WATER-BALANCE DURING AND AFTER MARATHON RUNNING

To describe the time course of plasma volume alterations and the chang es in the plasma concentrations of hormones regulating water balance i n relation to a marathon race, six experienced marathon runners (five men, one women) aged 28 (SD 6) years were studied during and for the 3 days following a treadmill marathon run at 68 (SD 5)% of maximal oxyg en consumption. Haematocrit, haemoglobin, plasma protein (Prot) and el ectrolyte (Na+, K+)concentration, osmolality (osm), plasma concentrati ons of renin (Ren), aldosterone (Aid) and atrial natriuretic peptide ( ANP) were determined at rest in a sitting position (T--30), and then a fter 30 min in an upright posture (R(0)), while running a marathon at 10 km (R(10)), 30 km (R(30)) and 42.2 km (R(end)), and after the marat hon at 30 min (T-30), 60 min (T-60), 120 min (T-120) and 24 h (TD+1), 48 h (TD+2) and 72 h (TD+3). The changes in plasma volume (PV), Prot, osm and Na+ observed during the race were nonsignificant. Significant increases in plasma concentration of K+ [4.8 (SD 0.6) vs 5.5 (SD 0.6) mmol . 1(-1) P < 0.01], Ren [38 (SD 57) vs 197 (SD 145) pmol . 1(-1) P < 0.02] and Aid [175 (SD 142) vs 1632 (SD 490) pmol . 1(-1); P < 0.01 ] were observed at R(end). A significant increase of ANP (P < 0.05) wa s only found after R(10). Body mass significantly decreased by 2.0 kg (P < 0.01) during the race in spite of the ingestion of 1.46 (SD 0.34) 1 of a 5% glucose solution. Urinary volume and Na+ excretion dropped significantly after the completion of the marathon in comparison with the day before [2600 vs 1452 ml . day(-1) (P < 0.02) and 161.3 vs 97.1 mmol . l(-1) (P < 0.05)]. At TD+1 and TD+2 a significant increase in PV was noted, compared to T--30. The lack of a decrease in PV during t he marathon may have been due to the production of 402 g of metabolic water and by the release of 1280 g of water stored in glycogen complex es in muscle and liver. Thus, the hormone response during the marathon may have been due to the effects of the exercise itself and not to th e effects of dehydration. The postmarathon PV expansion may be explain ed by a protein shift to the intravascular space and by renal sodium r etention.