Concerns have been raised about higher dose supplements of vitamin D after a Canadian study found users had lower bone mineral density (BMD) than people who took standard 400 IU daily doses.
A double-blind, randomised clinical trial conducted at a single Canadian centre compared the effects of daily doses of 400 IU, 4000 IU and 10,000 IU of vitamin D in 311 healthy adults aged 55-77 years of age.
The study participants had baseline 25-hydroxyvitamin D (25[OH]D) of 30-125 nmol/L. Calcium supplementation was provided to participants with dietary intake of less than 1200mg per day.
After three years, radial BMD was significantly lower in people who took 4000IU a day (-3.9mg HA/cm3) and 10,000 IU/day (−7.5mg HA/cm3 ) compared with people who took 400 IU per day.
Tibial BMD was also significantly lower (−4.1mg HA/cm3) with the daily dose of 10000 IU but not with the 4000IU/day dose compared to the 400IU dose.
There were no significant differences in bone strength (failure load) at either the radius or tibia.
The study authors said the findings raised concerns given that most vitamin D supplementation recommendations range up to 2000 IU daily, and 3% of US adults consumed daily doses of at least 4000 IU of vitamin D.
“For maintenance of bone quality in healthy vitamin D–sufficient adults, these results do not support a skeletal benefit of vitamin D doses well above the recommended dietary allowance,” they wrote in JAMA.
The negative effects on BMD at higher doses might be related to the observed combination of an increased plasma marker of bone resorption (CTx) and suppression of PTH seen in the high dose group, they postulated.
And there was growing evidence that the benefit of vitamin D supplementation for bone was only seen in the treatment of vitamin D deficiency.
“There is also evidence that very high intermittent (monthly or annual) doses of vitamin D may be harmful, with increased risk of falls or fracture.”
“If high-dose vitamin D does stimulate an increased rate of bone loss, this could have greater clinical significance in older individuals with osteoporosis,” they wrote.