In a well controlled and designed study, Baker et al¹ clearly showed the effects of ingesting large volumes of water or a carbohydrate/electrolyte sports drink (CES) on serum Na⁺ concentration ([Na⁺]) during exercise in older, active adults (54–70 years). The data show that ingesting any hypotonic fluid, be it water or a Na⁺-containing sports drink (about 18 mmol/l), in volumes that are similar to or more than weight losses produces a fall in [Na⁺]—a finding that mirrors the study of Twerenbold et al published in this journal.² Readers should know that any fluid of concentration less than 150 mmol/l is considered to be hypotonic to the plasma.

In the study of Baker et al,¹ the ingestion of a CES produced a lower …