The vitamin that mediates intestinal calcium absorbtion, bone calcium metabolism and probably, muscle activity. Occurs in and is isolated from fish liver oils. Vitamin D acts through a receptor that is a member of the ligand-dependent transcription factor superfamily. Modulates the proliferation and differentiation of both normal and cancer cells. Has antiproliferative and antimetastatic effects on breast, colon, and prostate cancer cells. Activated vitamin D receptors in intestine and bone maintain calcium absorbance and homeostasis.