Reference
Guest N, Corey P, Vescovi J, El-Sohemy A. Caffeine, CYP1A2 genotype, and endurance performance in athletes [published online ahead of print March 5, 2018]. Med Sci Sports Exerc.
Study Objective
To determine whether variation in the CYP1A2 gene, which affects caffeine metabolism, modifies ergogenic (athletic power and endurance) effects of caffeine in a 10-km cycling trial.
Design
A split-plot randomized, double-blind, placebo-controlled trial
Participants
One-hundred thirteen male athletes who competed in sports characterized by either endurance (eg, marathon, triathlon, cycling, cross-country skiing), power (eg. boxing, volleyball, dragon-boat, powerlifting), or a mixture of power and endurance (eg, soccer, rugby, basketball, swimming).
All athletes included in the study were training or competing in their sport at least 8 hours per week for 9 of 12 months per year for at least 3 years. Each athlete had to abstain from extraneous caffeine during the entire period of this trial. Athletes who could not complete the study due to a sports-related injury or school or work commitments, were unwilling to abstain from caffeine, would not consent to the required testing, or did not complete the required forms were excluded from the final analysis.
Study Parameters Assessed
Anthropometric data, maximum aerobic capacity (VO2 peak), questionnaire on general health, caffeine intake, sports history, and saliva for DNA determination.
Physical tests prior to the cycle time trial (TT) were vertical jump, handgrip, and Wingate 4 (a test that measures anaerobic power and capacity). The cycle TT was a 10-km ride and began when the participant’s serum lactate rose above 2.5 mmol/L during the Wingate 4 in the pre-TT. An Ergomedic 839 E stationary bike was set at a resistance tailored to the participant’s results from the VO2 peak test. Participants rated their perceived exertion on a scale of 6 to 20 (20=extremely difficult) at 5 and 9 km. They were also asked whether they thought they received caffeine or placebo. This study was conducted at the Goldring Centre for High Performance Sport, University of Toronto, Toronto, Canada.
Based on the results from the present clinical trial, only AA genotypes will benefit from caffeine ingestion prior to an athletic event.
Genotyping was performed using an Oragene ON-500 kit DNA isolation. All participants were grouped into AA (fast), AC (heterozygous slow), or CC (homozygous slow) genotype. They were genotyped for the −163A>C polymorphism in the CYP1A2 gene (rs762551).
Intervention
Each athlete completed 3 trials, approximately 1 week apart, receiving a different intervention each visit. The athletes were given a capsule of either placebo or 2 or 4 mg/kg caffeine 25 minutes before each trial. They sat quietly completing questionnaires before their warm-up, exercise tests, and time trial. Blood pressure and heart rate were taken 3 minutes after ingestion (of placebo or caffeine capsule) and again 20 minutes later, before warm-up exercises. The order of treatment was random for each participant.
The caffeine was anhydrous caffeine (American Chemicals, Ltd, Montreal, Quebec, Canada); placebo was dextrose.
Results
Of the 113 athletes who entered the trial, 8 dropped out due to either a sports-related injury, school or work demands, unwillingness to abstain from caffeine, or relocation. Four were excluded because of incomplete data. The remaining 101 athletes were 25±4 years and weighed 81.3±12.4 kg.
Classified by sport type, 42% of the men were endurance athletes, 42% were power athletes, and 16% were mixed (combination). By genotype, the men were AA (n=49; 48.5%), AC (n=44; 43.5%), and CC (n=8; 7.9%). There were no significant differences by genotype and sport type for height, weight, age, body fat, VO2 peak (L/min), VO2 peak (mL/kg/min), dietary caffeine per day (mg/d), and sport caffeine (mg/d).
There was a significant effect of treatment (P=0.04) for all participants who took caffeine 4 mg/kg, with a 3% drop in TT (0.5 minutes) compared to placebo. There was not significant difference between 2 and 4 mg/kg caffeine or between 2 mg and placebo. Stratification by caffeine dose and genotype was significant (P=0.002) as was caffeine-gene interaction (P<0.0001).
Genotype AA (fast metabolizers) had a significant positive effect from caffeine with TT performance decreased 4.8% (0.8 minutes) at 2 mg/kg (P=0.0005) and 6.8% (1.2 minutes) at 4 mg/kg (P<0.0001), but there was no significant difference between 2 and 4 mg/kg. Genotype AC (slow heterozygous metabolizers) had no significant caffeine effect on TT performance (P=0.43). Genotype CC (slow homozygous metabolizers) at 4 mg/kg had a significant decreased TT performance by 13.7% (2.5 minutes, P=0.04) compared to placebo, with no significant differences between 2 mg/kg and either 4 mg/kg or placebo. Genotype CC had the greatest change in TT performance but the worst athletic performance.
Among the AA genotype, 35 (71%) and 40 (82%) out of 49 men performed better during 2 or 4 mg/kg, respectively, compared to placebo. Among AC genotype, 26 (59%) and 28 (64%) out of 44 men performed better during 2 or 4 mg/kg caffeine, respectively, compared to placebo. Among CC genotype, 2 (25%) and 1 (12%) out of 8 men performed better during 2 and 4 mg/kg caffeine, respectively, compared to placebo.
Heart rate values were not significantly different between caffeine and placebo treatment doses in genotype AA. In genotype AC there was a significant 2.5% (4 beats per minute [bpm]) increase with 4 mg/kg compared to 2 mg/kg and placebo (P=0.007 and P=0.005, respectively). In genotype CC there was a significant 2% (3 bpm) decrease in 4 mg/kg compared to 2 mg/kg and placebo (P=0.05 and P=0.03, respectively).
Treatment blinding was assessed by collecting questionnaires from 86 of the 113 athletes after the TT. Of the 172 caffeine trials, 3% (54) correctly identified they had received caffeine. Among the other 118 caffeine trials, 81% (96) reported no caffeine and 19% (22) reported “maybe caffeine.” Only 3% (3) correctly identified all 3 trials (2 with caffeine, 1 with placebo).
Familiarization is the learning or visit effect that was expected in a trial where only 6% were experienced cyclists. Acoss the 3 treatment arms there was not a familiarization effect (P=0.73) and in the AA and CC genotypes the caffeine effect was significant compared to the familiarization effect (P<0.0001 and P=0.04, respectively).
Key Findings
In this 3-arm trial involving 2 caffeine doses and a placebo, caffeine had a significant positive effect in the timed cycle trial, but only for the AA genotype (fast metabolizers). Their TT results were significantly lower for each caffeine dose compared to placebo, but not significantly different between the 2 caffeine doses. In the CC genotype (slow metabolizers) caffeine had a significant negative effect on TT results compared to placebo, while in the heterozygous AC slow metabolizers TT results were not affected by any caffeine dose. No significant adverse effects of the interventions were reported.
Practice Implications
The CYP1A2 isoenzyme is responsible for caffeine metabolism among other substances.1 In genetic studies it is identified as rs762551 and consists of homozygous AA fast metabolizers, heterozygous AC slow metabolizers, and homozygous CC slow metabolizers. The metabolic rate for caffeine of genotype AC is generally between that of AA and CC. This enzyme metabolizes xenobiotics including caffeine, and is induced by tobacco, chargrilled meats, broccoli, Brussels sprouts, cauliflower, modafinil, insulin in diabetics, and omeprazole. It is inhibited by fluoroquinolones, fluvoxamine, verapamil, Hypericum perforatum, and weakly by cimetidine, Echinacea spp, Curcuma longa, cumin, and naringenin in grapefruit juice.1
There have been several studies looking at the effect of caffeine on athletic performance. In 2008, Jenkins et al published a study of 13 cyclists and found that caffeine at 2 and 3 mg/kg increased performance by 4% (P=0.02) and 3% (P=0.077), respectively.2 They noted considerable inter-individual variability; only 11 of the 13 participants benefited from caffeine ingestion.
In 2015, Paton et al reported that chewing gum containing 3-4 mg/kg caffeine improved performance in 20 male and female cyclists.3 However 13 (65%) were deemed positive responders, 5 (25%) negative responders, and 2 (10%) nonresponders. The difference was assumed to be due to rate of caffeine metabolism or absorption.
In 2012, Womack et al reported on the effects of ingesting 6 mg/kg anhydrous caffeine or placebo 1 hour before a 40-km ergometric cycle trial.4 They classified the participants as AA (homozygous) or C (allele) carriers. The AA genotype significantly benefited while the C type did not.
A number of studies assessing the influence of CYP1A2 genotype on caffeine effects in athletes have found no association. Algrain et al compared chewing gum containing 225 mg caffeine with placebo for a 15-minute cycle trial. Participants were both female and male recreational cyclists of genotype AA (n=10) or C (n=9), but the study found no significant differences from caffeine (P>0.258) or across genotypes (P>0.861).5 The amount of caffeine in their chewing gum may have been too low to have clinical effect. Pataky et al compared 6 mg/kg caffeine with placebo and 25 mL of 1.14% caffeine mouth rinse for a 3-km cycle time trial in 38 recreational cyclists with AA or AC genotype. Both AA and AC genotypes benefited from caffeine but only AC performed better. The rinse and rinse plus ingestion arms performed better early (1,000 h) compared to the late group, after 1,000 h.6 Salinero et al compared 3 mg/kg caffeine with placebo in 21 female and male participants of genotype AA (n=5) or C (n=16). Caffeine significantly increased peak power (P=0.008) but had no effect by genotype (P>0.05), reaction time between caffeine vs placebo (P=0.681), or on genotype performance. Nervousness was reported by 31.3% of C genotypes but not by the AA genotypes. Caffeine was more likely to cause nervousness, insomnia, gastrointestinal upset, hyperactiveness, irritability, muscular pain and/or headache in the C vs the AA genotype.7
The mechanism of caffeine’s effects on athletic performance is not yet clear. Caffeine may reduce cardiac blood flow during exercise by blocking adenosine receptors,8 which in turn could reduce performance in slow metabolizers as well as cause vasoconstriction to both the heart and skeletal muscles. Resting cardiac blood flow may not be affected compared to cardiac blood flow during exercise.
How should you advise your athletic patients or your citizen-athlete? The International Olympic Committee (IOC) and World Anti-Doping Agency (WADA) limits have dropped from 15 to 5 mcg caffeine per liter of urine as of January 1, 2018.9 Their ban on caffeine was removed in 2004 but may be reinstated in 2018 or 2019. This limit is approximately equal to 300 mg of caffeine or 1 to 3 8- to 12-ounce (236-355 mL) cups of coffee consumed within 1 hour before an athletic event. The National Collegiate Athletic Association (NCAA) has banned caffeine.10 Their urine limit of 15 mcg/mL, or the ingestion of about 500 mg of caffeine or 6 to 8 cups of brewed coffee 2 to 3 hours before competition, is considered a positive test that would ban a competing athlete. To date I am not aware of any citizen races that ban or test for caffeine.
If caffeine ingestion causes you to experience nervousness, insomnia, gastrointestinal upset, hyperactiveness, irritability, muscular pain, and/or headache then you may have the genotype that does not benefit or, even worse, performs poorly after caffeine ingestion. If you are an Olympic or NCAA athlete, your allowed caffeine intake is limited and may soon be completely banned. If you have coronary artery disease, caffeine could make your physical performance worse. A recent umbrella review of 218 meta-analyses concluded that coffee presented a statistical harm only in pregnancy and as a possible fracture risk for women.11 Overall the authors of the review concluded that caffeine was safe, more likely to benefit than to harm, and 3 to 4 cups per day was beneficial. The weakness of this meta-analysis was that most of the data was observational, not trial data. Genotyping was not discussed in this umbrella review.
Based on the results from the present clinical trial, only AA genotypes will benefit from caffeine ingestion prior to an athletic event. The article does not address the issue of caffeine's effects in major endurance events (eg, marathons), as the time trials lasted approximately 16 to 20 minutes. Patients can determine what CYP1A2 genotype they have by using commercially available genetic testing services or through their naturopathic doctor, but in Ontario naturopathic doctors currently cannot legally discuss these matters with patients, advise them to get genetic testing, or order genetic testing.
The study reviewed here was generally well-written, but it suffered from mathematical errors when calculating the results of reviewed studies and some errors in reference accuracy. Author Ahmed El-Sohemy is the founder and holds shares in Nutrigenomix, Inc., and author Nanci Guest serves on the Scientific Advisory Board of Nutrigenomix.12 The remaining authors declared no conflicts.
Summary
Caffeine ingestion is common inside and outside of the athletic world. This study looked at the randomized effects of caffeine in 2 doses, 2 and 4 mg/kg, vs placebo in athletes who competed in endurance, power, or mixed (combination) sports at least 9 of 12 months per year for at least 8 hours per week for at least 3 years. The athletes were genotyped to the CYP1A2 gene (rs762551) as AA (fast metabolizers), AC (slow metabolizers), and CC (homozygous slow metabolizers). Caffeine at both doses improved performance in genotype AA, had no effect on genotype AC, and reduced performance in genotype CC.
