High salt intake as an additional risk factor for protein losses in head down tilt bed rest

Abstract

Human spaceflight is associated with a loss of body protein. Additionally astronauts tend towards a high intake of sodium chloride (NaCl), which has recently been shown to induce low grade metabolic acidosis. In several patterns, e.g. chronical renal failure, metabolic acidosis is associated with protein catabolism. We therefore hypothesized that high dietary intake of NaCl enforces protein losses in head down tilt bed rest (HDBR). Eight healthy male subjects participated in a 14-day bed rest study in the metabolic ward of the DLR – Institute of Aerospace Medicine, Cologne, Germany. The study was carried out in a cross over design, consisting of two phases. The diet was identical in both study phases with the exception of NaCl-intake: Every subject received a low NaCl diet in one phase and a high in another one. Plasma pH did not change significantly (p=0.061), but plasma bicarbonate and base excess decreased (both p<0.001) with high NaCl intake in HDBR. Nitrogen balance in HDBR was negative, as expected in immobilization with low NaCl diet ( 0.47 ± 2.3 g/d). However, high NaCl intake in HDBR exacerbated the negative nitrogen balance to 1.26 ± 2.3 g/d (p<0.001). We conclude that high NaCl intake induces low grade metabolic acidosis during HDBR. Low grade metabolic acidosis may be a reason for an increased protein turnover reflected by an exaggerated negative nitrogen balance in HDBR. Accordingly, a high NaCl intake may exacerbate loss of body protein in µG.