Purpose: The objective of this research is to study plasma electrolyte and hematological changes in adolescent runners completing a standard 42.2-km marathon run. Methods: Fifty adolescents (30 healthy males and 20 healthy females) between ages 13 and 17 yr participated in the study. The participants had to undergo a routine physical examination including ECG records. Blood was taken before the race, immediately after the race, and 24 h after the race to determine complete blood cell count and electrolyte concentration. Results: Forty-seven runners completed the race with a mean finishing time of 4 h 57 min (range: 3 h 17 min 09 s to 6 h 14 min). None of the participants experienced an adverse medical event during or after the race. Only slight changes in plasma electrolytes without any cases of hyper- or hyponatremia and a marked increase in white blood cell count were demonstrated immediately after the race. At this time, the hemoglobin concentration and hematocrit were decreased (P < 0.05), indicating an approximately 11% increase in plasma volume. Some of these changes were still present 24 h postrace. No differences were observed between sexes. Conclusions: This is the first study evaluating plasma electrolyte and hematological changes in a relatively large sample of young runners completing a standard marathon. The presented findings indicate that well-trained and educated adolescent marathon runners are not at risk to develop clinically significant electrolyte or hematological changes.
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