Assessment of water balance in high school athletes and non-athletes

Water is the main component of cells, tissues, and organs, and plays vital roles in macronutrient hydrolysis and overall cell function regulation. While low water intake may be sufficient for hydration, several studies suggest that most children fail to meet water intake guidelines, leading to underhydration as indicated by elevated hydration biomarkers. We conducted a cross-sectional observational study to examine water intake and output among high school students, comparing non-athletes and athletes. We enrolled 26 non-athletes and 28 athletes in this study. Water balance was assessed by water intake from foods and beverages, metabolic water production, and non-renal water loss (NRWL). Moreover, urine was compared. Increases of total water intake from food and beverage consumption in athletes were significantly higher than those in non-athletes. Furthermore, urinary output was lower and NRWL was 2-3 times higher in athletes than non-athletes. These findings suggest that large NRWL induces hypohydration in athletes. J. Med. Invest. 72 : 42-46, February, 2025.