Ultra long-term sodium balance studies during the Mars500 campaign

Background: A direct relationship between salt intake and blood pressure is widely accepted. However, the relationship between changing salt intake, salt excretion, and total-body sodium (TBNa) is inferred but not known. Without such basic knowledge, the mechanisms leading to the blood pressure elevation cannot be determined.

Purpose: We tested the hypothesis whether or not long-term dietary salt restriction from 12g/day to 9g/day and 6g/day significantly lowers blood pressure in healthy volunteers during the Mars500 campaign. We sought to perform long-term balance studies to monitor sodium retention, sodium losses, TBNa and their relationships to any changes in blood pressure.

Methods: We investigated daily Na+ balance and blood pressure in crewmembers participating in the Mars500 campaign, a simulated space flight to Mars. The normal young men lived in the Mars500 simulator in a completely self-sustaining, thermoconstant environment for 105 and 520 days, and consumed a diet with a salt content of 12g per day for the first 40 days of the study, which then was gradually decreased to 9g per day for the 2nd third, and for to 6g per day for the 3rd third of the isolation period. The longer duration of the 520-day study allowed re-exposition to the original 12g salt intake level.

Results: Reduced salt intake lowered blood pressure in the subjects; with reexposition, blood pressure increased to initial levels. The blood pressure-lowering effect of reduced salt intake was uncoupled from changes in sodium balance, while increased and decreased potassium balance reflected low- and high-salt intake respectively. Salt balance, body weight, extracellular fluid, and TBNa were independent of salt intake. Ultra long-term constancy in salt intake did not lead to a constant urinary sodium excretion, although salt excretion over each diet period was 90–95% of intake. Infradian rhythms in urinary sodium excretion were found, which were independent of salt intake, but dependent on urinary aldosterone excretion. These rhythms seemed to be synchronized with night-shift work during the Mars105 day study.

Conclusions: We conclude that even moderate salt reduction induces relevant blood pressure decrease in healthy & normotensive subjects. The findings on day-to-day sodium balance must recast thinking about how homeostasis of internal environment composition and blood pressure are achieved. The findings will lead to novel studies of long-term salt regulation and disposition that could have mechanistic value concerning the blood pressure-related effects.