Microgravity inhibits intestinal calcium absorption as shown by a stable strontium test

Background Little is known about the onset and degree of biochemical and fu nctional alterations in calcium metabolism during microgravity. Objective To evaluate the effect of microgravity on intestinal calcium abso rption and calcium-regulating hormones under metabolic ward conditions. Materials and methods Fractional calcium absorption (Fc(240) 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 sp ace. Moreover, a sequence of blood samples was collected for the determinat ion 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 mug vitamin D-2 wa s given. Personal ultraviolet (UV) light exposure was measured during the w hole mission using a biologically weighting UV dosimeter. Results Fc(240) 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 cal citriol levels fell from 40.6 pg mL(-1) (mean pre-flight level) to 1.3 pg m L(-1) on flight day 18 and returned into the normal range after recovery. S erum 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 thre e weeks of microgravity. Changes are associated with a severe suppression o f circulating calcitriol levels, but are independent of exogenous vitamin D supply and serum PTH levels.