**Abstract**

Mankind has explored space since many years ago for distinct purposes. The space environment has its special features including microgravity (weightlessness), radiation, vacuum, and extreme temperature. It is fascinating to hear space traveling or even living on another planet, but it may cause dramatic changes in the human body. The skeleton has the primary role for the human body on the Earth and also in space. Osteoporosis is the principal feature in spaceflights occurring sooner or later. It will pose health risks. The aging of the population is the cause for osteoporosis to be more prevalent in the future. It seems that aging will happen sooner in space for human beings that are accustomed to living on the Earth. A complex of all reported changes that would occur in space, the picture of a closely resembling aged man emerges. Vitamin D can be synthesized and acts as a hormone. Its receptors are evident in more than 30 human tissues. Vitamin D has positive effects on the skeleton and other systems of the body in many regards. Multiple actions have been claimed for vitamin D (real or false), many aspects of which are not fully understood. The issue applies more about the vitamin and space, which has been tried to describe in this chapter.

**Keywords:** vitamin D, microgravity, spaceflight, bone loss, health risks
