Effect of exercise, heat stress, and hydration on immune cell number and function

Purpose: The purpose of this study was to determine the effect of thermal stress and hydration status on immune function during exercise. Methods: Ten trained men completed four cycle ergometer rides at 55% V(overdot)O-2peak under the following conditions: EN (euhydrated neutral; 22degreesC, 30% RH), DN (dehydrated neutral), EH (euhydrated hot; 38degreesC, 45% RH), and DH (dehydrated hot). During EN and EH, a carbohydrate/electrolyte beverage was consumed at a rate matching sweat loss, and during DN and DH, no fluid was ingested. Blood samples were drawn pre- and postexercise, and at 2 and 24 h of recovery. Cell counts were determined by automated counting and flow cytometry. Neutrophil activity was assessed as superoxide production, lymphocyte function was determined via PHA-stimulated mitogenesis, and natural killer (NK) cell activity was measured with a Cr-51-release assay. Cortisol was assayed via RIA. Results: Lymphocytes proliferation was depressed 2 h after exercise in all conditions (P < 0.05); however, when expressed on a per cell basis, function was greater in the DH and EH conditions. NK activity (V(overdot)(max).10(3) cells) was greater post compared with preexercise in all conditions (EH = 25.5 +/- 16.8, DH = 26.2 +/- 10.5, EN = 19.3 +/- 11.0, and DN = 16.5 +/- 8.7) but was not different between conditions. Leukocyte, neutrophil, lymphocyte, and NK cell counts were also elevated postexercise with the former two remaining elevated 2 h postexercise in the EH and DH conditions. Cortisol was greater postexercise in EH (22.1 +/- 1.3) and DH (27.7 +/- 1.3) compared with EN (17.8 +/- 2.1) and DN (18.9 +/- 1.6 mug.dL(-1)). Conclusion: Euhydration did not affect cell number or function when compared with a dehydrated state; however, the hot environment caused more severe disturbances in these measures compared with a neutral environment.