BackgroundUnaccustomed intense endurance exercise is associated with short-term suppression of natural immunity. However, it is not established whether intensified endurance training alters resting immune status or increases the risk of upper respiratory infection (URI).PurposeThis study examined the effect of intensified endurance training for performance enhancement on resting immune status in nine healthy, male competitive cyclists.DesignData were collected during 4 weeks of usual training (baseline), followed by prescribed cycle training that consisted of volume-building at customary training intensity (V phase, 6 weeks), unaccustomed very high intensity interval training at 100% maximal heart rate (I phase, 18 days), and an unloading taper (U phase, 10 days).MethodsThe main performance criterion was a simulated 20 km time-trial. Aerobic capacity measures included power output at ventilatory threshold (POTvent) and maximal oxygen uptake (VO2max). Markers of immune status (lymphocyte subset counts, serum cytokine levels, and new URI cases) and physiological indicators of training stress (cycling economy, 24-hour urinary cortisol excretion, and serum testosterone concentration) were evaluated in the rested state, 36 to 44 hours postexercise, during baseline, and after each training phase.ResultsTime-trial performance, POTvent, VO2max, and cycling economy improved significantly (p < 0.001) after the V phase, and remained higher than baseline (p < 0.001) after the I and U phases. As compared with the V phase, performance time was faster after the U phase (p < 0.01). In contrast, lymphocyte counts, cytokine levels, incidence of URI, cortisol excretion, and serum testosterone concentration were not significantly different from baseline in any phase.ConclusionsCycling efficiency and performance improved while resting immune status was maintained throughout the 10-week training program. This study provides encouraging data in support of immunological robustness during intensified endurance training.