The study, carried out between November 2003 and March 2004 and presented this week at a meeting of the American Association of Clinical Endocrinologists (AACE), involved 1,554 otherwise-healthy postmenopausal women in the United States who were receiving antiresorptive therapies for osteoporosis. The participants were aged between 47 and 103 years.
Researchers measured their serum 25-hydroxyvitamin-D concentrations and found them to be suboptimal - that is, less than 30ng/mL - in 52 percent of the group. The women also completed a questionnaire to obtain information on factors that might affect their vitamin D status.
The women were mostly educated, active and white - a bias that has led the association to place blame for the situation at the feet of clinicians, who it says are overlooking the importance of vitamin D supplementation.
"These results underscore a need for improved physician and patient education regarding optimization of vitamin D status in the care of postmenopausal women with osteoporosis," it said.
The National Academies Food and Nutrition Board currently recommends a daily intake (RDI) of vitamin D of 200 international units per day (IU/d) for young adults, 400 IU/d for those aged 51 to 70 years, and 600 IU/d for those over age 70.
But the AACE considers these guidelines to be too low. In its Medical Guidelines for Clinical Practice of the Prevention and Treatment of Postmenopausal Osteoporosis, it says that adequate intake is 400 IU/d for all age groups, rising to 800 IU/d for the elderly.
At the recent conference of the American Academy of Dermatology, Dr Vincent DeLeo, associate professor of clinical dermatology at Columbia University, drew attention to evidence that darker-skinned people are less likely than whites to glean the vitamin D they need from incidental exposure to sunlight or diet.
He explained that melanin, a natural substance that is more prolific in darker skin, reduced the skin's ability to photosynthesize vitamin D.
This factor may make AACE's case even stronger: if its survey had been representative of the American population, including darker-skinned as well as mostly white women, it could have uncovered an even more serious vitamin D deficit.
Natural dietary sources of vitamin D include milk, egg yolk, oily fish and liver. However DeLeo warned that the skin cancer risks of trying to boost levels by upping sun exposure or undergoing artificial tanning outweigh the benefits. He said that people concerned about their intake should instead boost it with dietary supplements.