Effect of dietary sodium on fluid/electrolyte regulation during bed rest.
BACKGROUND
A negative fluid balance during bed rest (BR) is accompanied by decreased plasma volume (PV) which contributes to cardiovascular deconditioning.


OBJECTIVE
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 x kg(-1) x d(-1)) in 17 men who consumed either a 4.0 +/- 0.06 g x d(-1) (174 mmol x d(-1)) (CONT; n = 6), 1.0 +/- 0.02 g x d(-1) (43 mmol x d(-1)) (LS; n = 6), or 10.0 +/- 0.04 g x d(-1) (430 mmol x d(-1)) (HS; n = 5) sodium diet before, during, and after 21 d of 6 degrees 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 x 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.
Williams
W Jon
WJ
Wyle Laboratories Life Sciences, Systems and Services, Houston, TX, USA. wjwilliams52@yahoo.com
Schneider
Suzanne M
SM
Gretebeck
Randall J
RJ
Lane
Helen W
HW
Stuart
Charles A
CA
Whitson
Peggy A
PA
D000328:Adult; D000704:Analysis of Variance; D001510:Bed Rest; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D001834:Body Water / Q000502:physiology; D003627:Data Interpretation, Statistical; D006728:Hormones / Q000097:blood / Q000652:urine; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D010953:Plasma Volume / Q000187:drug effects / Q000502:physiology; D012964:Sodium / Q000652:urine; D012982:Sodium, Dietary / Q000008:administration & dosage*; D014882:Water-Electrolyte Balance / Q000187:drug effects* / Q000502:physiology*
D006728:Hormones; D012982:Sodium, Dietary; D012964:Sodium