Altered Fluid Balance During 100 Hours Of Exercise And Sleep Deprivation

Abstract

Plasma volume (PV) is important for endurance performance and minimizing risk of heat illness. PV declines with prolonged exercise and dehydration, but expands ∼7% and ∼23% by the day after a single exercise session and after multiple days of 4–10 hours exercise, respectively. PV volume also expands during sleep deprivation. It is unknown how plasma and other fluid volumes respond to adventure racing, which can nearly resemble a single exercise session sustained across several days. PURPOSE To determine the nature and extent of body fluid volume shifts induced by very prolonged (100± h), competitive exercise with severe sleep deprivation. METHODS Fluid volume measures were obtained from 12 athletes (35 ±11 yrs, 74 ±9 kg, cycling VO2 peak 54 ±6 mL·kg−1·min−1, 6 females) before, during and after the Southern Traverse Adventure race in 2003. Blood and urine were sampled 2 d before the race (fasted), during the first 15–30 h, and at the finish (96–125 h). Pre and end-race samples were obtained from 26 additional athletes. Change in PV was calculated from hemoglobin ([Hb]) and hematocrit, using flow cytometry. Plasma arginine vasopressin ([AVP]) was measured using radioimmunoassay. Urine specific gravity (USG) was measured by portable refractometer. Intracellular (ICFV) and extracellular volumes (ECFV) were estimated under fasted and rested conditions, standing, 2-d before and on the morning following racing, using bioimpedance (4-frequency, 8-polar tactileelectrode). Time effects were analysed using repeated-measures ANOVA and t-tests. RESULTS By 15–30 h of continuous racing PV had increased 10 ±7% (M±SD, P<0.01), despite a 2% decrease (P<0.01) in mean cell volume. By the race finish PV had increased 24 ±10% (P<0.01, n=37) and the reticulocyte count had dropped 25% (P=0.04). The hypervolemic response was unrelated to athletes' initial [Hb] (r=0.11, P=0.52), but was correlated with [AVP] (r=0.70; P<0.01). However, no [AVP] exceeded 3 pmol/L and end-race USG was not high (1.018 ±0.006). ICFV was stable across the race (from 32.1 to 32.0 L, P=0.11), and ECFV increased 7 ±3% (from 14.6 to 15.5 L; P<0.01). CONCLUSION Plasma volume expanded by approximately one quarter during very prolonged exercise and sleep deprivation, presumably due in part to ECF expansion. Hypervolemia was already evident after a day of racing in some athletes.