We read with great interest the recent article by Timmons et al. (1) on stress/inflammatory responses to exercise in boys and men. While we applaud their efforts to directly compare subjects from these two age groups to research maturational mechanisms of immune responses, one aspect of their data caused us some concern. The authors report that the exercise bout had no effect on circulating interleukin-6 (IL-6) in the boys. This was troubling because of the many cytokines previously reported to be altered acutely by exercise, IL-6 has proved to be the most reproducibly elevated (2). Moreover, we and others have shown IL-6 to be elevated with exercise in younger children and adolescents (3,4), and Perez Navero et al. (5) reported increased IL-6 following a soccer practice in 6- to 7-year-old boys.

One problem that might explain the unexpected negative results in boys lies, perhaps, with the sensitivity of their assay. This is a relevant concern with IL-6 because in healthy individuals at rest normal values in circulating blood are in the range of 1–20 or so pg/ml, whereas with infection or very strenuous exercise, these values can increase by several orders of magnitude.

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