Between 2009 and 2011, Titze and his colleagues conducted long-term sodium balance studies in Russian cosmonauts who were participating in a human space flight simulation program at a research facility in Moscow in preparation for a potential manned spaceflight to Moscow.
Unexpectedly, when dietary salt was increased from six to 12 grams a day, the men drank less water, not more. That suggested they conserved or produced more water.
In a subsequent study in mice, the researchers showed that high salt induces a catabolic state driven by glucocorticoids that break down muscle protein, which is converted into urea by the liver. Urea enables the kidneys to reabsorb water and prevent body water loss while the salt is excreted.
Muscle wasting is a high price to pay for avoiding dehydration. The alternative is bringing in more fuel – eating more. That may be why the men in the study complained they were hungry.
Water conservation in response to a high-salt diet may have pathological consequences. Increased levels of glucocorticoids are an independent risk factor for diabetes, obesity, osteoporosis, and cardiovascular disease.
“We have always focused on the role of salt in arterial hypertension. Our findings suggest that there is much more to know – a high salt intake may predispose to metabolic syndrome,” Titze said.
Support for the studies came largely from the German Federal Ministry for Economics and Technology, the Interdisciplinary Center for Clinical Research Junior Research Group in Erlangen, Germany, and from the American Heart Association and National Institutes of Health.
