(CNN) Taking daily vitamin D and fish oil supplements may help protect older adults from developing autoimmune disorders such as rheumatoid arthritis, psoriasis, thyroid diseases and polymyalgia rheumatica, an inflammatory disease that causes muscle pain and stiffness in the shoulders and hips, a new study found.
People age 50 and older taking 2,000 IU (International Units) of vitamin D3 for over five years had a 22% lower relative rate of confirmed autoimmune diagnoses, said 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.
That dosage is two to three times the recommended daily dose of vitamin D for adults, which is 600 IU for people up to 69 years old and 800 IU for those age 70 and up, according to the National Institutes of Health.
Once people had been taking vitamin D for at least two years, the prevention rate from autoimmune disorders rose to 39%, according to the study, published Wednesday in the journal BMJ.
The study also found a possible link between taking 1,000 milligrams of omega-3 fatty acid (fish oil) and a reduction in autoimmune disorders, but the association was not statistically significant until possible cases of autoimmune disease -- not just confirmed cases -- were factored into the analysis.
However, the study did find that taking both vitamin D and omega-3 fatty acid supplements, versus the placebo alone, decreased autoimmune disease by about 30%.
People should not just run out and start popping vitamin D pills to boost their chances of avoiding autoimmune disease, Costenbader warned, as there are significant consequences to taking too much of the supplement.
Unlike water-soluble vitamins, which the body can easily eliminate, vitamin D is stored in the fat cells of the body and can build up to toxic levels, leading to bone pain and kidney damage.
Because the body makes vitamin D when the skin is exposed to sunshine, and milk and other foods like cereals are often fortified with vitamin D, many experts say healthy, younger people are not likely to require vitamin D supplements, especially in amounts over the recommended level of 600 IU/day.
Levels do drop in older age, but "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," Costenbader said. "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."
Costenbader's study analyzed 25,871 men and women age 50 and older who were participating in VITAL, a randomized, double-blind, placebo-controlled research study designed to see whether taking daily dietary supplements of vitamin D3 (2000 IU) or omega-3 fatty acids (1,000 mg of Omacor fish oil) would reduce the risk for developing cancer, heart disease and stroke in people with no prior history of these illnesses.
That trial showed no benefits from the extra supplementation in preventing either cardiovascular disease or cancer.
Because prior research has shown both vitamin D and omega-3 fatty acids derived from seafood can have a positive effect on inflammation and immunity in autoimmune disorders, Costenbader decided to use the same trial to investigate whether the supplements might prevent such diseases.
Autoimmune disease occurs when the body's natural defense system suddenly sees normal cells as invaders and begins destroying those cells by mistake. In rheumatoid arthritis, for example, the immune system attacks the lining of joints, creating inflammation, swelling and pain. With psoriasis, overactive T-cells -- which are among the body's best defenders -- cause inflammation that creates raised, scaly patches on the skin.
In Type 1 diabetes, the body's defenders destroy the insulin-producing cells of the pancreas. There's even some evidence to show that inflammation throughout the body might be part of the progression of Type 2 diabetes.
Autoimmune disorders can develop at any stage of life but do appear more among older adults, particularly women, Costenbader said.
To date, no large randomized clinical trials (considered the gold standard of research) had investigated whether fish oil and vitamin D could actually prevent the development of autoimmune diseases.
"This is the first direct evidence in older adults that taking vitamin D or omega-3 fatty acids -- or a combination -- for five years reduces autoimmune disease incidence, with more pronounced effect after two years of supplementation," Costenbader said.
At five years into the research, the study could not tease apart which of the 80 or more autoimmune diseases might benefit most from vitamin D and fish oil supplements, Costenbader said, but research is continuing. The study is now in its seventh year, she said. and more data should be released in the future.