Dr. Michael Holick, from Boston University Medical School, a professor of medicine, physiology and biophysics won the Linus Pauling Prize for Health Research for his work investigating how vitamin D deficiency increases the risk of cancer, autoimmune diseases, infectious and cardiovascular disease. This deficiency has become epidemic and more than 50 percent of the children and adults in the United States are vitamin D deficient. Through his work, Holick determined that anyone living north of 35 degrees latitude can't make enough vitamin D in the skin during winter exposure to sunlight. He has also influenced fortification of more foods and brought the deficiency to the forefront. Though using sunscreen is necessary it has contributed to the problem since Vitamin D is produced in the body by sunshine.
So most of us should take a supplement of vitamin D3. It is easy to take and very beneficial.