Absorption of calcium is inefficient if it"s by passive diffusion. If you have enough vitamin d in your system, you will have enough of the transport proteins and the absorption will be more efficient. The rda takes this inefficient absorption into account. The different roles vitamin d with respect to calcium: stimulates the release of calcium from bone, reabsorbs calcium from urine. Bone calcium vs blood calcium: if dietary calcium is low, bone calcium is sacrificed to maintain blood calcium. If it"s high, then you don"t have to sacrifice bone. Calcium isn"t just about bone, it"s also for: muscle contraction, blood pressure regulation (like k, neurotransmitter release, required for blood clotting. Inverse association b/w calcium intake and colon cancer: calcium binds with toxins in the colon and render them insoluble and biologically inactive. The mineral component of bone: hydroxyapatite = the inorganic matrix of the bone: It"s a mineral that provides strength and rigidity.