Dh. Zappe et al., FLUID RESTRICTION PRIOR TO CYCLE EXERCISE - EFFECTS ON PLASMA-VOLUME AND PLASMA-PROTEINS, Medicine and science in sports and exercise, 25(11), 1993, pp. 1225-1230
The purpose of this study was to test the hypothesis that changes in e
xercise intensity dominate the PV response to cycle exercise in the he
at independent of the initial plasma volume (PV) and total circulating
protein (TCP) content. The two experimental treatments (counterbalanc
ed design) were performed by nine trained male cyclists (age = 23 +/-
1 yr, VO2peak = 63 +/- 4 ml. kg-1.min-1) in both a euhydrated (EU) and
hypohydrated (HP, 24-h fluid restriction) state. Blood volume was mea
sured (carbon monoxide dilution) 30 min prior to each test and subsequ
ent changes in PV were calculated from serial venous blood samples usi
ng hematocrit and hemoglobin concentration. Following 20 min of seated
rest in a warm environment (T(db) = 30-degrees-C, 50-60% RH), each su
bject cycled in a semi-reclining posture for 60 min at three successiv
e intensities (60, 120, and 180 W for 20 min each, representing almost
-equal-to 22, 37, and 53% VO2peak). Fluid restriction reduced (P < 0.0
5) body weight by 1.4 0.3 kg (1.8 +/- 0.4%), PV by 353 +/- 73 ml (8 +/
- 2%) TCP by 20 +/- 7 g (7 +/- 2 %), and elevated serum osmolality by
6 +/- 2 mOsm.kg-1 (2 +/- 1%). After 20 min of passive heat exposure (p
rior to exercise), TCP content remained lower (P < 0.05) in HP (17 +/-
5 g) compared with EU as PV increased (P < 0.05) in EU (222 +/- 27 ml
) but not (P > 0.05) in HP (122 +/- 35 ml). During exercise, TCP decre
ased (P < 0.05) in HP by 14 +/- 3 g, which further increased the diffe
rence (P < 0.05) between EU and HP. However, during exercise the overa
ll loss of PV was similar between EU (472 +/- 54 ml; 10.7 +/- 1%) and
HP (470 +/- 43 ml, 11.2 +/- 1%), respectively. These data support the
hypothesis that in trained subjects, despite differences in TCP conten
t between hydration states, hypohydration induced by 24-h fluid restri
ction does not affect the subsequent rate of PV loss during cycle exer
cise in a warm environment. This implies that, during cycle exercise,
hydrostatic forces (rather than oncotic forces) drive PV exchanges.