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Nutrition and bone metabolism

Background of our research 

Astronauts bear a high risk for several bone diseases. The absence of gravitation and the consequential unloading of weight-bearing bones is of course the main reason for increased bone resorption and decreased bone formation during space flight. Additionally several nutrients affect the skeletal system either in a positive (calcium, vitamin D, vitamin K) or in a negative (dietary salt, overdose of vitamin A) way. In order to investigate the physiology of those nutrients we perform so called (metabolic balance studies) in our metabolic ward (AMSAN). These studies are also performed under conditions of prolonged bed rest in a 6° head-down-tilt position (HDTBR: head-down-tilt bed rest). HDTBR simulates weightlessness and induces many of the physiological changes similar to those seen with space flight. The test subject's bed-head is tilt down for 6° which results in a body fluid shift in the upper part of the body. The simultaneous immobilisation results in an unloading of bones.

Recent data from different space missions have shown that astronauts have a high intake of sodium chloride (dietary salt) during space flight. One reason for their preference for salted foods might be a reduced sense of taste during weightlessness. A known consequence of high salt intake is the concomitant retention of water to keep the sodium concentration in the extracellular fluids stable (osmotically active sodium retention). Moreover the sodium secretion via kidney increases.Observations in weightlessness as well as in our metabolic ward provided evidence for another mechanism of sodium regulation in healthy humans: if a certain level of dietary salt intake is exceeded, sodium will be retained in an osmotically inactive form. Sodium is no longer solved in body fluids, but is linked to other chemical compounds of the body. In our Salty Life (6, 7 and 8)   studies we examined the amount and form of sodium being retained by measuring the changes in body weight as well as the absorption and excretion rates of sodium, water and potassium. We will start observations on the international space station (ISS) in July 2008 for further discussion of these findings in space flight (SOLO-Sodium retention in microgravity). Because a high dietary salt intake increases the renal calcium excretion it is known as a risk factor for osteoporosis. In different studies Salty Life 6 and slty Life 7 we were able to show that a high dietary salt intake induces an increased bone resorption which may lead to an increased degradation of bones if bone formation stays constant. Our investigations show that this sodium chloride induced mechanism probably results from changes in the acid base balance. Therefore our research focuses also on the acid base balance and its influence on different body systems under different conditions.

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