Vitamin D claimed to have no impact on bone health

Researchers from the Universities of Auckland and Aberdeen have ruffled a few feathers in the medical world, claiming that vitamin D supplementation does not improve bone mineral density in adults or prevent fractures and falls.

Vitamin D supplements have long been recommended for older people to treat or prevent osteoporosis, with some early evidence suggesting benefits for bone health. However, recent large-scale reviews have reported no effect of vitamin D supplementation on bone mineral density, falls or fractures.

Seeking answers of their own, study authors Dr Mark J Bolland, Dr Andrew Grey and Dr Alison Avenell pooled data from 81 randomised controlled trials, the majority of which studied vitamin D alone (ie, not prescribed in conjunction with calcium supplements) and were of one year or less. Most also included women aged over the age of 65 (77% of trials) who received daily doses of more than 800 IU per day (68% of trials). The results of the meta-analysis were published in The Lancet Diabetes & Endocrinology.

The researchers found there was no clinically meaningful effect of vitamin D supplementation on total fracture, hip fracture or falls. There was reliable evidence that vitamin D does not reduce total fractures, hip fractures or falls by 15% — a clinically meaningful threshold. Even when lower thresholds were assessed, there was still reliable evidence that vitamin D does not reduce falls by 7.5% and total fractures by 5%.

In secondary analyses looking at bone density, there were small differences for lumbar spine, femoral neck and total body, but none of these were clinically relevant. In addition, the authors conducted more than 60 subgroup analyses to verify their findings.

The authors thus conclude that there is little justification to use vitamin D supplements to maintain or improve musculoskeletal health, except for the prevention of rare conditions such as rickets and osteomalacia in high-risk groups, which can occur due to vitamin D deficiency after a prolonged lack of exposure to sunshine. They say that clinical guidelines that continue to recommend vitamin D supplementation for bone health should be changed to reflect the best available evidence.

“Our meta-analysis finds that vitamin D does not prevent fractures, falls or improve bone mineral density, whether at high or low dose,” said Dr Bolland. “Clinical guidelines should be changed to reflect these findings. On the strength of existing evidence, we believe there is little justification for more trials of vitamin D supplements looking at musculoskeletal outcomes.”

The authors did, however, note that data were collected differently for falls in different trials, which might affect the study findings, and that while all trials were rated for risk of bias, there were methodological limitations in some of the trials. Additionally, smaller trials of shorter duration tended to find stronger effects of vitamin D compared to larger trials of longer duration.

The reaction from outside experts has also been mixed. Dr Robert Clarke from the University of Oxford acknowledged that the report included all available trials of vitamin D — but said such trials included too few participants, used an insufficient dose of vitamin D and had an insufficient duration of treatment.

“Hence, the study lacked the ability to reliably test the effects of vitamin D on risk of hip fracture,” he said.

Professor Martin Hewison from the University of Birmingham meanwhile noted that “almost all” previous trials have shown that supplementation is only effective if you are vitamin D deficient to begin with — and very few subjects in the 81 studies assessed in the meta-analysis would be considered vitamin D deficient according to UK guidelines, ie, with baseline 25-hydroxyvitamin D levels less than 25 nmol/L.

“As such, the benefits of vitamin D supplementation are difficult to determine from the current study even though it involves a large number of individuals,” he said.

“Much of the data in the meta-analysis reflect subjects who already had adequate levels of vitamin D. Many individual vitamin D supplementation trials do not screen for baseline vitamin D deficiency and this may account for disappointing results.

“What the current study … illustrates is that more studies are required that target vitamin D supplementation where it is needed — in people with vitamin D-deficiency.”

Professor Adrian Martineau from the Queen Mary University of London agreed, calling the small minority of vitamin D deficient participants a “major limitation” and noting that the authors failed to obtain data from individual trial participants for their meta-analysis.

“This limited their ability to determine whether individuals with low baseline vitamin D levels benefit more from vitamin D supplementation than those with higher baseline levels,” he said.

“We know from meta-analyses that have managed to obtain individual participant data that the health benefits of vitamin D supplementation tend to be most marked in people who have the lowest vitamin D levels to start with. While it’s easy to pool published summary statistics relating to treatment effects to come up with a negative overall answer, this simplistic ‘one size fits all’ approach is a blunt tool that has limited ability to explore whether the effects of a given intervention vary according to key characteristics of individuals who take part in clinical trials, such as whether or not they are deficient in a micronutrient at baseline.

“Public Health England’s recommendation that everyone needs 10 micrograms (400 international units) of vitamin D per day are designed specifically to elevate vitamin D levels out of the deficient range (ie, to bring them over 25 nmol/L). Achieving this target in the whole UK population would save lives by preventing the most extreme manifestations of vitamin D deficiency (seizures and heart failure in infants), which occur every year in the UK. The findings of the new paper do not provide any reason to revisit or reconsider this sound advice.”

Image credit: ©stock.adobe.com/au/emf_images