To the Editor:

We would like to thank Cooper and colleagues for their letter regarding our recent article (1). In their letter, Cooper and colleagues raised a concern regarding our method of measuring plasma interleukin-6 (IL-6) and the sensitivity of the chosen assay. The authors are correct to point out that the ELISA kit from ENDOGEN suggests a standard curve between 10.24 and 400 pg/mL. We too were aware that plasma levels of IL-6 in humans tend to be low and, therefore, included a dilution to 5.12 pg/mL and excluded the highest 400 pg/mL concentration from each standard curve. With this approach our correlation coefficients for the standard curve regression equations ranged from 0.9954 to 0.9971 demonstrating excellent linearity and, therefore, providing confidence in extrapolating values between 0 and 5.12 pg/mL.

However, based on the concern of Cooper and colleagues and their assertion that exercise should increase circulating levels of IL-6 in children, we returned to our raw data and analyzed the absorbance units derived from the plate reader, before submitting to the standard curve equations. With this approach, our conclusions remained the same insofar as the absorbances after exercise (p = 1.0) and during recovery (p = 0.9) were not different than at rest for the boys, but were significantly higher after exercise (p = 0.001) and during recovery (p = 0.0009) than at rest in the men.

In their letter, Cooper and colleagues point out that IL-6 is reproducibly elevated with exercise in children and apparently dismiss our findings as “unexpected negative results.” We would like to respectively address the evidence as provided by the authors. First, the study by Perez Navaro et al. (2) apparently measured IL-6 levels in saliva, which is not relevant to the discussion of circulating IL-6 levels. Second, Nemet et al. (3) investigated cytokine responses in boys ranging in age from 14 to 18.5 y (mean age of 16.5). It is very likely that most, if not all, of these boys were in advanced stages of puberty. If, as we hypothesized in our paper, maturity status plays a role in the magnitude of exercise-induced immune changes, one would expect this group to respond similarly to adults. Therefore, it is interesting to note that their reported cytokine levels, 1.4 pg/mL before exercise to 11.0 pg/mL after exercise are strikingly similar to our own results from men (Figure 3a). Finally, the paper by Scheet et al. (4) reported an exercise-induced change in circulating IL-6 of 1.6 pg/mL in children 10 y-old. Unfortunately, it was not indicated whether changes in plasma volume had occurred or were accounted for, which would have further minimized the degree of change. Moreover, another paper (5) from Dr. Cooper's laboratory reported an exercise-induced change of 1 pg/mL in 8- to 15-y-olds. Therefore, we do not regard our findings as “negative results,” but rather consistent with the available data, regardless of measurement method, suggesting that young children may be relatively resistant to major cytokine changes with exercise.

In closing, much more research is required to delineate the cytokine, in particular IL-6, response to exercise in young children. This should prove to be a fruitful area of research in light of the emerging role for IL-6 in metabolic adaptations to exercise in adults. The influence of growth and maturation on differences or similarities in these responses may have important implications for several childhood diseases.