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Vitamin D supplements may not strengthen bones

“Bad news for osteoporosis sufferers: Vitamin D supplements ‘do not help bone health’,” The Independent warns.

The claim comes after the publication of a major study into the effects of vitamin D supplements on bone density.

Bone density weakens as we get older – with post-menopausal women being at particular risk due to the effects that changes in hormone levels can have on bone density. This can increase the risk of fractures, such as hip fractures.

Vitamin D supplements – which are estimated to generate millions of pounds of profit for the dietary supplements industry – have been marketed as a way of preventing bone weakening. But the study in question throws doubt on this claim.

The study pooled the findings of 23 published studies. The results showed that vitamin D increased bone density by a small amount in just one site (femoral neck) of five sites tested. The effect was very small, and was reported to be unlikely to be clinically significant for preventing osteoporosis or fracture.

The conclusion that taking vitamin D does not appear to increase bone density on its own seems credible. Although the study didn’t directly test a link to bone fracture it did point to other research that showed that vitamin D might also be ineffective in this scenario.

The UK guidance on vitamin D supplementation is being reviewed and will take into account the best available evidence to inform its recommendations.

Where did the story come from?

The study was carried out by researchers from the University of Auckland, New Zealand, and was funded by the Health Research Council of New Zealand.

The study was published in the peer-reviewed medical journal The Lancet.

The media reporting was broadly accurate with some media outlets focusing on the science while other stories focused more on the cost implications of potentially wasteful use of vitamin D supplements in the NHS in England.

The Daily Telegraph reports that “the NHS currently spends more than £80m per year on prescriptions for vitamin D-based medicines”. However, this figure also includes treatment costs for people with diagnosed vitamin D deficiency, so the £80m figure is inaccurate.

What kind of research was this?

This was a meta-analysis of randomised controlled trials looking at whether vitamin D affects bone mineral density.

Vitamin D has several important functions including helping to regulate the amount of calcium in the body. This makes it biologically important in the formation and density of bones.

A meta-analysis of randomised controlled trials is a common method of trying systematically to identify all known research on a topic and to summarise it into a single conclusion. There have been a lot of research studies published on the effect of vitamin D on bone density, so this approach is an appropriate way of pooling the findings into an overall measure of effectiveness. 

Vitamin D supplements are sometimes given alongside calcium to prevent and treat osteoporosis, a condition where the bones become less dense (they lose bone mineral density), becoming more frail and likely to break. Fracture, especially hip fracture, can cause hospitalisation and is linked, particularly in the elderly, to an increased risk of dying from subsequent complications while in hospital.

However, the study authors reported that a number of recent research studies have questioned the effectiveness of vitamin D supplements alone for increasing bone density, preventing bone breaks and preventing osteoporosis. So the researchers aimed to review all the literature on the subject to see whether vitamin D supplementation affects bone mineral density.

What did the research involve?

The researchers searched electronic databases for randomised controlled trials assessing the effects of vitamin D (D3 or D2, but not vitamin D by-products) on bone density published before July 8 2012 (the date of the search for the literature).

The researchers included all randomised trials comparing interventions that differed only in vitamin D content and which included adults (average age >20 years) without other metabolic bone diseases.

They pooled data with a random effects meta-analysis with weighted mean differences and 95% confidence intervals (CIs) reported. This is an appropriate and standard method of meta-analysis.

The primary endpoint they were interested in was the percentage change in bone mineral density from baseline, that is, whether bones maintained their density, and whether or not they got denser through using vitamin D supplements.

What were the basic results?

The literature search identified 23 studies that were relevant to the topic and were analysed in the meta-analysis. The studies included 4,082 participants, 92% of whom were women, with an average age of 59 years and the vitamin D interventions lasted an average of 23.5 months (just under two years). Nineteen studies had mainly white participants.

Bone mineral density was measured at one of five sites:

  • lumbar spine (the bottom section of the spine in the lower back)
  • femoral neck (the top of the femur near the hip joint and commonly the place where a hip fracture occurs)
  • total hip
  • trochanter (another part of the femur near the top)
  • forearm

An overall total body bone density was also calculated.

The baseline levels of vitamin D varied a lot between the studies – the average level ranged from 30 nanomole (nmol) per litre to more than 75nmol per litre.

In 12 studies calcium supplements were also given to participants in both arms of the trial, balancing out any effect due to calcium.

Out of the 23 studies:

  • Six found a statistically significant benefit of vitamin D on bone density at one specific site – the femoral neck. Only one study showed benefit at more than one site.
  • Two found a statistically significant detrimental effect of vitamin D on bone density.
  • The rest, the majority, found no significant differences in bone density.

When they were pooled in a meta-analysis the results showed a small (0.8%) increase in bone density using vitamin D measured at the femoral neck only (weighted mean difference 0.8%, 95% CI 0.2–1.4). However, the characteristics of the underlying studies feeding into this result differed significantly (this is known as significant heterogeneity). For example, participants in the various studies were of different ages, ethnic groups or had different underlying conditions. This means that pooling the results may not be the most appropriate thing to do. No statistically significant effect at any other site was reported, including the total hip.

The authors noted that there was positive publication bias for femoral neck and total hip. This means that studies finding that vitamin D was effective were more likely to be published, and those that found no effect were less likely to be published. As a result there wasn’t an accurate balance of findings in the published literature – it was biased towards positive results.

The researchers used a range of statistical models to take into account the possible influences (confounders) on bone density. These included:

  • age
  • study duration
  • number of participants
  • sex
  • vitamin D concentration/dosage
  • weight
  • baseline bone mineral density

Even after taking these confounders into account no significant beneficial effects of vitamin D on bone density were seen, aside from the small increase in femoral neck (which, as mentioned, may have been a distorted result due to publication bias).

How did the researchers interpret the results?

The authors’ central conclusion was that “continuing widespread use of vitamin D for osteoporosis prevention in community-dwelling adults without specific risk factors for vitamin D deficiency seems to be inappropriate”.

They are quoted in The Independent as saying “our data suggest that the targeting of low-dose vitamin D supplements only to individuals who are likely to be deficient could free up substantial resources that could be better used elsewhere in healthcare”.

Conclusion

This meta-analysis of randomised controlled trials found that there was a statistically significant increase in bone mineral density at only one site (femoral neck) through giving vitamin D. This effect was very small, and was reported to be unlikely to be clinically significant at preventing osteoporosis or fracture.

This, the media and study authors suggest, calls into question the reportedly widely held belief that vitamin D supplementation is beneficial for preventing and treating osteoporosis and preventing bone fracture.

So does this contradict the current UK government recommendation that over-65s take a daily supplement?

Well, it is important to point out that preventing hip fracture is not the only reason people are advised to take vitamin D supplementation. The supplements have a variety of other important roles in the body and may be beneficial for other reasons, for example treating vitamin D deficiency due to an underlying condition such as Crohn’s disease.

Similarly, osteoporosis is known to have many influencing factors such as genetics, diet and the environment, which are not accounted for in this study. So vitamin D is just one of the potentially influential factors on osteoporosis risk.

The study authors acknowledge that their research was limited by restrictions common to the individual studies included. Some were unblinded, short-term or used low doses of vitamin D, and most participants had adequate calcium intakes. They also highlighted strengths, including the fact that the total number of participants was large, most individual studies were well powered and there were wide ranges of baseline vitamin D concentrations, vitamin D doses and dosing regimens covered.

Overall, the study findings appear to be relatively reliable.

The implication of the research was that giving vitamin D supplements to healthy individuals was a wasteful and inefficient use of healthcare resources and that “targeting of low-dose vitamin D supplements only to individuals who are likely to be deficient could free up substantial resources that could be better used elsewhere in health care”.

The UK guidance on vitamin D supplementation is under review and it would be highly surprising if this new evidence were not considered.

The bottom line is that this evidence suggests that vitamin D supplements alone may not be effective at increasing bone density in healthy people. Any changes to your medications should be discussed with your GP.

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