Authors: Williams, W. Jon; Schneider, Suzanne M.; Gretebeck, Randall J.; Lane, Helen W.; Stuart, Charles A.; Whitson, Peggy A.

Source: Aviation, Space, and Environmental Medicine, Volume 74, Number 1, January 2003 , pp. 37-46(10)

Publisher: Aerospace Medical Association

Abstract:

Williams WJ, Schneider SM, Gretebeck RJ, Lane HW, Stuart CA, Whitson PA. Effect of dietary sodium on fluid/electrolyte regulation during bed rest. Avait Space Environ Med 2003; 74:37-46.

Background: A negative fluid balance during bed rest (BR) is accompanied by decreased plasma volume (PV) which contributes to cardiovascular deconditioning. Hypothesis: We hypothesized that increasing dietary sodium while controlling fluid intake would increase plasma osmolality (POSM), stimulate fluid conserving hormones, and reduce fluid/electrolyte (F/E) losses during BR; conversely, decreasing dietary sodium would decrease POSM, suppress fluid conserving hormones, and increase F/E losses. Methods: We controlled fluid intake (30 ml · kg⁻¹ · d⁻¹) in 17 men who consumed either a 4.0 ± 0.06 g · d⁻¹ (174 mmol · d⁻¹) (CONT; n = 6), 1.0 ± 0.02 g · d⁻¹ (43 mmol · d⁻¹) (LS; n = 6), or 10.0 ± 0.04 g · d⁻¹ (430 mmol · d⁻¹) (HS; n = 5) sodium diet before, during, and after 21 d of 6° head-down BR. PV, total body water, urine volume and osmolality, POSM, and F/E controlling hormone concentrations were measured. Results: In HS subjects, plasma renin activity (−92%), plasma/urinary aldosterone (−59%; -64%), and PV (−15.0%; 6.0 ml · kg⁻¹; p < 0.05) decreased while plasma atrial natriuretic peptide (+34%) and urine antidiuretic hormone (+24%) increased during BR (p < 0.05) compared with CONT. In LS, plasma renin activity (+166%), plasma aldosterone (+167%), plasma antidiuretic hormone (+19%), and urinary aldosterone (+335%) increased with no change in PV compared with CONT (p < 0.05). Total body water did not change in any of the subjects. Conclusions: Contrary to our hypothesis, increasing dietary sodium while controlling fluid intake during BR resulted in a greater loss of PV compared with the CONT subjects. Reducing dietary sodium while controlling fluid intake did not alter the PV response during BR compared with CONT subjects.

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Keywords: fluid and electrolyte hormones; plasma volume; total body water; osmolality

Document Type: Research article

Publication date: 2003-01-01

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