Changes in total body water content during running races of 21.1 km and 56 km in athletes drinking ad libitum.

Results: Significant BM loss occurred in both races (21.1 km; 21.4 6 0.6 kg; P , 0.000 and 56 km; 22.5 6 1.1 kg; P , 0.000). Total body water was reduced in the 56-km race (21.4 6 1.1 kg; P , 0.001). A negative linear relationship was found between percentage change (%D) in TBW and %D in BM in the 56-km runners (r = 0.6; P , 0.01). Plasma osmolality and [TP] increased significantly in the 56-km runners (6.8 6 8.2 mOsm/kg H2O; P , 0.05 and 5.4 6 4.4 g/L; P , 0.01, respectively), but all other biochemical measures were within the normal range.