If you have psoriatic arthritis, you may have heard that people with the condition—which causes painful, swollen, stiff joints—often have low levels of vitamin D. In a 2015 study published in Arthritis Research & Therapy, researchers found that 40.9% of participants with psoriatic arthritis had a vitamin D deficiency, compared to 26.7% of control participants. Other autoimmune diseases have also been linked to low levels of the sunshine vitamin (so-called because the body produces vitamin D when it's exposed to sunlight—you can also get some vitamin D from food, but sunshine is the main source). In the same study as above, 40.5% of rheumatoid arthritis patients were found to have a deficiency, as did 57.8% of psoriasis patients in earlier research from the British Journal of Dermatology.

Experts believe inflammation may have something to do with this. Autoimmune diseases like psoriatic arthritis and psoriasis involve an inflammation process, explains Waseem Mir, MD, a rheumatologist at Lenox Hill Hospital in New York City. "We think that inflammation causes a decrease in vitamin D," he told Health. "[It’s] not because [people] don’t have enough vitamin D in their body, but they’re not processing it correctly."

It makes sense. The body needs vitamin D to absorb calcium. Both calcium and vitamin D work to promote healthy bones, and vitamin D also seems to be involved in proper functioning of the immune system. And the bones and immune system are both compromised in people who have psoriatic arthritis.

RELATED: 31 Ways to Manage Your Psoriatic Arthritis

Can vitamin D treatment help?

Dr. Mir points out that there's been some research to suggest that vitamin D treatment may help ease joint pain in certain people with psoriatic arthritis. One such treatment is vitamin D supplements. If you have psoriatic arthritis, speak to your doctor about your vitamin D levels to find out if supplements are right for you.

Another treatment is phototherapy (careful exposure to ultraviolet rays), but there isn't enough research to recommend this for psoriatic arthritis patients. And although you can feel free to add more vitamin D-rich foods to your plate (think: fatty fish like salmon and tuna, certain kinds of mushrooms, and fortified milk) it's unlikely that you'd be able to reverse a true deficiency through diet alone.

In his own practice, Dr. Mir usually prescribes liquid vitamin D to patients, which he says helps get a better response.

"That is the most effective," he says. "A lot of it is absorbed through the mouth."

However, it is possible to get too much vitamin D, which can result in a build-up of calcium in the blood and possibly lead to nausea, vomiting, and kidney problems. Speak to your doctor before adding any new supplements to your diet.