Blood volume responses in partially dehydrated subjects working in the cold

The effects of high-salt diets, cold, and heavy exercise have been examined in 33 Marine Corps volunteers living and working in a cold chamber. Temperatures varied from -29 degrees C during working hours to -4 degrees C, simulating partly warmed shelters, at night. Dehydrated operational rations were eaten and fluid intakes were controlled. When quantities of water were consumed at or above the amounts determined in previous studies in this series to prevent symptoms of cold-induced dehydration with these diets, ie., 2.5-3.0 L/d with diets containing 24 g of NaCl/d or about 2.0 L/d with 8 g NaCl/d, blood volumes increased 5-10%, primarily as a result of plasma volume expansion. This finding apparently confirms the results of earlier workers who have noted plasma volume increases in subjects undergoing vigorous exercise training. In subjects receiving 0.5 L/d or more below recommended quantities of water, intravascular volume increases developed slowly and quite erratically during 5 d of exercise. Intravascular volume increases responded within 1-2 d when fluid levels became appropriate for either high- or low-salt intakes. Besides confirming recommendations concerning water needed with dehydrated diets under conditions of the experiment, the observation of intravascular fluid volume increases during exercise training--despite limiting fluid intakes--raises the question of whether there are any physiological advantages from this adaptive mechanism.