Senior author Dr. Michael Holick, director of the General Clinical Research Center at Boston University School of Medicine, said increased skin pigmentation, application of a sunscreen, aging and clothing have a dramatic effect on previtamin D3 production in the skin.

Holick said 45 nursing home residents who were taking a multivitamin that contained 400 IU of vitamin D2 -- derived from fungal and plant sources -- showed a dramatic decline in their 25(OH)D levels, an accurate measure of the amount of vitamin D in the body, from the end of summer to the beginning of the following summer.

The study, published in the March issue of the Journal of Bone and Mineral Research, found 49 percent were vitamin D deficient in August, 67 percent were deficient in November, 74 percent in February and 78 percent in May.

Most experts agree that a minimum of 1,000 IU of vitamin D per day is necessary to maintain circulating concentrations of vitamin D, the researchers said.