Vitamin D supplements may have no bone benefits for those getting the recommended amounts, a small study of postmenopausal women suggests.
Together vitamin D and calcium play an important role in bone health such as avoiding rickets. Vitamin D is also under investigation for cancer prevention, cardiovascular disease, diabetes, and immunity. In 2011, the Institute of Medicine, which advises the U.S. and Canadian governments, found the evidence that vitamin D supplements can be beneficial was inconsistent beyond bone health.
Researchers randomly assigned 159 postmenopausal women to take vitamin D3 and calcium, one of the supplements, a placebo or two placebos to evaluate the influence of the supplements on bone turnover — a measure of bone loss.
Vitamin D and calcium interact to suppress bone turnover, said the study's lead author, Dr. John Aloia of Winthrop University Hospital in Mineola, N.Y. The study was designed to look at the physiology behind this.
"I think too much of a good thing is a statement that really applies here," Aloia said.
"Our study was in normal healthy, postmenopausal women and in those women, vitamin D in high amounts had no effect on bone turnover."
In total, 120 women completed the six-month study. The doses were 1,200 milligrams of calcium daily and 4,000 IU of vitamin D daily, which the study's authors said reflects advice that may be given to women currently.
Last year, the U.S. Preventive Services Task Force reviewed research on the supplements and concluded that taking 400 IU of vitamin D or less and 1,000 milligrams of calcium seems to slightly increase the risk for kidney stones in postmenopausal women and should not be taken.
The latest findings come amid recommendations from groups such as Osteoporosis Canada that advise vitamin D supplements for specific patient populations who are at risk, said Dr. David Hanley, who chaired the group's vitamin D guidelines committee.
Hanley called the study interesting but noted that the variables the researchers measured weren't the typical ones like fractures.
Health Canada uses the Institute of Medicine's recommendations for adults of between 600 IU and 800 IU of vitamin D depending on age.
"The Health Canada recommendation for vitamin D intake really could not be achieved without taking a modest dose of a supplement, at least 400 units per day," Hanley said, noting the typical dietary intake of Canadians is a little over 200 units per day.
Health Canada's tolerable upper limit that can be taken without medical supervision by adults is 4,000 IU.
Osteoporosis Canada's recommendation of between 800 to 2,000 IU for adults over 50 is focused on people at risk of vitamin D deficiency or those who have disorders, particularly osteoporosis, that might be made worse by vitamin D deficiency, Hanley said.
Aloia recommended that women be cautious about the possibility of vascular side-effects from too much calcium and should consult their physicians about whether their diet is adequate.
The study was funded by Merck and the Empire Clinical Research Investigator Program.