Salt presents a mystery to experts
Remarkable results of Mars500 metabolism study
The human body is much more flexible than previously thought when it comes to balancing its sodium metabolism. This conclusion from the results of a long-term study came as a surprise even to Jens Titze and his group. The team led by the Professor of Electrolyte and and Circulatory Research at the Department of Medicine 4 and leader of the junior research group 2 of the Interdisciplinary Centre for Clinical Research at the Erlangen University Hospital have examined not only the standard 24-hour rhythm but also a period of six to nine days for the balance of salt supply and excretion. ‘What surprised us even more was that other tissue apart from the kidneys is also responsible for the correct salt content,’ Titze says. ‘Cell Metabolism’ published the results on Tuesday, 8 January 2013, in the print and online versions of the journal. Jens Titzes study is one of eleven projects in Germany funded by the Federal Ministry of Economics and Technology via the German Aerospace Center.
The Russian-German-European Mars500 mission, a simulated journey to our neighbour planet that took place from 2009 to 2010, presented the unique opportunity to collect metabolic data from the participants first on 105 and then on 205 consecutive days. Food intake and excretion in the closed-off ‘space capsule’ could be strictly controlled and monitored. It was possible to make daily comparisons of the prescribed salt intake and the concentration of sodium in the participants’ urine. According to the commonly accepted theory, the sodium contained in salt and taken in via food leaves the body within 24 hours – but the two measured values were in no way identical. There were even large fluctuations in the daily excretions even though the intake remained unchanged.
This means that urine collected over the course of 24 hours does not suffice to assess how much table salt a person has consumed in the same period. This shakes the foundations of a common diagnostic procedure. ‘As doctors who collect 24-hour samples, we found this hard to believe at first. I used this procedure routinely myself. I accused patients of consuming way too much salt when we found 16 grams of salt yet again, and with 6 grams or less I doubted that the collection was carried out properly,’ Jens Titze admits. No assurances to the contrary by the patients could convince him. ‘I was unable to imagine it, but since Mars500, I can.’ A change between three daily meals with different salt content did nothing to change the result.
‘Where we assumed constancy, the body produces rhythmic variabilities, and their extent is very, very surprising,’ the project leader comments. By comparison the interaction of table salt and certain hormones in the urine samples seems simple at first glance. Cortisol, seen as having a supporting role so far, directly mirrors the sodium content; the known regulator aldosterone, which retains sodium salts in the body and excretes potassium salts, is inversely related to this. But this too was ‘very astounding’ according to the research team. Cortisol has so far only been seen as a supporter of aldosterone. The new findings, however, clearly show the two hormones as adversaries. Titze speaks of a ‘cortisol mystery’ that needs solving urgently.
The fact that the ‘super-regulators’ of the sodium metabolism, the kidneys, could lose this unchallenged position has the biggest implications. Blood passes their purifying filter and the remaining concentrations of substances are passed on to all parts of the body supplied with blood. But apparently the organism does not simply rely on this process alone. In addition, some kinds of tissue have developed own methods of actively securing the preferred amount of salt. This process is not measured by days or weeks. Skin and muscles apparently store sodium over months, completely independently from salt intake and without any change in weight or volume. But how can a substance be such a phantom in the human body? Until the experts detect what exactly is going on, we will have to use the expression of Titze’s team member Friedrich Luft, who called the sodium metabolism ‘ghostly’.
‘What are our examinations to determine salt intake – in everyday clinical practice and in field studies – even worth in the face of this new information?’ Jens Titze asks himself and all nephrologists. At least the group is not alone when it comes to presenting proof for the need to change views on this topic. Skin storing sodium has been discussed since the 1990s. Weekly rhythms similar to those of the Mars travellers have been observed in blue-green algae and single cells. And after the initial shock has worn off, medical research using human test subjects could be revived: mind-boggling results are possible even without gene knockout models and cell rows.
Prof. Dr. Jens Titze