Ingesting carbohydrate (CHO) beverages at regular intervals during prolonged strenuous exercise is associated with fewer numbers of circulating neutrophils and attenuated neutrophil functional responses (Bishop et al. 2000). These effects are thought to be largely mediated through the influence of CHO in blunting the stress hormone response to exercise, particularly that of cortisol (Gleeson & Bishop, 2000). However, there is little information concerning the effect of fluid intake alone on immune responses to prolonged exercise, yet reports suggest that restricting fluid intake during exercise augments the plasma cortisol response (McGregor et al. 1999). Therefore, the aim of the present study was to investigate the influence of regular fluid ingestion compared with no fluid ingestion during a bout of prolonged cycling on plasma cortisol, circulating neutrophil and lipopolysaccharide (LPS)-stimulated neutrophil degranulation responses.

Following Loughborough University Ethical Committee approval, nine recreationally active males (means ± S.E.M.: age 21 ± 0 years, body mass 72.6 ± 1.6 kg,V_O2,max 55.5 ± 1.9 ml kg^-1 min^-1) volunteered to participate in the study. On two occasions, separated by 1 week, subjects reported to the laboratory following an overnight fast and were assigned to either the fluid (F) or no fluid (NF) trial. On the F trial subjects consumed 5 ml (kg body mass)^-1 of artificially sweetened lemon flavoured water 5 min before cycling for 2 h on a stationary ergometer at 65 %V_O2,max. Subjects consumed a further 2 ml (kg body mass)^-1 of the flavoured water at 15 min intervals throughout the exercise. On the NF trial no fluid was consumed before or during exercise. On both trials subjects consumed 5 ml (kg body mass)^-1 of flavoured water at 5 min post-exercise. The order of the trials was randomised. Laboratory conditions were 19 ± 0 °C and 58 ± 2 % humidity. Venous blood samples were obtained from a superficial forearm vein at 10 min pre-exercise, immediately post-exercise and at 1 h post-exercise. Plasma cortisol was measured using ¹²⁵I radioimmunoassay. Blood neutrophil counts were performed using a Sysmex SE9000 cell counter. The in vitro neutrophil degranulation response (elastase release) to bacterial LPS was assessed as described by Blannin et al. (1997). Results were analysed using a two-factor (time X trial) repeated measures ANOVA with post-hoc Tukey and paired t tests applied where appropriate. Statistical significance was accepted at P < 0.05.

Mean sweat rate (calculated from net body mass loss) during both exercise trials was 0.76 ± 0.05 l h^-1. Immediately post-exercise, plasma cortisol concentration had increased significantly from pre-exercise values on both trials (F: pre-exercise, 453 ± 42 nM, post-exercise, 592 ± 46 nM, P < 0.01; NF: pre-exercise, 416 ± 29 nM, post-exercise, 670 ± 63 nM, P < 0.01) and was significantly higher on the NF trial compared with the F trial (P < 0.05). Numbers of circulating neutrophils increased similarly on both trials over the sample time points to 12.6 ± 0.9 X 10⁹ cells l^-1 and 12.9 ± 0.5 X 10⁹ cells l^-1 at 1 h post-exercise on the F and NF trials, respectively (main effect of time, P < 0.001). LPS-stimulated elastase release per neutrophil decreased similarly on both trials in response to the exercise (F: pre-exercise, 167 ± 23 fg cell^-1, post-exercise, 100 ± 10 fg cell^-1; NF: pre-exercise, 173 ± 13 fg cell^-1, post-exercise, 99 ± 12 fg cell^-1, main effect of time, P < 0.001). Plasma volume changes were -5.0 ± 0.6% (F) and -6.5 ± 1.0% (NF) (n.s.). Adjusting the data for changes in plasma volume did not alter the relationships observed. These data suggest that in ambient environmental conditions fluid ingestion alone has negligible influence on circulating neutrophil and LPS-stimulated neutrophil degranulation responses to prolonged exercise.