The standard protocol to assess hydration status is by measuring body mass in the early morning without controlling fluid intake. However, obtaining first-morning body mass is not necessarily feasible for many situations, for example, most physical activities take place in the afternoon. Thus, first-morning body mass might not be practical to assess hydration status. PURPOSE: To investigate first-morning body mass versus afternoon body mass in free- living and controlled euhydration. METHODS: 9 males (age: 21 ± 2; mass: 79.7 ± 17.8 kg) and 5 females (age: 22 ± 2; mass: 60.5 ± 13.6 kg) visited the laboratory in the morning (7:00-9:00am) and afternoon (2:00-4:00pm) for six days to measure their nude body mass and urine specific gravity (USG). Participants were in the free-living (FL) condition for the first three consecutive days, and then in a euhydrated (EUH) state (USGRESULTS: There were no interactions between FL and EUH with morning and afternoon in USG (Morning-FL, 1.017±0.005; Afternoon-FL, 1.012±0.006; Morning-EUH, 1.011±0.004; Afternoon-EUH, 1.007±0.004; p=0.390). No statistically significant differences were found between morning and afternoon in both FL and EUH controlled (Morning-FL, 72.7±18.3 kg; Afternoon-FL, 72.0±18.1 kg; Morning-EUH, 72.9±18.1 kg; Afternoon-EUH, 73.1±18.1 kg, p=0.661). CONCLUSION: There is no difference between morning and afternoon body mass, regardless of the hydration status. This means that first morning body mass is no more, or less, accurate than afternoon.



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