Vitamin D supplements and flu
“Vitamin D is better than vaccines at preventing flu,” reported_ The Times_ . The newspaper said that children’s risk of getting flu could be halved if they take vitamin D, a finding that has implications for flu epidemics.
This trial of 430 schoolchildren in Japan found that taking daily vitamin D supplements in winter decreased their risk of seasonal flu, compared to taking an inactive tablet. The news headline is misleading, as the vitamin D supplementation was only compared with this inactive placebo, not with flu vaccines.
Vitamin D is produced through exposure to natural daylight and is also present in various food sources. Most people should therefore be able to gain all the vitamin D that they need without taking supplements. Exceptions to this include pregnant women, the elderly or people who cover up their skin or rarely go outdoors. These people are advised to take 10 micrograms (0.01mg) a day.
It is important not to take above the recommended daily dose of any supplements. The FSA recommends that taking 25 micrograms (0.025 mg) or less of vitamin D supplements a day is unlikely to cause any harm.
Where did the story come from?
This study was carried out by Mitsuyoshi Urashima and colleagues from the Jikei University School of Medicine in Tokyo and other hospital departments of Japan. The medical school provided financial support. The study was published in the peer-reviewed American Journal of Clinical Nutrition.
Overall, The Times reported this study accurately. But its headline (“Vitamin D better than vaccines”) is misleading, as it suggests that the vitamin was compared to vaccines. It has only been compared with an inactive placebo, so it has not been proved that the vitamin is more effective than flu vaccines.
What kind of research was this?
This was a randomised, placebo-controlled trial investigating how vitamin D supplementation affects the incidence of seasonal flu in schoolchildren.
This type of study, a randomised controlled trial, is the best way to investigate the effectiveness of a treatment. The trial was double blinded, meaning that neither participants nor researchers knew who was getting which treatment. Randomisation should balance out differences between the groups; important ones in this case being the varying amounts of vitamin D each child received naturally through their diet and daylight exposure, and their variable exposure to people with flu.
As the trial was only four months long, its findings do not indicate the longer-term effects of taking vitamin D supplements.
What did the research involve?
The trial was conducted in 12 hospitals in Japan between December 2008 and March 2009. Researchers enrolled 430 healthy schoolchildren aged between 6 and 15 (with an average age of 10). They did not include children who were taking vitamin D supplements as part of treatment for a specific disease. However, children could be included if they were taking vitamin and nutrient supplements for general health.
General medical questionnaires were completed by the parents that included information on the child’s health and family medical history. Parents were given tablet bottles containing either vitamin D or a placebo and told that the children should take three tablets, twice daily (total dose 1,200 international units [IU] of vitamin D, or inactive placebo).
The researchers assessed the participants’ compliance by seeing how many tablets there were left at follow-up sessions (one bottle should have been consumed within 15 days).
After the study, parents completed follow-up questionnaires on whether their children had caught influenza A (diagnosed by nose and throat swab by a doctor), which was the main outcome of interest to the researchers. Influenza B and other illnesses were secondary outcomes. They also questioned the child’s compliance with study drugs, typical consumption of oily fish, eggs and shiitake mushrooms, outdoor activities, school absence and other possible adverse effects of the tablets.
What were the basic results?
Of the children who were enrolled, 334 (77.7%) completed the study, with a similar number of children dropping out from both the treatment and placebo groups. Compliance was reported to be 96% and was similar between groups. Influenza A was diagnosed in 49 children; 18 in the vitamin D group and 31 in the placebo group; which was calculated as a 42% risk reduction from taking vitamin D (relative risk (RR) 0.58, 95% confidence interval (CI) 0.34 to 0.99).
In subgroup analyses, a number of patterns in risk reduction were found. Risk of flu was significantly less in the treatment group only between days 30 and 60 of the study, not before or after this period.
Risk reduction was greatest in children who had not taken other vitamin D supplements before or during the study. The researchers found that 6% of those who developed flu in the treatment group and 16.5% of those with flu in the placebo group had never taken additional supplements of vitamin D (RR of flu with treatment: 0.36, 95% CI 0.17 to 0.79).
Risk reduction was also only significant for those who started nursery school after the age of three. This could be broken down into 7.5% of those with flu in the treatment group and 20.5% of those with flu in the placebo group (RR of flu with treatment: 0.36, 95% CI 0.17 to 0.78).
How did the researchers interpret the results?
The researchers conclude, “Vitamin D supplementation during the winter may reduce the incidence of influenza A, especially in specific subgroups of schoolchildren.”
Conclusion
This randomised controlled trial of 430 schoolchildren found that taking daily vitamin D supplements for the four winter months reduced children’s risk of being diagnosed with seasonal flu compared to taking an inactive tablet. There are a few important points:
- The Times headline, “Vitamin D better than vaccines”, is misleading, as it implies that the vitamin has been trialled against vaccines. It has only been compared with an inactive placebo drug, so there is no evidence that the vitamin is more effective than seasonal flu vaccines or any other vaccine, including the swine flu vaccine.
- Subgroup analyses found that there were some differences in the effect of vitamin D depending on certain characteristics, such as if the child had never taken supplements before, or if they started school later than their peers. However, there were very small numbers of children with flu in each of these groups (eight in the treatment group who had never before taken supplements vs 22 in the placebo group). When calculating differences between such small numbers of cases there is a higher likelihood of chance findings.
- The trial did not last long enough to examine the long-term safety effects of taking vitamin D. In particular, the researchers say they did not measure the potentially adverse effects of vitamin D on calcium metabolism. People taking supplements should not take more than the recommended daily maximum.
- The sample was relatively small and the research needs to be replicated in larger numbers of people over a longer period to confirm these results.
- Although compliance was reportedly high in this study, this may be an issue in real-life situations. Few children are going to be happy taking three tablets twice a day on a regular basis.
Vitamin D is produced through our exposure to natural daylight and is also present in various food sources including oily fish, dairy and fortified cereals and margarine. Most people should therefore be able to gain all the vitamin D that they need through natural sources, without needing to take supplements. Exceptions to this include pregnant women, the elderly or people who cover up their skin or rarely go outdoors.
It is important not to take more than the recommended daily dose of any supplement.
