"Vitamin D supplements may be pointless for preventing heart disease and cancer," reports the Mail Online. Vitamin D, known as the "sunshine vitamin" because our skin makes it from contact with sunlight, is needed to make strong bones.
Vitamin D, known as the "sunshine vitamin" because our skin makes it from contact with sunlight, is needed to make strong bones.
This study tested 25,871 adults over the age of 50 in the US over 5 years. Half of those tested took high daily doses of vitamin D.
While the results seem pretty conclusive, it's worth bearing in mind that these chronic diseases can take a long time to develop, and 5 years may not be long enough to see the potential effects.
In the UK, certain groups of people are advised to take daily 10mcg vitamin D supplements year-round. Others are advised to consider taking supplements from October to March.
Find out more about vitamin D
The study was carried out by researchers from Brigham and Women's Hospital, Harvard Medical School and the Harvard TH Chan School of Public Health in the US.
It was funded by the US National Institutes of Health and published in the peer-reviewed New England Journal of Medicine on an open access basis, so it's free to read online.
The Mail Online carried a reasonably accurate story, although their description of the design of the placebo-controlled trial wasn't right (they may have been confused by the study's separate arm looking at omega 3 oil supplements).
This was a randomised controlled trial (RCT) of high-dose vitamin D. RCTs are the best way to find out if a treatment works.
Researchers followed them for an average of 5.3 years, questioning them annually about diagnoses of cancer, heart attack or stroke. They also checked to see if they had died and, if so, of what cause.
About 65% of people had their vitamin D levels measured at the start of the study. The average level was 77nmol/l, above the 50nmol/l recommended for bone strength. Only 12.7% had levels below 50nmol/l.
Researchers looked for potential variations in effect caused by ethnic group, age, sex, body mass index (BMI), baseline vitamin D level, use of omega 3 oils, other use of vitamin D supplements and baseline risk factors.
The numbers were split fairly evenly between those who'd taken vitamin D supplements and those who hadn't.
In the vitamin D group, 793 people were diagnosed with cancer, and 824 people in the placebo group – so close that the difference could easily be down to chance (hazard ratio [HR] 0.96, 95% confidence interval [CI] 0.88 to 1.06).
In the vitamin D group, 396 people in the vitamin D group were diagnosed with a heart attack or stroke, and 409 people in the placebo group – so close that the difference could easily be down to chance (HR 0.97, 95% CI 0.85 to 1.12).
While this may seem like a big difference, it's still not enough to be statistically significant – in other words, not a big enough difference that we can be sure it was caused by vitamin D (HR 0.83, 95% CI 0.67 to 1.02).
The finding about cancer death that the Mail Online referred to was a separate analysis done after the study, which found a 25% reduction in risk of death from cancer, but only if you excluded the results from the first 2 years of follow-up (HR 0.75, 95% CI 0.59 to 0.96).
We need to be cautious about this finding because the study wasn't set up to look at this result, and making lots of different comparisons using the same set of results can throw up unreliable results.
The researchers concluded: "Daily supplementation with high-dose vitamin D for 5 years among initially healthy adults in the US did not reduce the incidence of cancer or major cardiovascular effects."
The results from the study suggest that vitamin D supplements are unlikely to be of use in preventing heart attacks, strokes or cancers.
This large, well-run RCT was set up to investigate whether these conditions could be prevented by giving people supplements of 50mcg vitamin D (5 times the amount recommended in the UK).
If vitamin D had a significant effect, you'd expect to see this reflected in these results. But this study had a few limitations.
Most people in the study had vitamin D levels above the recommended levels, which means they may not have required additional vitamin D.
But it would be unethical to carry out a study where people known to have low levels of vitamin D weren't given treatment to correct that.
The study's ethnic background mix was 71% white, 20% black and 4% hispanic, which is representative of the US population, but not necessarily of the UK population.
Find out more about healthy living
While vitamin D may not reduce the risk of cancer or cardiovascular disease, supplements are still recommended for certain groups of people in the UK for healthy bones, teeth and muscles. That's because these groups may not get enough vitamin D from sunlight.