• Bone resorption;
  • calcitriol;
  • calcium absorption;
  • microgravity;
  • strontium test;
  • vitamin D

Little is known about the onset and degree of biochemical and functional alterations in calcium metabolism during microgravity.

To evaluate the effect of microgravity on intestinal calcium absorption and calcium-regulating hormones under metabolic ward conditions.

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 µg vitamin D2 was given. Personal ultraviolet (UV) light exposure was measured during the whole mission using a biologically weighting UV dosimeter.

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.

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.