Clinical trial finds vitamin D supplements do not ward off colds and flu

Respiratory infections are generally highest during winter months when people’s vitamin D levels are lowest. However, QIMR Berghofer researchers have found that supplementing the general population does not reduce the number of infections.

As we brace for cold and flu season and the onslaught of seasonal respiratory illnesses during the cooler months, Australians often turn to supplements like vitamin D in the hope of warding off winter ills.

The QIMR Berghofer-led D-Health clinical trial is the largest of its kind to study the relationship between vitamin D supplements and respiratory infection to date, and the results from the multi-year study have recently been published.

Researchers analysed self-reported health data from 16,000 Australians aged between 60 and 84, who were given either a capsule of 60,000 international units of vitamin D, or a placebo, every month for up to 5 years.

Professor Rachel Neale from QIMR Berghofer’s Cancer Aetiology and Prevention Group was the lead researcher on the study, which found vitamin D supplements did not protect most people from developing colds, flus and other acute respiratory infections.

‘Our clinical trial showed that people who took vitamin D supplements were infected with colds and flus at the same rate as those who were given placebo tablets,’ Professor Neale said.

However, participants who received vitamin D supplements reported they had cold and flu symptoms for slightly less time than participants who did not take the vitamin. They also had severe symptoms for less time and needed less medication to manage their symptoms.

‘The findings suggest that vitamin D might give the immune system a little boost, but supplementing the general population with vitamin D is unlikely to protect people from getting sick in the first place and won’t markedly improve the speed of recovery.’

Professor Neale said it was important to note that most of the D-Health participants were not vitamin D deficient at the start of the study. She commented, ‘It is possible that treating people who are vitamin D deficient would reduce their risk of respiratory tract infection.’

‘This study suggests that taking supplements will not have significant benefits for warding off colds and flus in people who are not vitamin D deficient.’

Professor Neale said the trial results are timely as people around the globe try to understand how best to improve their immunity in the face of the COVID-19 pandemic.
‘The studies that have been published do suggest that having higher vitamin D levels may reduce the risk and severity of COVID-19, but many of these studies have had fairly serious limitations, so the results are not entirely reliable,’ she said.

‘The D-Health Trial was completed before the COVID-19 pandemic began so does not provide specific answers about this. However, our findings about the reduced and severity of respiratory tract infection suggests that taking a vitamin D supplement may boost the immune system. The existing evidence suggests that it is prudent to avoid being vitamin D deficient, but more is not necessarily better.’

Professor Neale said anyone who was concerned about their vitamin D levels should speak to their doctor.

Footnote: The D-Health Trial was funded by the National Health and Medical Research Council of Australia and the study results have been published in the scientific journal The Lancet Diabetes and Endocrinology.

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