Dr. Michael Holick, MD, of Boston University and author of The Vitamin D Solution, says that vitamin D deficiency is epidemic throughout the USA through all age groups. In fact, 75% of US teens and adults are deficient. Unless you live south of the 30th parallel, which runs along the panhandles of Florida and Texas, it is very difficult to get enough vitamin D from the sun, especially in the winter months, because the sun never gets high enough in the sky for the ultraviolet rays to penetrate the atmosphere.
Furthermore, less and less people get out in the sun because of inside activities and entertainment. Kids and teenagers, for example, once active outdoors, find more reasons to stay indoors than to go outside because of digital entertainment. Similarly, the elderly rarely get enough sun exposure because of inactivity. Thus, it has become important to supplement this important nutrient even during summer months, since serum levels of most patients tested will be woefully inadequate all throughout the year, and because nearly every disease has a vitamin D deficiency connection.
While supplementation seems to be the answer, a Boston study found the 36% of young adults 18-29 years old had vitamin D levels below 20 ng/mL at the end of winter in spite of consuming fortified milk or a multivitamin daily. Furthermore, an article appearing in JAMA (Feb. 11, 2013) indicated that researchers found that OTC vitamin D supplements from a dozen manufacturers tested at a range that was as low as 52% of their advertised vitamin D content. Duffy MacKay, vice president of the scientific and regulatory affairs at the Council for Responsible Nutrition (CRN) in Washington, DC - a trade group representing supplement manufacturers and ingredient suppliers, said that "there is no excuse for supplements to deviate from their labels." Yet, this goes on in the industry routinely because there are no FDA regulatory standards on supplement label claims. That is where GMP certified products that are tested both internally and externally ensures products that do not deviate from label claim.
The Vitamin D Council has recently raised their supplement recommendations from a minimum of 4,000 IUs per day to 5,000. Hence, active people might consider supplementing at least 1,000 IUs per day during the summer months if they get outside regularly, and more if they are less active. During the winter months, it is not unreasonable to supplement as much as 10,000 IUs per day, and more if serum levels are extremely low.