Lorino, A. J., L. K. Lloyd, S. H. Crixell, and J.L. Walker. “The effects of caffeine and athletic agility.” Journal of Strength and Conditioning Research. 20(4) (2016): 851-854.
Shearer, J., and T. E. Graham. “Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal.” Nutrition Reviews. 72(S1) (2014): 121-136.