A report in the Archives of Internal Medicine shows more than three out of four Americans are not getting enough of the so-called sunshine vitamin.
Part of the problem may be that fewer people are getting sun exposure, which helps trigger the production of vitamin D.
Also, many experts now agree that current recommendations for vitamin D supplements are too low.
The Institute of Medicine is in the process of revising its guidelines for intake levels. In the meantime, researchers suggest people get at least 1,000 international units of vitamin D daily.