Lifestyle and exercise

'No evidence' that vitamin D protects against colds

There is “no proof vitamin D stops colds,” BBC News has reported, as “scientists say they can find no convincing evidence to show that taking vitamin D supplements will fend off a cold”.

This news comes from a well-designed trial into whether vitamin D reduced the incidence or severity of upper respiratory tract infections (URTIs) in healthy adults. URTIs are infections that affect the nose, sinuses and throat and include the common cold and flu.

Some have argued that vitamin D may play a role in protecting against cold. This is because natural levels of vitamin D drop during the winter (vitamin D is primarily produced when the skin is exposed to sunlight). This drop in vitamin D levels may lead to a drop in immune function, making people more vulnerable to URTIs. It has been suggested that taking supplements is a way to boost immune function and protect against infection.

To test this theory, researchers gave vitamin D to 161 healthy adults for 18 months while a further 161 were given a dummy pill (placebo). Every month the study participants were asked about the number and severity of URTIs they had. The results showed there was no difference in the number of URTI episodes, or the severity of the infection, between the groups during this period.

Significantly, the study mainly included people with normal or near-normal levels of vitamin D, so there may still be a role for supplements in those who are already deficient in vitamin D.

For most of us, there remains no easy way to avoid getting winter sniffles other than washing our hands regularly to avoid germs.

Where did the story come from?

The study was carried out by researchers from universities and medical departments in New Zealand and the Harvard Medical School in the US. It was funded by the Health Research Council of New Zealand.

The study was published in the peer-reviewed Journal of the American Medical Association.

The BBC coverage was well balanced. It included an opposing view from Professor Ronald Eccles, a “leading UK cold expert”, who said he takes vitamin D as a precaution in the winter months. However, in the BBC article he also said that supplementation was pointless unless the person’s vitamin D levels were lower than normal. 

What kind of research was this?

This was a randomised double blind placebo-controlled trial (RCT) that examined the effect of vitamin D supplementation on the incidence and severity of upper respiratory tract infections (URTIs).

URTIs are infections which affect the nose, sinuses and throat and include the common cold, tonsillitis, sinusitis, laryngitis (inflammation of the vocal cords) and influenza (flu).

The researchers reported that previous observational research has linked low vitamin D levels to a higher incidence of URTIs. In observational studies, researchers have no control over exposures and instead observe what happens to groups of people, so the studies are more prone to bias. For example, observational studies often rely on the participants self-reporting, which can make results more subjective and open to bias. 

Previous results from RCTs, a more vigorous study design than observation studies, have proven inconclusive.

What did the research involve?

This randomised, double-blind, placebo-controlled trial recruited 322 healthy adults between February 2010 and November 2011 in Christchurch, New Zealand.

Participants were then randomly assigned to receive either vitamin D supplementation (a group of 161 people) or placebo (the other 161). Vitamin D supplementation was given orally at an initial dose of 200,000 IU (international unit), for the first month then at monthly doses of 100,000 IU thereafter. A dose of 100,000 IU of vitamin D a month is equivalent to 2.5mg. The placebo was administered at an identical dosing schedule and was identical in appearance but contained no active ingredients. Both the vitamin D and placebo treatments were given on a monthly basis for 18 months.

The participants met research staff monthly to receive their dose of placebo or vitamin D. Neither the participants nor the researchers knew whether the participant was receiving vitamin D or placebo. During the visit, researchers asked questions about episodes of URTI during the preceding month. The participants were also asked to contact study staff whenever they experienced a URTI, defined as the sudden onset of one or more cold-like symptoms, such as a runny nose, nasal stuffiness, sore throat or a cough that the participant did not attribute to an allergy.

The researchers were primarily interested in the effect of the vitamin D supplementation on the number of URTI episodes. They also measured duration of URTI episodes, their severity and the number of days of work the participants missed due to the URTI episode. 

The analysis compared the frequency, duration, severity and time off work due to URTIs in the vitamin D group with the placebo group. The analysis was based on the initial treatment allocation, a so-called “intention to treat” analysis, which is the most appropriate comparison. In an intention-to-treat analysis the participants are analysed in the groups they were organically randomised into, regardless of whether they received or adhered to the allocation intervention. This gives a more realistic estimate of the effect of the treatment in the real world where not everyone will follow the exact treatment prescribed. For example, some people may stop taking the drug earlier than advised.

What were the basic results?

A high proportion of the participants (294, or 91%) completed the study and 18-month follow-up, with only three missed monthly appointments throughout the study.

There were 593 URTI episodes in the vitamin D group compared with 611 in the placebo group. This was not a statistically significant difference and equated to 3.7 URTIs per person in the vitamin D group and 3.8 URTIs per person in the placebo group (risk ratio 0.97, 95% confidence interval 0.85 to 1.11).

There was also no significant difference between the vitamin D group and placebo for the number of days missed from work as a result of the URTIs, their severity or the duration of symptoms. These findings remained unchanged when the researchers took into account variation due to the season and the vitamin D levels of the participants at the start of the study (before they were given supplements or placebo).

How did the researchers interpret the results?

The researchers concluded that a “monthly dose of 100,000 IU of vitamin D in healthy adults did not significantly reduce the incidence or severity of URTIs”.


This well-designed double-blind placebo controlled trial showed that a monthly dose of 100,000 IU (2.5mg) of vitamin D did not significantly reduce the incidence or severity of URTIs in healthy adults with normal vitamin D levels.

The study had many strengths, including:

  • the relatively large study size
  • the 18-month study duration (longer than previous studies)
  • the relatively high dose of vitamin D given
  • the method of recording URTI episodes (monthly interview and notifications)

This study provides relatively strong evidence that monthly vitamin D supplementation is no better than a dummy pill for preventing URTIs in healthy adults who had near-normal vitamin D levels to start with.

However, the study authors pointed out that:

  • The baseline levels of vitamin D in both groups were relatively normal on average. It is possible that supplementation may have an effect if people were deficient in vitamin D. Only a small number of participants in this study had low levels of vitamin D to start with, and this was not enough to reliably compare their results with the rest to explore this possibility.
  • It is possible that daily vitamin D supplementation may have different effects from the monthly dose used in the trial, but this is speculative. Further research would be needed to confirm this.

The main conclusion of this research is that healthy adults who are not deficient in vitamin D will not effectively reduce their risk of, or severity of, coughs, colds and sore throats by taking a monthly dose of vitamin D.

The effect of vitamin D supplementation on adults who are vitamin D deficient is still largely unknown and requires further research.

Making sure you wash your hands and eat a healthy diet remains the best advice on avoiding coughs and colds.

NHS Attribution