The fluid and electrolyte regulation experiment with seven subjects was designed to describe body fluid, renal, and fluid regulatory hormone responses during the Spacelab Life Sciences-1 (9 days) and -2 (14 days) missions. Total body water did not change significantly. Plasma volume (PV; P < 0.05) and extracellular fluid volume (ECFV; P < 0.10) decreased 21 h after launch, remaining below preflight levels until after landing. Fluid intake decreased during weightlessness, and glomerular filtration rate (GFR) increased in the first 2 days and on day 8 (P < 0.05). Urinary antidiuretic hormone (ADH) excretion increased (P < 0.05) and fluid excretion decreased early in flight (P < 0.10). Plasma renin activity (PRA; P < 0.10) and aldosterone (P < 0.05) decreased in the first few hours after launch; PRA increased 1 wk later (P < 0.05). During flight, plasma atrial natriuretic peptide concentrations were consistently lower than preflight means, and urinary cortisol excretion was usually greater than preflight levels. Acceleration at launch and landing probably caused increases in ADH and cortisol excretion, and a shift of fluid from the extracellular to the intracellular compartment would account for reductions in ECFV. Increased permeability of capillary membranes may be the most important mechanism causing spaceflight-induced PV reduction, which is probably maintained by increased GFR and other mechanisms. If the Gauer-Henry reflex operates during spaceflight, it must be completed within the first 21 h of flight and be succeeded by establishment of a reduced PV set point.

Comparison was made between three methods for calculating the protein content of 68 foods. The 3 methods of calculation were: (1) multiplication of Kjeldahl nitrogen by 6.25; (2) multiplication of Kjeldahl nitrogen by factors varying from 5.30 to 6.38 depending on food type; and (3) summation of amino acid content as determined by chemical analyses. New conversion factors were calculated based on amino acid analyses. Substantial differences (20 to 40%) were found in protein content for many foods depending on the calculation method. The protein content of six typical menus for mature American males was calculated using the three methods. The resulting protein content of menus did not vary substantially (less than 3%) as a function of calculation method since relatively large differences in various foods tended to cancel out. Conversion factors based on amino acid analyses are recommended whenever more accurate approximation of protein content of individual food is required.

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