text: skin tans due to calcium homeostasis in the intestine and activation of subcutaneous vitamin d.  how? i'm unsure what link exists between calcium absorption and the melanocytes in the skin. uvb rays are needed for your skin to produce vitamin d, but it also stimulates melanocytes to darken your skin, which allows less uvb rays in. vitamin d created by your skin and from diet is activated by the liver and kidneys, then stimulate increased absorption of calcium through in the intestines. prior to fortification of milk with vitamin d, there was a higher occurrence of rickets amount people of darker complexion and people who didn't get enough sun exposure.