In fact, calcium relies on magnesium (and Vitamin D) to help build bone and prevent unnecessary breakdown of bone, particularly as we get older. (twohootsabouthealth.com)
What the alendronate drugs do is by stopping the normal breakdown of bones, they favor the buildup of old bone that becomes brittle over time and becomes so hard and dense that the normal blood supply can not get into the bone. (mindbodygreen.com)
They are also high in potassium which prevent the breakdown of bone that comes with age. (sogoodblog.com)