Histamine appears to be a main component of the “exercise response” and blocking its actions seems to have differential outcomes on exercise performance. In humans, blocking histamine’s actions via oral antihistamines has no effect on short-duration, high-intensity exercise. Conversely, antihistamines decrease the speed and duration components of endurance tasks of murine species. It is unknown if the difference in performance between humans and rodents is species specific or related to exercise duration. PURPOSE: To test the effects of antihistamines on cycling time trial performance preceded by rest or exercise. It was hypothesized that, in comparison to a placebo condition, antihistamines would have no effect on 10-km time trial performance when the trials are preceded by rest, but antihistamines would increase the amount of time needed to complete a time trial when preceded by an endurance exercise session. METHODS: Two competitive female cyclists completed six 10-km cycle time trials. Two time trials served as familiarization tests. In the following 4 time trials, the first two were preceded by 2 hours of seated rest and the last two by 2 hours of steady state cycling at 50% VO2peak. The volunteers consumed either placebo or antihistamines (540 mg Fexofenadine, 300 mg Ranitidine) pills in a semi-randomized order in a way that placebo and antihistamine conditions were represented in the pre-time trial rest and exercise paradigms. Time to completion of the 10-km time trials was the main outcome variable. Rating of perceived exertion (RPE), blood lactate and glucose, as well as isometric quadriceps muscle strength were recorded before and after the 120-min rest/exercise as well as following the time trials. No statistical comparisons were made between conditions given the low number of completed volunteers. RESULTS: In comparison to placebo conditions, antihistamines increased the time to completion of 10-km time trials by 3.7±0.8 s (mean±SE) when preceded by rest and 11.2±0 s when preceded by exercise. Associated with the slower completion times were greater blood glucose (placebo: 88±6, antihistamine 97±6 mg/dl), lactate (placebo: 6.6±1.1, antihistamine 8.7±1.3 mmol/L) and percent decrease in quadriceps strength (placebo: 1.9 ± 2.0%, antihistamine: -6.8 ± 5.2%). RPE at the completion of the time trials were similar between conditions (19-20). CONCLUSION: The results of the present study suggest that antihistamine use may have a detrimental effect on endurance performance which might be related to decreased muscle strength or increased lactate production. More subjects are necessary for a statistical backing of these initial observational findings.

Support provided by NIH grant HL115027

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