Moderate sodium restriction does not alter lower body negative pressure tolerance

Space travel with exposure to microgravity leads to a significant reduction in orthostatic tolerance on return to Earth, for which countermeasures are only partially successful. The purpose of this study was to examine the effect of moderate dietary sodium restriction on tolerance to LBNP.

Eight healthy men, age 25.1+/-1.3 yr, volunteered for the study. Subjects were exposed to presyncopal LBNP after consuming their "typical" diet (C) for 5 d and after consuming a sodium restricted (SR) diet for 5 d. Diet sequence was randomized and adherence was verified by 24-h urine collection on the 4th and 5th days of each diet.

All subjects reached presyncope during the LBNP, regardless of diet. Urinary sodium excretion was 3390+/-950 mg on the C diet and 1174+/-560 mg on the SR diet. Urinary potassium was not different between the diets. Cumulative stress index scores were 655+/-460 (mm Hg x min) on the C diet and 639+/-388 (mm Hg x min) during SR. Cardiac volumes, BP and total peripheral resistance were not different at any stage of the LBNP between the diets, nor were catecholamines. Plasma renin activity, determined by radioimmunoassay, was significantly higher during SR at rest, and during all stages of LBNP in comparison with the control diet.

Moderate dietary sodium restriction is not detrimental to orthostatic tolerance.