Taking a combination of vitamin D supplements and omega-3 fish oil appears to carry a lower risk of developing autoimmune disease, finds a trial of older adults published by the British Medical Journal (BMJ).

The researchers say the clinical importance of these findings is high, given that autoimmune diseases, such as rheumatoid arthritis, psoriasis, and thyroid diseases, are all common conditions among the older population, and there are currently no other known effective therapies to reduce the risk of developing such conditions.

More than 25,000 participants took part in the trial

While vitamin D and omega-3 fatty acids derived from seafood are known to have a beneficial effect of inflammation and immunity, no large randomised trials have tested whether these supplements can lower the risk of autoimmune disease.

So, researchers set out to test the effects of these supplements on rates of autoimmune disease in 25,871 adults living in the USA. 

The average age of the participants was 67 years old, 51% were women and 71% were non-Hispanic white.

As well as details about their age and ethnicity, participants also provided details about their income, education, lifestyle, weight, medical history, diet and supplement use. 

Participants were then randomly allocated to receive either Vitamin D and fish oil supplements or placebos, and were asked to report any diagnosed autoimmune disease during the five-year follow-up period. 

These included rheumatoid arthritis, polymyalgia rheumatica (pain and stiffness in the muscles around the shoulders, neck and hips), thyroid disease, and psoriasis.

Vitamin D and omega-3 fatty acid supplements decreased autoimmune disease by around 30% compared to placebo alone

During the follow-up period, autoimmune disease was diagnosed in in 123 participants in the vitamin D group compared with 155 in the placebo group - a 22% lower relative rate.

In the omega-3 fatty acid group, 130 confirmed cases were diagnosed compared with 148 in the placebo group (a 15% reduction), but this was not a statistically significant result.

However, when 'probable' cases were included (when signs and symptoms were present but there wasn't sufficient documentation to confirm an autoimmune disease) omega-3 fatty acid supplements did significantly reduce the rate by 18% compared with placebo.

Paired together, vitamin D and omega-3 fatty acid supplements decreased autoimmune disease by about 30% versus placebo alone. 

The study authors are continuing with their research

The researchers say that further research should now be conducted to test these interventions in younger populations, and those with high autoimmune disease risk. They are also continuing with their research to see the effect of supplementation over time. 

They write: “We are continuing to follow participants for two years in an extension study to test the time course of this autoimmune disease reduction effect."