ABSTRACT
BACKGROUND: Little is known about the onset and degree of biochemical and functional alterations in calcium metabolism during microgravity. OBJECTIVE: To evaluate the effect of microgravity on intestinal calcium absorption and calcium-regulating hormones under metabolic ward conditions. MATERIALS AND METHODS: Fractional calcium absorption (Fc240 in percentage of dose administered) was determined pre-flight, in-flight and post-flight, by use of a stable strontium test in one cosmonaut who spent 20 days in space. Moreover, a sequence of blood samples was collected for the determination of serum parathyroid hormone (PTH), 25-hydroxyvitamin D, calcitriol and serum C-telopeptide (CTx, biomarker of bone resorption) levels. During all periods of data collection, calcium intake was held constant at a minimum level of 1.000 mg day(-1) and a daily supplement of 16.6 microg vitamin D2 was given. Personal ultraviolet (UV) light exposure was measured during the whole mission using a biologically weighting UV dosimeter. RESULTS: Fc240 was markedly reduced on flight day 19 (4.4%) as compared to pre-flight and post-flight data (13.4% and 17.2%, respectively). Serum calcitriol levels fell from 40.6 pg mL(-1) (mean pre-flight level) to 1.3 pg mL(-1) on flight day 18 and returned into the normal range after recovery. Serum CTx increased during the flight, while serum PTH and 25-hydroxyvitamin D levels did not change significantly. CONCLUSIONS: Intestinal calcium absorption can be diminished after only three weeks of microgravity. Changes are associated with a severe suppression of circulating calcitriol levels, but are independent of exogenous vitamin D supply and serum PTH levels.
