The diffienciency of vitamin D has been found to be the leading cause of osteoporosis. Vitamin D deficiency is affecting 10 million Americans. Experts say osteoporosis will become more prevalent unless more vitamin D is taken by the average person, especially men and women over 50. Deficiency of Vitamin D can also cause insomnia and arthritis. The best source of vitamin D is sunshine. Depending whether your skin is light or dark, 15 to 30 minutes of sun exposure in mid morning or late afternoon is the best supplement. Egg yolks, enriched soy and rice milks, margarines and breakfast cereals are some of the foods that also provide vitamin D.