Vitamin K status in spaceflight and ground‐based models of spaceflight

Bone loss is a well‐documented change during and after long‐duration spaceflight. Many types of countermeasures to bone loss have been proposed, including vitamin K supplementation. The objective of this series of studies was to measure change in vitamin K status in response to microgravity under a variety of spaceflight and spaceflight analog (model) conditions, including long‐duration spaceflight studies ( n  = 15), three bed rest studies ( n  = 15, 49, and 24), and a 14‐day saturation dive ( n  = 6). In crew members who flew 2–6 months on the International Space Station, in‐flight and postflight plasma phylloquinone concentrations were unchanged from the preflight mean. Consistent with this finding, urinary γ‐carboxyglutamic acid (GLA), a measure of vitamin K‐dependent protein turnover, did not change in response to flight. Serum undercarboxylated osteocalcin (%ucOC), a measure of vitamin K function, was generally unchanged in response to flight. Spaceflight findings were corroborated by findings of no changes in phylloquinone, urinary GLA, or %ucOC during or after bed rest in three separate bed rest studies (21–90 days in duration) or after a 14‐day saturation dive. The data presented here do not support either a need for vitamin K supplementation during spaceflight or the suggestion of using vitamin K as a bone loss countermeasure in spaceflight. © 2011 American Society for Bone and Mineral Research.