Vitamin D, too, especially if you don't eat dairy. I managed to develop a deficiency which caused seriously low bone density.

The only reason I found out was because I got a compression fracture of one of the T vertebrae while snowboarding (just riding, too---no crash or anything like that). My point being that I felt otherwise completely normal so somebody with a sedentary lifestyle may not notice until it gets really bad.

Vitamin D facilitates calcium absorption; my calcium levels were normal. I think regular diets are more resistant to lower vitamin D levels because they contain more calcium on average.