Why was this study done?

Astronauts and athletes can experience a drop immune system function due to various stressors experienced during long flights. Dehydration is a common stressor to both athletes and astronauts. This study was designed to understand the role of proper hydration and its relationship to the immune system.

What did the study find?

Plasma osmolality (concentration of electrolytes), and not plasma volume, appears to be a driving force for a component of the immune system (CD4/CD8 T-lymphocyte ratio), so maintaining proper osmolality is important for the health of both athletes and astronauts.

CD4+/CD8+ T-lymphocyte ratio: effects of rehydration before exercise in dehydrated men
Authors: Greenleaf JE, Jackson CG, Lawless D. Med Sci Sports Exerc. 1995 Feb;27(2):194-9.
PubMed Citation PMID: 7723642.
PDF Manuscript: Med Sci Sport Exerc, 1995.