If the calciferol is not adequate to raise the serum calcium level, the kidney converts the calciferol to the most active form, dihydroxy vitamin D. Amount needed is 400 I.U. or 10 micrograms. Since we live in a society that has us wearing clothing, taking baths, staying indoors, etc., vitamin D is added to milk. Severe deficiencies are rickets in children and osteomalacia in adults. For descriptions of these diseases read page 234 in your textbook. This can easily occur if supplements are given and vitamin enriched cereals are eaten in large quantity.
Vitamin D is the most toxic of all the vitamins.
Sources are exposure to sunlight, fish liver oils, fortified milk and margarine, and eggs. Absorption of vitamin D made on the surface of the skin takes 6 to 8 hours to be absorbed. When you or your children spend time in the sun be sure to allow time for the vitamin D to be absorbed before you bathe. Storage is in body fat. It is NOT added to milk that is made into cheese and yogurt, or put into other foods.