Autoimmune diseases, where the immune system attacks healthy cells by mistake, are a growing problem. In the industrialized world, they are the third leading cause of overall morbidity and the leading cause of morbidity among women. Many times, there’s no effective cure for such diseases and the best available approach is only aimed at reducing symptoms.

Previous studies have suggested that vitamin D could have an effect on autoimmune disease. To put that to the test, led by Jill Hahn, researchers from Harvard and Brigham and Women’s Hospital, Boston, carried out a study on almost 26,000 participants (mean age 67.1 years), analyzing the effect that vitamin D and omega-3 supplements have on autoimmune disease.

They split participants randomly into two groups, giving them either vitamin D supplements or a placebo. They then looked at the differences between these two groups. It was a randomized, double-blind trial — in which neither the participants nor the researchers knew who was in which group, to remove any potential bias and influence from the study.

The team then tracked participants for five years.

Overall, a dose of 2000 international units (IU) of vitamin D per day reduced the risk of developing an autoimmune disease by 22% compared to the placebo group. Surprisingly, when researchers looked at only the last three years of the intervention, the vitamin D group had 39% fewer participants that developed autoimmune diseases.

This dose is larger than the standard 400-800 IU recommended by most health organizations. However, consuming 1,000 IU or even more has been linked to various health benefits, and there seem to be no negative side effects up to 4,000 IU.

We get vitamin D from two sources: from foods, and from the Sun. Foods such as fatty fish, cheese, or mushrooms are rich in vitamin D. Your body also produces vitamin D when exposed to sunlight, which is why it’s recommended that especially in places with lower exposure to sunlight, either foods or supplements rich in vitamin D should be considered.

Previous studies have suggested that vitamin D levels are correlated with the risk and severity of autoimmune diseases, as well as several other diseases such as MS, heart disease, and influenza. A Danish study even found a 49% reduction in rheumatoid arthritis risk for each 30 g increase in daily fatty fish intake. However, the mechanisms through which vitamin D exhibits these protective effects are unclear, and researchers warn that you shouldn’t rush to take as much Vitamin D as you can.

“I would say everybody should talk to their doctor first before taking 2000 international units of vitamin D on top of whatever else you’re taking,” study author Dr. Karen Costenbader, a professor of medicine at Harvard Medical School in the division of Rheumatology, Inflammation and Immunity and the director of the lupus program at Brigham and Women’s Hospital in Boston, told CNN. “And there are certain health problems such as kidney stones and hyperparathyroidism (a rise in calcium levels), where you really shouldn’t be taking extra vitamin D.” Speaking to NewScientist, Costenbader added that there are several known potential mechanisms, such as, for instance, vitamin D aiding the immune system tell the difference between healthy cells and disease-causing microbes.

The study also reported a possible link between omega-3 supplements and a reduction in autoimmune disorders, though this was not immediately apparent. It was only when researchers also considered the possible cases of autoimmune disease (and not just the confirmed cases) that the association emerged.

Now, the researchers are working on another survey that also includes younger participants to assess the impact of vitamin D on autoimmune diseases and see how long the benefits last.

The study was published in the British Medical Journal.