No additive effect of acetaminophen when co-ingested with caffeine on cycling performance in well-trained young men.

Abstract

We investigated the effect of caffeine and acetaminophen on power output during a 6-min performance-test, peripheral fatigue, and muscle protein kinase A (PKA) substrate-phosphorylation. Fourteen men (age(mean±SD): 26±6 years; V̇O2max: 63.9±5.0 mL∙min-1∙kg-1) completed four randomized trials with acetaminophen (1500 mg), caffeine (5 mg∙kgbw-1), combined caffeine and acetaminophen (caffeine+acetaminophen) or placebo. Mean power output during the 6-min performance-test (placebo mean:312±41 W) was higher with caffeine (+5 W;95%CI: 1 to 9;P=0.017) and caffeine+acetaminophen (+6 W;95%CI: 0 to 12;P=0.049) than placebo, but not with acetaminophen (+1 W;95%CI: -4 to 7;P=0.529). Decline in quadriceps maximal isometric voluntary torque immediately after the performance-test was lower (treatment×time; P=0.035) with acetaminophen (-40 Nm;95%CI:-53 to -30;P<0.001) and caffeine+acetaminophen (-44 Nm;95%CI: -58 to -30;P<0.001) than placebo (-53 Nm;95%CI: -71 to -39;P<0.001) but was similar with caffeine (-54 Nm;95%CI: -69 to -38;P<0.001). Muscle phosphocreatine content decreased more during the performance-test (treatment×time;P=0.036) with caffeine+acetaminophen (-55 mmol∙kgdw-1;95%CI: -65 to -46;P<0.001) than placebo (-40 mmol∙kgdw-1;95%CI: -52 to -24;P<0.001). Muscle net lactate accumulation was not different from placebo (+85 mmol∙kgdw-1;95%CI: 60 to 110;P<0.001) for any treatment (treatment×time;P=0.066), being +75 mmol∙kgdw-1 (95%CI: 51 to 99;P<0.001) with caffeine, +76 mmol∙kgdw-1 (95%CI: 58 to 96;P<0.001) with acetaminophen, and +103 mmol∙kgdw-1 (95%CI: 89 to 115;P<0.001) with caffeine+acetaminophen. Decline in muscle ATP and glycogen content and increase in PKA substrate-phosphorylation was not different between treatments (treatment×time;P>0.1). Thus, acetaminophen provides no additive performance enhancing effect to caffeine during 6-min maximal cycling. In addition, change in PKA activity is likely not a major mechanism of performance improvement with caffeine.