CALCIUM AND VITAMIN D: IS SUPPLEMENTATION AN EFFICIENT COUNTERMEASURE TO BONE LOSS IN IMMOBILIZATION?

摘要

Reduction of mechanical stress on bone inhibits osteoblast-mediated bone formation and increases osteoclast-mediated bone resorption, and leads to what has been called disuse osteoporosis. Prolonged therapeutic bed rest, immobilization and spaceflight are common causes of disuse osteoporosis. During spaceflight dietary calcium intake of astronauts is very moderate when they eat at libitum. In contrast to postmenopausal osteoporosis - where bone loss is mainly due to estrogen deficiency - immobilization in spaceflight or during bed rest causes bone density reduction by decoupling of bone formation and resorption processes. A huge amount of data exists supporting the use of calcium in combination with vitamin D in the prevention and treatment of osteoporosis in postmenopausal women and during growth. Although sufficient calcium and vitamin D intake is mandatory to prevent bone loss because of respective malnutrition it is questionable if increasing calcium intake above the recommended intake levels is a solution for immobilized people. Results from bed rest experiments and data from space missions, where astronauts' calcium intake was at a high level, indicate that even high calcium intake does not counteract disuse induced bone loss. In contrast to the apparently positive effect of high calcium intake and vitamin D supplementation in postmenopausal women and in adolescence, high calcium intake alone may neither be applicable to humans in microgravity nor to immobilised people as it does not effectively counteract disuse-induced bone loss.