VIENNA — More than half of postmenopausal women being treated for osteoporosis in the United States and the rest of the world have vitamin D levels that are inadequate for skeletal health, according to two recent cross-sectional surveys totalling 2,821 such women in 20 countries.
“Wherever we look in the world, patients are not getting enough vitamin D to maintain calcium homeostasis. … This is a missed opportunity. When we're giving bone-active drugs to patients with osteoporosis, if we don't think about vitamin D inadequacy then we miss the opportunity to ensure that our patients have optimal gains in bone mineral density. And these studies show that the problem is very, very common,” David Hosking, M.D., declared at the annual European congress of rheumatology.
The North American survey involved 1,536 community-dwelling postmenopausal women being treated for osteoporosis. Of these, 52% were found to have a serum 25-hydroxyvitamin D (25[OH]D) level below 30 ng/mL, which most experts define as the cutoff for vitamin D inadequacy from the standpoint of facilitating calcium absorption in the intestine. Both surveys showed the prevalence of secondary hyperparathyroidism began rising as 25(OH)D dropped below 30 ng/mL.
The body senses when intestinal calcium absorption is inadequate. It responds by triggering secondary hyperparathyroidism. The resultant increased parathyroid hormone production leads to greater bone remodeling.
“If you're young, that probably doesn't matter desperately much, but if you're elderly or you're a postmenopausal woman in negative calcium balance, then that amplifies your rate of bone loss,” explained Dr. Hosking of Nottingham (England) City Hospital.
In the international survey, the overall prevalence of vitamin D inadequacy was 59%. (See box.)
“The Middle East was a real surprise. Here, where there's lots of sunshine, we'd imagine that all the patients would be able to make adequate levels of vitamin D. But because it's so searingly hot, hardly anybody goes out in the midday sun and the prevalence of vitamin D deficiency is very high,” he observed at the meeting, which was sponsored by the European League Against Rheumatism.
The prevalence of vitamin D inadequacy in the two surveys was unaffected by latitude. This finding suggests that the casual, “let-the-sun-take-care-of-it” approach to vitamin D that's widespread in more equatorial areas is misplaced, the physician said.
The Institute of Medicine's recommended daily vitamin D intake is 400 IU for individuals aged 51–70 years and 600 IU for those older than 70 years. Yet only 60% of postmenopausal osteoporosis patients in the U.S. survey claimed to be taking at least 400 IU/day. Their rate of vitamin D inadequacy was 45%, as compared with 63% in those not taking a daily supplement of at least 400 IU of vitamin D. But that 45% prevalence is still unacceptably high, Dr. Hosking said, adding that it probably reflects poor compliance with vitamin D therapy.
In the North American survey, multivariate analysis identified eight variables as independent predictors of vitamin D inadequacy: age older than 80 years; obesity; nonwhite race; the use of drugs known to affect vitamin D metabolism; education less than 12th grade; lack of exercise; a daily vitamin D intake lower than 400 IU; and a lack of physician counseling regarding the importance of vitamin D to bone health. Patients with five or more risk factors had a 90% prevalence of inadequate vitamin D.
Two of these risk factors—failure to take an adequate daily vitamin D supplement and lack of physician counseling regarding vitamin D's importance to skeletal health—are readily remediable through patient and physician education. That's the approach Dr. Hosking considers most effective. Some experts advocate modestly increasing sun exposure; however, the skin becomes less efficient at converting sunlight to vitamin D with advancing age, and skin cancer is an issue.
Dr. Hosking was skeptical about taking a public health approach to encourage people to eat more vitamin D-rich foods, as the list of such foods is limited.
“I think for us within osteoporosis, it's much easier to target patients who need vitamin D supplements as part of our treatment because, after all, we're giving them a preparation to control their bone disease, so it's practical to deal with vitamin D at that stage,” he said.
His analysis of the survey data was funded by Merck Sharp & Dohme Ltd. He is a consultant to the company as well as a member of its speakers' bureau.
KEVIN FOLEY, RESEARCH/JULIE KELLER, DESIGN