Low energy intake, low calcium intake, low plasma 25-hydroxy-vitamin D or low calcitriol levels, and high salt intake might support the development of space osteoporosis. Therefore, my colleagues and I monitored the daily energy and calcium intakes in eight astronauts during their respective space missions (Spacelab D2, Euromir 94, Euromir 95). In most of these astronauts, energy intake was reduced by more than 20% compared with their calculated energy expenditure. In all three missions, the average daily calcium intake of the eight astronauts was 25% lower than the German recommended daily allowances of 900 mg/d for healthy people without osteoporosis risk. In some astronauts, the calcium intake was extremely low at 53 and 74 mg/d. Sodium intake in these astronauts varied from 39 mEq/d to a very high intake of 462 mEq/d. As a consequence of these results, we examined in the 21-d Mir 97 mission a preventative dietary approach of high calcium intake of at least 1000 mg/d with vitamin D supplementation (650 IU/d of Ergocalciferol) and constant sodium intake (180 mEq/d). Total serum calcium concentration and urinary calcium excretion significantly increased during this mission. Synthesis of 25-OH-cholecalciferol synthesis was markedly reduced because of inadequate ultraviolet light, whereas total 25-OH-Vitamin D levels were unchanged. However, parathyroid hormone and calcitriol levels decreased significantly. Sodium excretion decreased significantly, resulting in positive sodium balances. Based on these results, dietary calcium and vitamin D do not stabilize bone turnover because markers of bone formation were reduced and markers of bone resorption were increased. We concluded that, in contrast to terrestrial conditions, adequate or even high calcium and vitamin D intakes during microgravity do not efficiently counteract the development of space osteoporosis. Conversely, vitamin K (Konakion) seemed to counteract microgravity-induced reduction of bone formation markers. In the 179-d Euromir 95 mission, investigators administered 10 mg of vitamin K from inflight day 86 to day 136 in one astronaut. During and after supplementation, bone formation markers increased significantly during this part of the mission. Therefore, vitamin K seems to play a significant role in bone turnover during space flight.