'Add vitamin D to food to prevent colds and flu', sa...

"Adding vitamin D to food would reduce deaths and significantly cut NHS costs," The Guardian reports.

A review of existing data estimates that supplementing food with vitamin D would prevent millions of cold and flu cases, and possibly save lives.

Researchers looked at data from 25 previous studies where vitamin D was compared with a placebo.

The studies explored the effect of vitamin D in preventing acute respiratory tract infections. These are infections of the body's airways, such as colds, flu, bronchitis and pneumonia. More than 10,000 people were involved in total.

Their analysis suggests daily or weekly vitamin D supplementation was useful in preventing respiratory tract infections. Perhaps unsurprisingly, supplementation was particularly beneficial for people who had very low levels of vitamin D.

The researchers concluded that these results add to the body of evidence that fortifying widely eaten foods with vitamin D would improve public health.

But this opinion is not shared by all experts in the UK. Professor Louis Levy, head of nutrition science at Public Health England (PHE), said: "The evidence on vitamin D and infection is inconsistent, and this study does not provide sufficient evidence to support recommending vitamin D for reducing the risk of respiratory tract infections."

As this debate is ongoing, it would seem sensible to stick to the relatively new guidelines about vitamin D – that is, everyone should consider taking supplements during the winter months.

Where did the story come from?

The study was carried out by researchers from a number of institutions, including Queen Mary University, Winthrop University Hospital in the US, and the Karolinska Institutet in Sweden.

No manufacturers of vitamin D supplements were involved in this research. Funding was provided by the National Institute for Health Research.

The study was published in the peer-reviewed British Medical Journal on an open access basis, so it's free to read online.

The findings of this research have been widely covered by the UK media, and the reporting has been accurate.

A number of quotes are provided from experts across all media sources to provide both sides of the argument for vitamin D supplementation and the reaction to this particular review.

The British Medical Journal itself includes an editorial from independent experts arguing that food should not be routinely fortified with vitamin D.

What kind of research was this?

This systematic review and meta-analysis investigated the use of vitamin D supplementation as a way of preventing acute respiratory tract infections such as flu, bronchitis and pneumonia.

This type of review is the best way of gathering all available evidence on a topic – but the findings can only be as reliable as the studies included. The researchers therefore only included high-quality evidence from randomised controlled trials (RCTs).

What did the research involve?

The researchers searched four literature databases and two clinical trial registries to identify RCTs that looked at the overall effect of vitamin D supplementation on risk of acute respiratory tract infection.

To be included in the review, studies had to compare vitamin D3 or D2 with a dummy pill (placebo).

They also had to be double blind, meaning that neither the participant nor the doctor knew which pill they were taking.

Finally, the trial needed to set out to look at the rates of respiratory infection, rather than this being an incidental finding.

The studies included in the analysis were all considered to be high quality according to a validated assessment using the Cochrane risk of bias tool, which assesses bias and skewed reporting.

The primary outcome of the meta-analysis was incidence of acute respiratory tract infection of any location.

When analysing the pooled data, the researchers adjusted for the potential confounding effects of age, sex and duration of the study.

They also performed subgroup analyses, where a data set is split into smaller groups to check for possible patterns, to see if other factors such as asthma and body mass index affected the results.

But the researchers weren't able to analyse the results according to whether people had chronic obstructive airways disease (COPD) or if they had been given a flu vaccine.

What were the basic results?

In total, 25 RCTs from 14 countries, including the UK, were included. These involved a total of 11,321 participants, aged from 0-95 years.

After pooling the findings, vitamin D supplementation was found to reduce the risk of acute respiratory tract infection by 12% (adjusted odds ratio [aOR] 0.88, 95% confidence interval [CI] 0.81 to 0.96).

By splitting the participants into smaller subgroups, a statistically significant protective effect was seen for those who had daily or weekly vitamin D supplementation without large one-off doses (aOR 0.81, 95% CI 0.72 to 0.91) but not for those receiving one or more large one-off doses (aOR 0.97, 95% CI 0.86 to 1.10).

Among those receiving daily or weekly vitamin D, protective effects were stronger for those with lower vitamin D levels at the start of the study and people with asthma.

There were no serious adverse events or deaths linked to supplementation.

How did the researchers interpret the results?

The researchers concluded: "Our study reports a major new indication for vitamin D supplementation: the prevention of acute respiratory tract infection.

"We also show that people who are very deficient in vitamin D and those receiving daily or weekly supplementation without additional bolus doses experienced particular benefit.

They added: "Our results add to the body of evidence supporting the introduction of public health measures such as food fortification to improve vitamin D status, particularly in settings where profound vitamin D deficiency is common."

Conclusion

This was a systematic review and meta-analysis investigating the use of vitamin D supplementation as a way of preventing acute respiratory tract infections such as flu, bronchitis and pneumonia.

The study found vitamin D supplementation to be useful in the prevention of acute respiratory tract infection. People who are very deficient in vitamin D and those receiving daily or weekly supplementation without additional large one-off doses had a larger benefit.

This study has both strengths and limitations. It is very well designed and includes high-quality evidence. The researchers made efforts to reduce the risk of bias and investigate possible areas where bias may exist in their study.

They provided the following limitations:

Analysis suggests the results may have been subject to some degree of publication bias, so some small trials showing adverse effects of vitamin D may not have been included.
The study was not adequately powered to detect effects of vitamin D supplementation in some subgroups, such as people with COPD.
Data relating to adherence to supplementation was not available for all participants.

PHE guidelines published in the summer of 2016 recommend adults and children over the age of one should consider taking a daily supplement containing 10 micrograms (mcg) of vitamin D, particularly during autumn and winter.

People who have a higher risk of vitamin D deficiency are being advised to take a supplement all year round.

But PHE currently does not recommend the routine fortification of common foodstuffs with vitamin D.

In the words of Professor Louis Levy, head of nutrition science at PHE: "The evidence on vitamin D and infection is inconsistent, and this study does not provide sufficient evidence to support recommending vitamin D for reducing the risk of respiratory tract infections."

Another way to protect yourself during the winter is to make sure you have the seasonal flu jab if you're vulnerable to the effects of flu.

Read more about who should have the seasonal flu vaccination.