Moderate sodium restriction does not alter lower body negative pressure tolerance

Hypothesis: Space travel with exposure to microgravity leads to a significa nt reduction in orthostatic tolerance on return to Earth, for which counter measures are only partially successful. The purpose of this study was to ex amine the effect of moderate dietary sodium restriction on tolerance to LBN P. Methods: Eight healthy men, age 25.1 +/- 1.3 yr, volunteered for the stu dy. Subjects were exposed to presyncopal LBNP after consuming their "typica l" 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 c ollection on the 4(th) and 5(th) days of each diet. Results: All subjects r eached presyncope during the LBNP, regardless of diet. Urinary sodium excre tion 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 in dex 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 we re not different at any stage of the LBNP between the diets, nor were catec holamines. Plasma renin activity, determined by radioimmunoassay, was signi ficantly higher during SR at rest, and during all stages of LBNP in compari son with the control diet. Conclusion: Moderate dietary sodium restriction is not detrimental to orthostatic tolerance.