NUTRIENT CONSUMPTION/METABOLISM Nutritional Interventions Related to Bone Turnover in European Space Missions and Simulation Models Martina Heer, PhD From the Institute of Aerospace Medicine, German Aerospace Center, DLR, Cologne, Germany 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 supplemen- tation (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 micro- gravity 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. Nutrition 2002;18:853– 856. ©Elsevier Science Inc. 2002 KEY WORDS: microgravity, constant nutrient intake, metabolic balances, energy, calcium, vitamin D, salt, vitamin K, bone metabolism INTRODUCTION In a microgravity environment or during short- or long-term bed- rest, the lack of mechanical forces on weight-bearing bones has a significant impact on calcium and bone metabolism. Because hu- mans during space flight wish to stay in a microgravity environ- ment for more than a few days, e.g., interplanetary missions such as a flight to Mars, nutrient supply becomes more important. Malnutrition often observed in missions 1–4 may become a physi- ologic problem that exacerbates existing muscle degradation and bone resorption, and it may affect cardiovascular function (just to mention the most important factors). Insufficient food supply, because of decreased hunger and thirst or as a result of an imbal- ance between intake and needs, leads to undernutrition, with known functional impairments. 5 In this article, preliminary results from nutritional interventions in immobilization during space flight or bedrest, although in a limited sample size, are summarized. ENERGY INTAKE Evidence indicates that decreased rates of whole-body and muscle protein synthesis and of bone formation markers can be observed within a few hours of fasting. 6,7 Energy intake and energy expen- diture studies during recent short-term and long-term missions have shown that most astronauts are likely to be in negative energy balance during flight. 1,3,8,9 Negative energy balances lead to body mass loss and concomitantly to a reduced body mass index. It is widely known that bone mineral density and body mass index have a very good correlation. 10,11 For example, the body mass index of patients with anorexia nervosa (AN) accurately predicts their bone mineral density. 12,13 In postmenopausal women who follow a diet to reduce body weight, bone mobilization, lowering of bone min- eral density, and osteoporotic fractures are observed. Changes in bone turnover also were seen in athletes with inadequate energy intake. 7 In male rowers, for instance, a decrease in bone formation markers was observed after 24 h of starvation. 6 In an extreme way, insufficient food intake is demonstrated in patients with AN. In This study received financial support from the German Aerospace Center (DLR), Directorate Raumfahrt. Correspondence to: Martina Heer, PhD, Institute of Aerospace Medicine, German Aerospace Center, DLR, 51147 Cologne, Germany. E-mail: martina.heer@dlr.de Nutrition 18:853– 856, 2002 0899-9007/02/$22.00 ©Elsevier Science Inc., 2002. 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