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


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.
Zittermann
A
A
Institut für Ernährungswissenschaft, Universität Bonn, Germany. a.zittermann@uni-bonn.de
Heer
M
M
Caillot-Augusso
A
A
Rettberg
P
P
Scheld
K
K
Drummer
C
C
Alexandre
C
C
Horneck
G
G
Vorobiev
D
D
Stehle
P
P
D000328:Adult; D018480:Astronauts; D002117:Calcitriol / Q000097:blood; D002118:Calcium / Q000378:metabolism*; D002128:Calcium Metabolism Disorders / Q000175:diagnosis / Q000209:etiology* / Q000378:metabolism; D003094:Collagen / Q000097:blood; D024042:Collagen Type I; D003404:Creatinine / Q000097:blood; D006801:Humans; D006887:Hydroxycholecalciferols / Q000097:blood; D007408:Intestinal Absorption; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood; D010455:Peptides / Q000097:blood; D011834:Radiation Monitoring; D013026:Space Flight; D013324:Strontium; D014466:Ultraviolet Rays; D014893:Weightlessness / Q000009:adverse effects*
D024042:Collagen Type I; D006887:Hydroxycholecalciferols; D010281:Parathyroid Hormone; D010455:Peptides; C052929:collagen type I trimeric cross-linked peptide; D003094:Collagen; D003404:Creatinine; D002117:Calcitriol; D002118:Calcium; D013324:Strontium