Carbohydrate (CHO) ingestion during exercise has been shown to attenuate elevations in the plasma IL-6 concentration (Nieman et al. 1998). However, IL-6 mRNA expression in the contracting muscle is unaffected by CHO ingestion. It has recently been suggested that during exercise the liver may produce IL-6 to stimulate its own glucose output (Starkie et al. 2001). The attenuation of hepatic glucose output following CHO ingestion during exercise may be mediated, at least in part, by a reduced hepatic IL-6 production. Since it has been demonstrated that CHO ingestion during exercise in the heat does not attenuate hepatic glucose output, it may be speculated that the plasma IL-6 concentration may also be unaffected by CHO ingestion during exercise in the heat. Therefore, the present study examined the effect of regular CHO ingestion on the plasma IL-6 response to prolonged cycling in the heat.
Seven healthy, moderately trained men (22.7 ± 0.4 years, 74.7 ± 3.1 kg; ΩO2,peak 59.0 ± 1.8 ml kg-1 min-1; mean ± S.E.M.) participated in the study, which was approved by the University of Birmingham Ethics Committee. On four occasions separated by at least 1 week, following an overnight fast, subjects cycled to exhaustion at ~62 % or ~74 % of ΩO2,peak in an environmental chamber maintained at 35°C (~30 % relative humidity). Immediately prior to the rides subjects ingested 8 ml kg-1 of either a 6.4 % CHO-electrolyte solution (CHO) or an artificially sweetened placebo (PLA); an additional 3 ml kg-1 of the same solution was consumed every 15 min during exercise. The order of the trials was randomised. Blood was sampled from an antecubital vein before exercise, at fatigue and at 1 and 2 h post-exercise. An additional blood sample was collected after 60 min during trials at 62 % ΩO2,peak. Results were examined using repeated measures ANOVA with post hoc Tukey test and paired t tests applied where appropriate. Performance times were examined using the Wilcoxon signed ranks test for non-parametric data.
CHO ingestion resulted in a significantly longer time to fatigue compared with PLA during rides at ~62 % ΩO2,peak (149 ± 13 vs. 123 ± 13 min; P < 0.05) but not during rides at ~74 % ΩO2,peak (62 ± 13 vs. 53 ± 8 min). The peak plasma IL-6 concentration was observed at fatigue in all trials (62 % ΩO2,peak CHO: 6.45 ± 2.24 pg ml-1; 62 % ΩO2,peak PLA: 5.74 ± 1.58 pg ml-1; 74 % ΩO2,peak CHO: 2.52 ± 0.21 pg ml-1; 74 % ΩO2,peak PLA: 2.59 ± 0.51 pg ml-1) with values significantly higher than pre-exercise (~1.1 pg ml-1 in all trials) in all conditions (P < 0.01). In addition, the plasma IL-6 concentration at fatigue was significantly higher in the lower intensity condition. Similar peak plasma cortisol concentrations were observed at fatigue in all trials (~520 nmol l-1) and plasma glucose concentration was well maintained in all trials.
In contrast to the majority of previous studies performed in comfortable ambient conditions, CHO ingestion during constant-load exercise in hot conditions did not attenuate the rise in the plasma IL-6 concentration. Furthermore, this effect was exercise intensity independent.
We gratefully acknowledge financial support from consumer healthcare GlaxoSmithKline.