Large doses of vitamin D don't improve bone density or strength, and may even result in a decrease in bone density, researchers have warned.
Vitamin D helps to keep bones, teeth and muscles healthy. Public Health England says that people should consider taking a daily supplement to ensure they have 10 micrograms or 400 international units (IU) of vitamin D every day.
Many supplements contain higher levels than this, but a new study published in the Journal of the American Medical Association (JAMA) suggests that there is no benefit in taking high doses of vitamin D -- and in fact high doses may actually compromise bone health.
The three-year study led by the University of Calgary in Canada followed 300 volunteers aged 55-70, with one third receiving 400 IU of vitamin D per day, one third receiving 4,000 IU per day, and another third receiving 10,000 IU per day.
The researchers wanted to test the hypothesis that, with increasing doses of vitamin D, there would be a dose-related increase in bone density and bone strength.
Volunteers had both their bone density and bone strength measured using standard dual-X-ray absorptiometry (DXA), as well as a new, high-resolution computed tomography (CT) scan of bone at the wrist and ankle, called an XtremeCT, used only for research. This allows researchers to take a detailed look at bone microarchitecture.
Adults slowly lose bone mineral density (BMD) as they age, and the DXA results showed a modest decrease in BMD over the duration of the study, with no differences detected between the three groups. However, the more sensitive measurement of BMD with high resolution XtremeCT showed significant differences in bone loss.
Over the three-year period, total BMD decreased by 1.4% in the 400 IU group, 2.6% in the 4,000 IU group and 3.6% in the 10,000 IU group.
"We were surprised to find that instead of bone gain with higher doses, the group with the highest dose lost bone the fastest," commented Dr Steve Boyd, one of the principal investigators of the study. "That amount of bone loss with 10,000 IU daily is not enough to risk a fracture over a three-year period, but our findings suggest that for healthy adults, vitamin D doses at levels recommended by Osteoporosis Canada (400-2,000 IU daily) are adequate for bone health."