The mean PEDro rating was a 9.3 out of 10. Sixteen of the studies scored a perfect 10. The most common PEDro item that studies failed to achieve was “blinding of therapists” and “blinding of assessors” (4 studies for each [17,34,35,61]). By failing to double-blind a study that is largely dependent on subject motivation and assessor encouragement, there is a risk that results are biased. It is important to note that failing to identify blinding procedures implies that blinding procedures were not used.
Out of the 33 trials, 21 used cycling (mean improvement = 4.4 ± 5.0%), 6 used running (mean improvement = 0.9 ± 0.7%), 4 used rowing (mean improvement = 1.1 ± 0.3%), 1 used swimming (mean improvement = 1.7%), and 1 used cross-country skiing (mean improvement = 1.1%) as the mode of exercise. Sixteen trials used a set intensity before the time-trial component (mean improvement = 4.7 ± 5.6%); the other 17 did not (mean improvement = 1.8 ± 1.4%). Fourteen of the articles used time to complete a set distance as the performance measure (mean improvement = 1.1 ± 0.5%), 10 used maximum work produced in a fixed time (mean improvement = 4.3 ± 4.1%), and 9 used time to complete a set number of revolutions (W) while cycling (mean improvement = 5.2 ± 6.2%). Women were included in 10 of the 29 trials. Only 1 study (2 trials) tested only women subjects (1); the 8 remaining trials with women did not statistically analyze differences between sexes.
Independent of ingestion timing, the average performance improvement with CAF was 3.2 ± 4.3% over placebo. The median improvement with CAF was 1.7%. Thirty of the 33 trials showed positive improvements in performance with CAF, but only 15 were statistically significant (p < 0.05). One study with 2 different CAF trials observed large performance improvements (14.5 and 17.3%) but were not statistically significant, possibly because of a type II error (15). Another study (18) observed a large, significant improvement (15%) over placebo. This improvement may be attributable to the combination of repeated CAF ingestion throughout the exercise protocol and/or the unique submaximal protocol in which subjects alternated between 60 and 75% o2max for 120 minutes before starting a 15-minute time trial.
Caffeine ingested before exercise resulted in a mean performance improvement of 2.3 ± 3.2% (Figure 2). Performance was improved 4.3 ± 5.3% when CAF was ingested both before and during exercise (Figure 2). Total CAF ingestion does not explain these differences (mean = 5.3 mg·kg−1, Table 1; 5.2 mg·kg−1, Table 2). Further, the range of performance improvement is similar when CAF is ingested before exercise (−0.7 to 14.5%, Table 1) and when ingested before and during exercise (−0.3 to 17.3%, Table 2).
The purpose of this systematic review was to critically evaluate studies that examined the effects of CAF on sport-specific endurance performance. We only reviewed articles that had time-trial components. This type of performance test is not only more valid in detecting treatment effects (33), but, more importantly, it is applicable to sport. For this same reason, we chose to include articles that were conducted in field settings or that used the treatment of CAF with other substances (i.e., CE solutions). Although ingesting CAF via capsules may prevent any interaction with other ingested foods or fluid, there is limited availability of ready-made CAF capsules that an athlete may ingest. There are quite a few studies examining CAF with CE solutions. This is not surprising because the American College of Sports Medicine recommends the use of CE solutions in long-duration exercise (58), a setting in which CAF is a commonly used ergogenic aid (23). Some have hypothesized that a high-carbohydrate diet may attenuate the increase in free fatty acids observed with CAF ingestion and, thus, modulate the ergogenic effect of CAF (63). This has not been observed in performance settings, and it likely has little effect. The ergogenic effect of CAF, especially in non-glycogen-limiting exercise, is thought to be more central in nature and not metabolic (20,40).
Degree of improvement with CAF does not seem to be consistently associated with mode of CAF delivery, timing, total exercise time, or the exercise mode employed (Tables 1 and 2). Total CAF ingested seems to be loosely associated with degree of improvement. It is generally observed that quantities above 3 mg·kg−1 body mass are needed for improvement. Quantities up to 6 mg·kg−1 are most commonly used, but this amount and greater amounts do not always result in performance improvements. Using a time-trial protocol, Bruce et al. (10) did not observe increased performance compared with placebo when increasing CAF ingestion from 6 to 9 mg·kg−1 body mass. In the studies we reviewed, 9 mg·kg−1 resulted in improvements no greater than 1.5% over placebo (Tables 1 and 2). Because some athletic governing bodies have restrictions against large amounts of CAF ingestion (i.e., National Collegiate Athletic Association), but none completely ban CAF use, it is recommended that CAF use not exceed 9 mg·kg−1. Performance improvements with CAF ingestion are maximized with amounts up to 6 mg·kg−1 and are not further improved with 9 mg·kg−1 (10,31). The ergogenicity of CAF up to 6 mg·kg−1 has been observed in a variety of settings, but factors such as one's habitual use of CAF may change the dose needed to elicit an ergogenic effect.
Although there are no known studies examining the effects of CAF habituation on time-trial performance, several studies have examined CAF habituation using other exercise performance protocols. Using a time-to-exhaustion protocol, Van Soeren and Graham (62) measured performance after subjects abstained from CAF for 0, 2, and 4 days. There was a trend for greater improvement with CAF ingestion after abstaining from CAF for 2 and 4 days (vs. 0 days). Similarly, using a time-to-exhaustion protocol, Bell and McLellan (5) showed that improvements in performance were greater for CAF nonusers (< 50 mg CAF per day) vs. users (≥ 300 mg of CAF per day). Therefore, it is possible that CAF habituation may modulate performance improvements with acute CAF ingestion.
It is not known how many days an endurance athlete should abstain from CAF to maximize its ergogenic effects, but animal studies show that increases in adenosine receptor number and affinity are maximized in 7 days (42). Therefore, we recommend that athletes abstain from CAF ingestion for no fewer than 7 days before competition. This should allow for withdrawal symptoms (which may negatively affect performance) to subside and allow sufficient time for adenosine receptor downregulation to occur (42), thus possibly maximizing the ergogenic effects of CAF. Although abstaining from CAF before use in an athletic setting is ideal, some may find it too difficult because of withdrawal symptoms (e.g., headaches, fatigue, lethargy, flulike symptoms) (5,62). Over-the-counter medicine may help alleviate these symptoms, but the interaction of these substances with CAF is unknown. When CAF is habitually consumed, one may improperly conclude that an increase in dosage may be sufficient to elicit an ergogenic effect similar to that experienced by a CAF-naïve individual. Unfortunately, this specific scenario has not been examined in a performance setting. Regardless of habituation level, the ingestion of large amounts of CAF may result in negative side effects (14); therefore, abstaining from CAF before use possibly will give the greatest chance of optimizing the ergogenic effect (5).
In conclusion, there are a number of high-quality research articles that have examined the effects of CAF on time-trial endurance performance. The expected performance improvements with CAF ingestion are varied, but they may be dependent on a number of factors including timing, ingestion mode/vehicle, and subject habituation. Given the available evidence, we recommend that endurance athletes abstain from CAF use at least 7 days before competition. Acute ingestion should occur no more than 60 minutes before and, if practical, during competition. The amount of CAF commonly shown to improve endurance performance is between 3 and 6 mg·kg−1 body mass; these amounts are equally effective when combined with a CE solution or water. Further research should seek to identify specific factors that mediate the large range of improvements commonly observed with CAF ingestion.
Caffeine is a widely used legal drug that has been shown to improve endurance performance and that, theoretically, could be used before training sessions when high-intensity exercise is desired. The degree of performance improvement is variable and likely influenced by the timing of ingestion, amount ingested, mode of ingestion, and how often an athlete consumes CAF on a daily basis (level of habituation). To maximize performance improvements with CAF ingestion, it is recommended that athletes consume up to 6 mg·kg−1 body mass no more than 60 minutes before exercise, but it also may be consumed during exercise. Caffeine seems to be equally effective when ingested in combination with CE solutions (e.g., sports drinks) or other modes of ingestion (e.g., gum), but other substances in caffeinated-coffee may counteract CAF's performance-improving properties. Evidence suggests that consuming CAF every day may dampen the degree of performance improvement observed when CAF naïve. Therefore, we recommend that athletes abstain from CAF no fewer than 7 days before its use in competition. Because some individuals react differently to CAF than others (i.e., CAF sensitive), it is recommended that athletes try CAF while training before using it in competition.
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