Changes in the body composition of monkeys during long-term exposure to high acceleration fields

Adult, 10-14 kg, male pig-tailed monkeys (Macaca nemestrina) were subjected to continuous centrifugation for 7 months in acceleration fields up to 2.5 g. Several major parameters of body composition, body-fluid distribution, and hematology were evaluated at various levels of high g by in vivo analytical techniques. Statistically significant decreases in total body mass, lean body mass, total body water, extracellular water content and interstitial water content proportional to the level of high g were also demonstrated. Body cell mass, total body fat, dry body mass, lean body solids, and red cell volume did not change significantly. Significant decreases in red cell count, blood hemoglobin level and venous hematocrit value accorded with the plasma volume and red cell volume findings. Total plasma protein concentration did not change significantly. The results indicate a substantial loss of total body mass in the form of body water. The loss apparently occurred from the interstitial spaces of the body in the face of a sizeable increase in plasma volume. Physiological significance of the body composition and fluid distribution changes at high g is discussed and compared to the changes described for astronauts at low g. Current theory of fluid-balance physiology favors the concept that interstitial volume and blood volume are tightly coupled and change in the same direction. However, it appears that the changes which occur where the ambient acceleration field is varied may represent an exception to the general hypothesis for the regulation of the extracellular fluid volume.