In fact, rather than a hypothesized increase in volumetric bone mineral density (BMD) with doses well above the recommended dietary allowance, a negative dose-response relationship was observed,, of the McCaig Institute for Bone and Joint Health at the University of Calgary (Alta.) and colleagues found.
The total volumetric radial BMD was significantly lower in 101 and 97 study participants randomized to receive daily vitamin D3 doses of 10,000 IU or 4,000 IU for 3 years, respectively (–7.5 and –3.9 mg of calcium hydroxyapatite [HA] per cm3), compared with 105 participants randomized to a reference group that received 400 IU (mean percent changes, –3.5%, –2.4%, and –1.2%, respectively). Total volumetric tibial BMD was also significantly lower in the 10,000 IU arm, compared with the reference arm (–4.1 mg HA per cm3; mean percent change –1.7% vs. –0.4%), the investigators reported Aug. 27 in.
There also were no significant differences seen between the three groups for the coprimary endpoint of bone strength at either the radius or tibia.
Participants in the double-blind trial were community-dwelling healthy men and women aged 55-70 years (mean age, 62.2 years) without osteoporosis and with baseline levels of 25-hydroxyvitamin D (25[OH]D) of 30-125 nmol/L. They were enrolled from a single center between August 2013 and December 2017 and treated with daily oral vitamin D3 drops at the assigned dosage for 3 years and with calcium supplementation if dietary calcium intake was less than 1,200 mg daily.
Mean supplementation adherence was 99% among the 303 participants who completed the trial (out of 311 enrolled), and adherence was similar across the groups.
Baseline 25(OH)D levels in the 400 IU group were 76.3 nmol/L at baseline, 76.7 nmol/L at 3 months, and 77.4 nmol/L at 3 years. The corresponding measures for the 4,000 IU group were 81.3, 115.3, and 132.2 nmol/L, and for the 10,000 IU group, they were 78.4, 188.0, and 144.4, the investigators said, noting that significant group-by-time interactions were noted for volumetric BMD.
Bone strength decreased over time, but group-by-time interactions for that measure were not statistically significant, they said.
A total of 44 serious adverse events occurred in 38 participants (12.2%), and one death from presumed myocardial infarction occurred in the 400 IU group. Of eight prespecified adverse events, only hypercalcemia and hypercalciuria had significant dose-response effects; all episodes of hypercalcemia were mild and had resolved at follow-up, and the two hypercalcemia events, which occurred in one participant in the 10,000 IU group, were also transient. No significant difference in fall rates was seen in the three groups, they noted.
Vitamin D is considered beneficial for preventing and treating osteoporosis, and data support supplementation in individuals with 25(OH)D levels less than 30 nmol/L, but recent meta-analyses did not find a major treatment benefit for osteoporosis or for preventing falls and fractures, the investigators said.
Further, while most supplementation recommendations call for 400-2,000 IU daily, with a tolerable upper intake level of 4,000-10,000 IU, 3% of U.S. adults in 2013-2014 reported intake of at least 4,000 IU per day, but few studies have assessed the effects of doses at or above the upper intake level for 12 months or longer, they noted, adding that this study was “motivated by the prevalence of high-dose vitamin D supplementation among healthy adults.”
“It was hypothesized that a higher dose of vitamin D has a positive effect on high-resolution peripheral quantitative CT measures of volumetric density and strength, perhaps via suppression of parathyroid hormone (PTH)–mediated bone turnover,” they wrote.
However, based on the significantly lower radial BMD seen with both 4,000 and 10,000 IU, compared with 400 IU; the lower tibial BMD with 10,000 IU, compared with 400 IU; and the lack of a difference in bone strength at the radius and tibia, the findings do not support a benefit of high-dose vitamin D supplementation for bone health, they said, noting that additional study is needed to determine whether such doses are harmful.
“Because these results are in the opposite direction of the research hypothesis, this evidence of high-dose vitamin D having a negative effect on bone should be regarded as hypothesis generating, requiring confirmation with further research,” they concluded.
SOURCE: Burt L et al. .