Dietary salt and urinary calcium excretion in a human bed rest spaceflightmodel

Background: Dietary salt is known to increase the excretion of urinary calc ium (Ca). To determine the potential role of dietary sodium (Na) on the cal ciuria associated with a spaceflight simulation model, we evaluated urinary Ca in two groups of bed rest subjects fed either high or low normal amount s of salt. Methods: We analyzed urinary Ca excretion expressed in terms of creatinine (UCa/Cr), fractional Ca excretion (FECa), and urinary cAMP (UCAM P) as an index of parathyroid function, in the urine of 30-50-yr-old male v olunteers for 6 degrees head down tilt bed rest studies. Dietary Na was in the high normal range (190 mmol.d(-1)) in 8 men for 7 d (HiNa), and in the low normal range (114 mmol.d(-1)) in 11 men for 30 d (LoNa) bed rest. Dieta ry Ca averaged 20 mmol.d(-1) in both studies. Results: Within the first 3 b ed 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 <plus/minus> 0.70 to 3.19 +/-. 0.93, p < 0.001); those in LoNa showed no change in UCa/Cr (0 .125 <plus/minus> 0.06 to 0.121 +/- 0.07, NS) or FECa (1.93 +/- 0.75 to 2.2 2 +/- 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 r est with HiNa, but not until the third week with LoNa diets (p < 0.05). Con clusion: These findings implicate high salt diets in Ca excretion in a spac eflight model and suggest that low normal salt diets may reduce early calci uria associated With spaceflight.