Changes in plasma volume during prolonged restriction of motor activity: effect of chronic hyperhydration

Prolonged reduction of number of steps taken per day by healthy subjects (hypokinesia), is associated with a decrease in plasma volume (PV), which may contribute to the development of several so-called hypokinetic disorders seen immediately during exposure to hypokinesia (HK). The purpose of this study was to determine whether a daily intake of fluid and salt supplementation (FSS), would attenuate this loss of PV. Thirty male long distance runners (20-25 years of age) completed this investigation. Ten volunteers placed under a normal ambulatory life (control subjects), ten volunteers were subjected to continuous restriction of motor activity without using FSS (hypokinetic subjects) and ten volunteers were submitted to continuous restriction of motor activity and took daily FSS (hyperhydrated subjects). For the simulation of the hypokinetic effect the hypokinetic and hyperhydrated volunteers were kept under 2.7 km/day (3,000 steps/day). Plasma volume decreased significantly (p < 0.05) in the hypokinetic volunteers during HK, from 3.010 +/- 81 to 2.693 +/- 52 ml. The volunteers who were submitted to combined HK and FSS, had a significant (p < 0.05) increase in PV during HK, from 2.940 +/- 72 to 3.327 +/- 72 ml. There were demonstrable significant differences in PV between the hyperhydrated and hypokinetic groups of volunteers. Thus, prolonged restriction of motor activity appears to have significant effects on the loss of PV, whereas chronic hyperhydration significantly increases PV.