Calcium Metabolism and a Mars Mission: New Problems

Results of ground-based and spaceflight experiments are analyzed using a mathematical model to predict long-term effects of the adaptation of human mineral metabolism that takes place during spaceflights of extended duration. The changes in calcium metabolism induced by spaceflight, both real and simulated, are summarized. These changes include a decline in the amount of calcium in the bone pool inexchangeable with free calcium and a decrease in the capacity of tissue and blood buffer systems to retain calcium. In the mathematical model of calcium metabolism, the rates of the main calcium fluxes were estimated for normal conditions and for stress conditions mimicking the 110th day of spaceflight according to the rate of calcium efflux from bone. This analysis showed that, under ordinary living conditions, a 1-h calcium load of 8 mmol/h would give rise to only a 20% transient increase in PCa++, causing a 17-fold increase in the calcium binding by tissue and blood buffer systems and an 11-fold increase in binding of the ion by blood buffers. The calcium content in woven bone was predicted to increase by 10–11% in response to this calcium load. The activity of the regulatory mechanisms in the model was varied. When the blood level of the active metabolite of vitamin D3 was set to one-tenth of its norm, the rate of compact bone resorption changed only slightly. As calculated in the model, the calcium input from woven bone can be normalized by raising the calcitonin level 20-fold or by completely blocking the secretion of parathyroid hormone, neither of which is realistically achievable, whether on land in experiments simulating weightlessness or during spaceflight. The results of experiments and the data of simulation using the model show that none of the active calcium regulators tested can offset a microgravity-induced decline in the ability of calcium stores to retain calcium.