Because body mass change (Delta Mb) does not represent all water losses and gains, the present field investigation determined if (a) ?Mb equalled the net effective body water change during ultra-endurance exercise and (b) ground speed and exercise duration influenced these variables. Thirty-two male cyclists (age range, 35-52 years) completed a 164-km event in a hot environment, were retrospectively triplet matched and placed into one of three groups based on exercise duration (4.8, 6.3, 9.6 h). Net effective body water loss was computed from measurements (body mass, total fluid intake and urine excreted) and calculations (water evolved and mass loss due to substrate oxidation, solid food mass and sweat loss), including (Delta EBWgly) and excluding (Delta EBW) water bound to glycogen. With all cyclists combined, the mean Delta Mb (i.e. loss) was greater than that of ?EBWgly by 1200 ± 200 g (P = 1.4 × 10-18), was similar to Delta EBW (difference, 0 ± 200 g; P = .21) and was strongly correlated with both (R2 = .98). Analysis of equivalence indicated that Delta Mb was not equivalent to Delta EBWgly, but was equivalent to Delta EBW. Due to measurement complexity, we concluded that (a) athletes will not calculate the effective body water calculations routinely and (b) body mass change remains a useful field-expedient estimate of net effective body water change.
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