"Calcium is lost from bones during space flight due to skeletal unloading and insufficient levels of vitamin D. Vitamin D plays a critical role in calcium absorption and metabolism. Sunlight, specifically ultraviolet radiation, is essential for synthesis of vitamin D. Spacecraft are heavily shielded to reduce crewmembers' exposure to the harmful types of radiation. Thus, the lack of ultraviolet light during space flight may decrease vitamin D pools in the body, which poses a concern over lengthy missions."

After commenting on your postVitamin D bonked the Dinosaurs

I felt like an idiot for suggesting that lack of sunlight (due to impactor) may have caused a Vit D deficiency in Dinosaurs leading to their extinction.

Rob