High doses of vitamin D do not increase bone density, and they may even lower it, researchers report.

In a double-blinded, three-year clinical trial published in JAMA, scientists randomized 311 healthy adults without osteoporosis to daily doses of 400, 4,000 or 10,000 units of vitamin D.

At the beginning and end of the study, they used CT scans to calculate bone density in the arm and leg of each participant, and estimated bone strength using mathematical techniques.

The researchers’ hypothesis was that vitamin D supplements would increase bone density and strength, but the findings were the opposite. Declines in bone mineral density in the participants’ arms were 1.2 percent for the 400 group, 2.4 percent for the 4,000 group, and 3.5 percent for those taking 10,000 units a day. In the leg bones, declines were statistically significant in the 4,000 and 10,000 groups, but not in the 400 group. Bone strength declines were not significant in any group.