What percentage of the U.S population is deficient in vitamin D?

Due to fortification of foods, especially cereals, only a small percentage of individuals in the U.S. are deficient in vitamin B12. However, because of malabsorption syndromes, celiac disease, gastric bypass operations and conditons of undernourishment (as a result of nutritionally empty calories) such as Alcoholism, it is estimated that up to 10% of individuals could be deficient in vitamin B12.