When bare skin is exposed to sunlight, it makes vitamin D, which our bodies need to absorb calcium and build strong, healthy bones. In the summer, it only takes about 10-15 minutes in the sun to generate all the vitamin D your body needs for the day. Unfortunately, it is reduced during the long winter months. This results in many turning to supplements to get the required vitamin D. However, some people take too many supplements. So, what's the right dose? And how much is too much?
A three-year study showed that it does not pay to take high doses of vitamin D. The study followed volunteers between the ages of 55 and 70 to test the hypothesis that with increasing doses of vitamin D, a dose-related increase in bone density and bone strength would occur. A third of the study participants received 400 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 new, high-resolution computed tomography (CT) scan of bone at the wrist and ankle called an XtremeCT, which was used for research only. The XtremeCT is the first of its kind in the world and allows researchers to take a closer look at bone microarchitecture. Participants received scans at the start of the study and at 6, 12, 24 and 36 months. To assess vitamin D and calcium levels, researchers also collected fasting blood samples at the start of the study and after 3, 6, 12, 18, 24, 30 and 36 months, as well as annual urine collections.
Bone mineral density (BMD) is determined by measuring the amount of calcium and other minerals in a particular segment of bone. The lower the bone density, the greater the risk of bone fractures.
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 between the three dose levels.
Overall BMD decreased by 1.4 percent in the 400 IU group, 2.6 in the 4,000 IU group and 3.6 in the 10,000 IU group over the three-year period. The conclusion was that, contrary to what had been predicted, vitamin D supplementation at higher doses was not associated with an increase in bone density or bone strength. Instead, the XtremeCT detected a dose-dependent decrease in bone density, with the largest decrease in the 10,000 IU per day group. More research is needed to determine whether high doses can actually harm bone health.
A secondary outcome of the study indicated a potential safety risk with taking high levels of vitamin D. While there were incidents in all three arms of the study, the researchers found that participants taking higher doses of daily vitamin D supplementation for three years (4000 IU and 10,000 IU) were more likely to develop hypercalciuria (increased calcium levels in the urine), compared with those given a lower daily dose. Hypercalciuria is not uncommon in the general population, but is associated with an increased risk of kidney stones and may contribute to impaired renal function.
Hyperalciuria occurred in 87 participants. The incidence varied significantly between the 400 IU (17%) 4000 IU (22%) and 10,000 IU (31%) study groups. When hypercalciuria was observed in study participants, calcium intake was reduced. After repeated tests, the hypercalciuria usually disappeared.
