Vitamin D deficiency has been linked to psoriasis. Vitamin D deficiency is not the cause of psoriasis, but low levels may increase flares and interfere with the body’s immune function and ability to keep the skin healthy. Vitamin D taken orally or topically can help treat psoriasis. Taken as a supplement internally, vitamin D helps boost the immune system. Since psoriasis is an autoimmune disease, this can help alleviate symptoms of the disorder. Applied topically, vitamin D can slow the rate of cell proliferation or cell growth and hence help to thin psoriasis plaques. You can also increase your vitamin D levels by exposure to sunlight—15 minutes a day for fair skin up to 30 minutes for darker skin—and from dietary sources. Foods high in vitamin D include fatty fish, such as tuna, mackerel, and salmon, and foods fortified with vitamin D, including some dairy products, orange juice, soy milk, and cereals. Vitamin D is fat-soluble, so it is essential to have your doctor monitor your levels to avoid unhealthy buildup, also called hypervitaminosis D, or vitamin D toxicity. Hypervitaminosis can cause a buildup of excess calcium in the blood, which can cause frequent urination and weakness and can lead to kidney concerns. This is typically only caused by excessive doses of  vitamin D supplements and is unlikely to occur from diet or sunlight exposure.