High doses of vitamin D may decrease bone density, study says

New research from the University of Calgary in Alberta, Canada, has found that taking high doses of vitamin D may result in a decrease in bone density, according to a study published in the Journal of the American Medical Association.

Vitamin D is primarily synthesized through sun exposure. During the summer months, it only takes about 10 to 15 minutes for the body to generate the vitamin D it needs for the day. However, those living in regions where sunlight is diminished in the winter months often struggle to meet their daily vitamin D requirements, often turning to supplements as a result.

Researchers found several discrepancies in dosage recommendations by national health organizations for vitamin D and decided to investigate the correct dose as well as how much is too much. The three-year study showed there is no benefit in taking high doses of vitamin D. The study followed 300 volunteers between the ages of 55 and 70 in a double-blind, randomized clinical trial to test the hypothesis that with increasing doses of vitamin D, there would be a dose-related increase in bone density and bone strength. A third of the study participants received 400 international units (IU) of vitamin D per day, a third received 4,000 IU per day, and a third received 10,000 IU per day.

Volunteers had both their bone density and bone strength measured using a high-resolution computed tomography (CT) scan of bone at the wrist and ankle, called an XtremeCT, used only for research. The XtremeCT the first of its kind in the world and allows researchers to look at bone microarchitecture in detail never seen before. Standard dual-X-ray absorptiometry (DXA) bone density was also obtained. Participants received scans at the start of the study and at six, 12, 24 and 36 months. To assess vitamin D and calcium levels, researchers also collected fasting blood samples at the beginning of the study and at three, six, 12, 18, 24, 30 and 36 months as well as urine collections annually.

Bone mineral density (BMD) is determined by measuring the amount of calcium and other minerals in a defined segment of bone. The lower the bone density, the greater the risk for bone fracture. Adults slowly lose 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 among the three dose levels.

Total BMD decreased over the three-year period by 1.4 percent in the 400 IU group, 2.6 percent in the 4,000 IU group and 3.6 percent in the 10,000 IU group. Researchers concluded that vitamin D supplementation at doses higher than those recommended were not associated with an increase in bone density or bone strength. Instead, the XtremeCT detected a dose-related decrease in bone density, with the largest decrease occurring in the 10,000 IU per day group. More research is required to determine if high doses may compromise bone health.