An Australian randomised controlled trial of vitamin D supplements, led by QIMR Berghofer Medical Research Institute has found they do not protect most people from developing colds, flu and other acute respiratory infections.

The trial, which is the largest of its kind to study the relationship between vitamin D and respiratory infection to date did show, however, that the supplements may shorten the length of infection slightly and help ease the severity of those illnesses. The study results have been published this week in the scientific journal The Lancet Diabetes and Endocrinology.

The researchers analysed self-reported health data from 16,000 Australians aged between 60 and 84, who participated in the QIMR Berghofer Medical Research Institute-led D-Health Trial. Lead researcher and head of QIMR Berghofer's Cancer Aetiology and Prevention group, Professor Rachel Neale, said 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.

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 D-Health Trial was completed before the COVID-19 pandemic began so does not provide specific answers about this. However, our findings about reduced length and severity of respiratory tract infection suggests there could be some benefit to the immune system of taking a vitamin D supplement, particularly in people who are deficient."