Dietary salt and urinary calcium excretion in a human bed rest spaceflight model.
BACKGROUND
Dietary salt is known to increase the excretion of urinary calcium (Ca). To determine the potential role of dietary sodium (Na) on the calciuria associated with a spaceflight simulation model, we evaluated urinary Ca in two groups of bed rest subjects fed either high or low normal amounts of salt.


METHODS
We analyzed urinary Ca excretion expressed in terms of creatinine (UCa/Cr), fractional Ca excretion (FECa), and urinary cAMP (UCAMP) as an index of parathyroid function, in the urine of 30-50-yr-old male volunteers for 6 degrees head down tilt bed rest studies. Dietary Na was in the high normal range (190 mmol x d(-1)) in 8 men for 7 d (HiNa), and in the low normal range (114 mmol x d(-1)) in 11 men for 30 d (LoNa) bed rest. Dietary Ca averaged 20 mmol x d(-1) in both studies.


RESULTS
Within the first 3 bed rest days, subjects in the HiNa study showed increases in UCa/Cr (0.1130 +/- 0.05 to 0.161 +/- 0.05, p < 0.002) and in FECa (1.95 +/- 0.70 to 3.19 +/- 0.93, p < 0.001); those in LoNa showed no change in UCa/Cr (0.125 +/- 0.06 to 0.121 +/- 0.07, NS) or FECa (1.93 +/- 0.75 to 2.22 +/- 0.63). After the 5th bed rest day UCa/Cr stabilized at similar levels in both dietary groups. UCAMP decreased 20% during the first week of bed rest with HiNa, but not until the third week with LoNa diets (p < 0.05).


CONCLUSIONS
These findings implicate high salt diets in Ca excretion in a spaceflight model and suggest that low normal salt diets may reduce early calciuria associated with spaceflight.
Arnaud
S B
SB
Life Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA. sarnaud@mail.arc.nasa.gov
Wolinsky
I
I
Fung
P
P
Vernikos
J
J
D000328:Adult; D001510:Bed Rest / Q000009:adverse effects*; D002118:Calcium / Q000097:blood / Q000652:urine*; D002136:Calcium, Dietary / Q000008:administration & dosage; D003404:Creatinine / Q000652:urine; D000242:Cyclic AMP / Q000652:urine; D004039:Diet, Sodium-Restricted / Q000009:adverse effects / Q000379:methods; D004435:Eating; D018475:Head-Down Tilt / Q000009:adverse effects*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood; D017673:Sodium Chloride, Dietary / Q000009:adverse effects*; D013026:Space Flight; D018561:Space Simulation / Q000009:adverse effects*; D013997:Time Factors
D002136:Calcium, Dietary; D010281:Parathyroid Hormone; D017673:Sodium Chloride, Dietary; D003404:Creatinine; D000242:Cyclic AMP; D002118:Calcium