In a minor way, it acts to increase the reabsorption of calcium by the kidney. It increases renal tubular reabsorption of phosphate. Other functions for Vitamin D are being discovered. Receptors for the active form of Vitamin D have been found not only in the intestinal, renal, parathyroid and skeletal tissues, but also in skin, breast, pancreas and connective tissue. It is thought that the active form of Vitamin D, which is present in the beta cells of the pancreas, may have a role in insulin secretion, possibly through maintenance serum calcium, which is important for adequate insulin secretion.
Vitamin D is found only in small and highly variable amounts in butter, cream, egg yolk and liver.
The best food sources are fish liver oils. At this time within the U.S., approximately 98 percent of all fluid milk and evaporated milk are fortified with Vitamin D. Most dried whole milk and evaporated milk are fortified as well as some margarines, butter, selected cereals and some infant products. The milk used to make cheese is usually not Vitamin D-fortified. Vitamin D deficiency in adulthood may result in osteomalacia. It is characterized by pronounced softening of the bones, which leads to deformities, especially of the limbs, spine, thorax and pelvis.