Fluid regulation during prolonged physical strain with water and food deprivation in healthy, trained men.

The aim of this study was to investigate fluidregulating mechanisms, with special regard to the role of plasma proteins in the control of plasma volume (PV), and the role of the superficial tissues as a water storage organ of the body during prolonged physical strain. 29 male subjects (mean age 22.2 +/- 2.8 years) were studied during a 5 day period of survival training with multifactorial strain including restricted water intake (11 H2O.day-1) and food intake (628 kJ.day-1) additionally to physical exercise and sleep deprivation (20 h within 5 days). Under field conditions the heart rate was monitored continuously, and body mass, body composition, thickness of the shell tissues, and blood parameters were measured at (T1), after 72 h (T2), after 120 h (T3) and in the recovery period after 48 h (T4) and 72 h (T5). The estimated energy expenditure was approximately 24,000 kJ.day-1. The mean decrease of body mass was 6.77 kg (9.5%) at T3 (p < 0.001), 0.95 kg (1.3%) at T4 (p < 0.05) and 0.68 kg (0.9%) at T5 (n.s.). A reduction of total body water of 3.8 1 was estimated at T3. Serum creatinine ([Cr]) was raised at T3 by 18.5% (p < 0.0001). No relationship was found between [Cr] and other parameters. The PV decreased by 3.7% (p < 0.0001) at T2, increased by 1.6% (p < 0.0001) at T3 and was not different to baseline at T4 (+0.2%; n.s.). Total protein concentration ([TP]) increased at T2 (11.7%; p < 0.0001) and T3 (2.6%; p < 0.01), and decreased (p < 0.0001) at T4 (8.2%) and T5 (5.7%). Plasma proteins shifted into the intravascular space at T2 and T3 and moved out of the intravascular space at T4 and T5. This gives support to the hypothesis that one of the counterregulatory mechanisms maintaining PV during prolonged exercise is provided by protein shifts from the extravascular into the intravascular space. Our data provide evidence that this mechanism assists PV homeostasis efficiently over a period of 120 h with multifactorial strain, even under conditions with a fluid loss of almost 8% of the total body water.