A new meta-analysis by medical researchers in New Zealand and Scotland has seriously called into question whether vitamin D supplementation has any effect on bone health. But judging by the reaction of other physicians and scientists, the long-standing practice of recommending daily vitamin D to older adults is not going anywhere – at least, not yet.
Two of the study’s three authors – Mark Bolland and Andrew Grey, both from the University of Auckland – have been leading investigations into vitamin D since 2012, and each time they have presented dubious conclusions about the benefits of the supplement. The recent work, published in The Lancet Diabetes & Endocrinology, was undertaken to analyze the wave of 30-plus clinical trials that were conducted since their last literature review in 2014. These additions have nearly doubled the amount of available evidence.
So, Bolland and Grey teamed up with Alison Avenell from the University of Aberdeen to examine findings from 81 randomized control trials that compared bone mineral density and rates of falls and/or fractures among people taking high or low doses of vitamin D to those taking a placebo, not taking a supplement, or those taking even lower doses of vitamin D.
“Our findings suggest that vitamin D supplementation does not prevent fractures or falls, or have clinically meaningful effects on bone mineral density,” they wrote, adding that they believe clinical guidelines should be amended.
“On the strength of existing evidence, we believe there is little justification for more trials of vitamin D supplements looking at musculoskeletal outcomes,” Bolland told the Guardian.
Adrian Martineau, a professor of Respiratory Infection and Immunity at Queen Mary University of London, is one of many experts that disagree with the sweeping conclusions of Bolland’s group.
“The authors of this study acknowledge that ‘trials of vitamin D supplementation in individuals with marked vitamin D deficiency… might produce different results’. This caveat has important implications, since more than 1 in 5 of the UK population is vitamin D deficient,” he said in a statement to the Science Media Centre (SMC). “The potential for vitamin D supplementation to reduce risk of fractures and falls should not be discounted until further trials in populations with 25-hydroxyvitamin D levels <25 nmol/L have been conducted.”
Blood levels of 25-hydroxyvitamin D, the chemical vitamin D3 is converted into by the liver, are used to determine whether someone has adequate or insufficient vitamin D in their body. In the UK health system, levels of 25 billionths of a mol per liter or below are considered deficient, whereas in the US, the cut-off ranges between 30 to 50 nmol/L.
Setting aside the question of whether or not supplements do anything, past research shows that 25-hydroxyvitamin D and 25-hydroxyergocalciferol (the molecule D2 is converted into) are essential to a variety of physiological processes, including maintaining bone integrity via calcium homeostasis.
“Almost all trials for vitamin D supplementation [for bone health] have shown that supplementation is only effective if you are vitamin D-deficient to begin with,” Professor Martin Hewison, deputy director of the Institute of Metabolism and Systems Research at The University of Birmingham, also told the SMC.
“What the current study in Lancet Diabetes and Endocrinology illustrates is that more studies are required that [focus on how to best apply] vitamin D supplementation where it is needed … repeated expansive meta-analyses will not achieve this.”
While Bolland’s study did include trials assessing vitamin D taken with calcium supplements, these investigations only compared calcium + vitamin D regimens to vitamin D only, not to placebo. This means the analysis overlooked any possible benefit of taking the vitamin directly with the bone-building mineral it helps the body utilize.