People seem to be getting less vitamin D in their diets.

Researchers said in the Archives of Internal Medicine that average blood levels dropped from 1994 to 2004.

In a news release on the work, researchers pointed out that a vitamin D deficiency is associated with bone loss, cancer, heart disease, infections and poor overall health.

Evidence suggests that people need 30 to 40 nanograms per milliliter, the release said.

Testing of more than 18,000 people found an average level of 30 nanograms in 1988. In testing of about 13,000 people from 2001 to 2004, the average level was 24 nanograms.

Dr. Adit A. Ginde of the University of Colorado Denver School of Medicine also found that the number of black people with very low levels rose from 9 percent to 29 percent. The number of black people with more than 30 nanograms dropped from 12 percent to 3 percent.

Researchers said part of the reason for the drop might be that people have been taught to avoid sun exposure. Sunlight exposure is a main determinant of vitamin D status in humans, they said.

Vitamin D has been said to fight the common cold and power muscles. Some also call it a "super nutrient" that helps protect against diabetes, hypertension, heart disease and certain cancers, as well as supporting the immune system.