High dietary protein restores overreaching induced impairments in leukocyte trafficking and reduces the incidence of upper respiratory tract infection in elite cyclists

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• High-intensity exercise training (or overreaching) is associated with an increased incidence of upper respiratory tract infection in athletes. We have recently shown that leukocyte trafficking, an important aspect of immune surveillance required to detect and eliminate infection, is impaired during high-intensity exercise training. The current investigation shows that a high dietary protein during high-intensity exercise training restores leukocyte trafficking, and therefore aspects of immune surveillance, to levels observed during normal-intensity exercise training. This restoration of immune surveillance with a high protein diet occurred in parallel with a reduced incidence of upper respiratory tract infections in athletes.

The present study examined whether a high protein diet prevents the impaired leukocyte redistribution in response to acute exercise caused by a large volume of high-intensity exercise training. Eight cyclists (VO2max: 64.2 ± 6.5 mL kg−1 min−1) undertook two separate weeks of high-intensity training while consuming either a high protein diet (3 g kg−1 protein BM−1 day−1) or an energy and carbohydrate-matched control diet (1.5 g kg−1 protein BM−1 day−1). High-intensity training weeks were preceded by a week of normal-intensity training under the control diet. Leukocyte and lymphocyte sub-population responses to acute exercise were determined at the end of each training week. Self-reported symptoms of upper-respiratory tract infections (URTI) were monitored daily by questionnaire. Undertaking high-intensity training with a high protein diet restored leukocyte kinetics to similar levels observed during normal-intensity training: CD8+ TL mobilization (normal-intensity: 29,319 ± 13,130 cells/μL × ∼165 min vs. high-intensity with protein: 26,031 ± 17,474 cells/μL × ∼165 min, P > 0.05), CD8+ TL egress (normal-intensity: 624 ± 264 cells/μL vs. high-intensity with protein: 597 ± 478 cells/μL, P > 0.05). This pattern was driven by effector-memory populations mobilizing (normal-intensity: 6,145 ± 6,227 cells/μL × ∼165 min vs. high-intensity with protein: 6,783 ± 8,203 cells/μL × ∼165 min, P > 0.05) and extravastating from blood (normal-intensity: 147 ± 129 cells/μL vs. high-intensity with protein: 165 ± 192 cells/μL, P > 0.05). High-intensity training while consuming a high protein diet was associated with fewer symptoms of URTI compared to performing high-intensity training with a normal diet (P < 0.05). To conclude, a high protein diet might reduce the incidence of URTI in athletes potentially mediated by preventing training-induced impairments in immune-surveillance.