'No evidence' that vitamin D prevents cancer or heart attacks
"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.
In recent years, scientists have investigated whether it's also helpful in preventing cardiovascular disease (heart attacks and strokes) or cancer.
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.
The results showed no difference in the number of people who developed cancer or cardiovascular disease, or in those who died from cancer, cardiovascular disease or any other cause.
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.
And even if vitamin D supplements have no benefit in preventing cancer or heart attacks, they're important for keeping bones, teeth and muscles healthy.
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.
Where did the story come from?
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).
They also stated that vitamin D "did seem to reduce cancer deaths (but not diagnoses) by around 25%". The full story around cancer deaths is actually more complicated than that.
The Independent ran a balanced story that also included information about the omega 3 trial, which similarly found little benefit for most people in terms of cardiovascular disease and cancer.
What kind of research was this?
This was a randomised controlled trial (RCT) of high-dose vitamin D. RCTs are the best way to find out if a treatment works.
In this case, the researchers wanted to see if taking high-dose vitamin D could prevent cancer and cardiovascular disease.
What did the research involve?
Researchers recruited men over 50 and women over 55 from across the US. In total, 25,871 people were eligible (they had no history of cancer or cardiovascular disease) and agreed to take part.
Researchers questioned them about risk factors for cancer and cardiovascular disease, and randomly assigned them to take either 50mcg vitamin D supplements or a placebo. Half were women.
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.
They recorded figures for different types of cancer, cardiovascular disease and cardiovascular treatments.
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.
What were the basic results?
Of the 25,871 people in the study, 1,617 (6.25%) were diagnosed with cancer and 805 (3.11%) had a fatal or non-fatal heart attack or stroke.
The numbers were split fairly evenly between those who'd taken vitamin D supplements and those who hadn't.
Cancer
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).
Cardiovascular disease
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).
Cancer deaths
In addition to these main results, researchers looked in more detail at deaths from cancer. They found 154 people died from cancer in the vitamin D group and 187 in the placebo group.
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.
How did the researchers interpret the 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."
Conclusion
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 were followed up for 5.3 years, which is a relatively short period of time for a study looking into effects on slowly developing conditions like cancer or cardiovascular disease.
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.
If you want to reduce your risk of cancer or cardiovascular disease, there are plenty of ways to keep healthy that are likely to be more effective than taking vitamin D pills.
They include:
- eating a healthy, balanced diet with plenty of vegetables, fruit and wholegrains
- keeping your blood pressure and cholesterol at healthy levels
- not smoking and not drinking alcohol above recommended levels
- exercising regularly
- keeping to a healthy weight
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.
