Most people who have psoriasis have used sunlight or UV light in one form or another at some time in the treatment if their condition, whether under the direction of a dermatologist or as self-treatment. It has long been known that psoriatic lesions react positively to UV light.
When I lived in South Africa I used the sun a lot. Being a surfer and a sailor, I spent many hours each year in or on the water, with my bare skin absorbing the sunshine. I have worked from home from soon after I was diagnosed, so on any sunny day I also spent lunch time naked in my back garden. Even the winter sun was able to help to keep my skin clear and it was only in late winter or early spring that my all-over tan would fade somewhat and my skin deteriorate to the point of showing lesions on my lower back and elbows. By mid to late summer they would be completely gone again.
After we moved to USA, the situation changed. The combination of a less healthy diet and weaker sunshine was bad for me. It proved to be easier to improve my diet than to convince the earth to let more UV light through for me to use. The hole in the ozone layer makes the sunshine stronger in the southern hemisphere than north of the equator. While the summer sun is hotter here in Virginia Beach than in Cape Town, the UV rays are weaker.
I found that to compensate I had to resort to using a sunbed to increase my UV exposure in the cooler months, with at least weekly visits to a tanning salon. Even that didn't fully do the job, although it did keep my skin nicely golden. Note that it is important to apply plenty of moisturiser to your skin multiple times a day if using a sunbed, or you will not only become golden but also crisply crinkled. I moisturised all over first thing each morning and after tanning. I also use a moisturising bar rather than soap in the shower.
Then one day a thought crossed my mind that maybe my body lacked Vitamin D rather than UV light. After all, our bodies produce Vitamin D from UV light. So, I went on-line and Googled Vitamin D and psoriasis. I found one reference to research that is being done into the effects of increasing Vitamin D intake to treat psoriasis, which said that the preliminary results were positive. I had also read elsewhere that there is new thinking and research into the Vitamin D requirements of the human body, which shows that the Recommended Daily Allowance that has been used for decades is very inadequate.
That prompted me to try Vitamin D supplements over the past 6 months or so. I started early autumn, when my skin should have been starting to deteriorate. In previous years I would have started to visit the tanning salon by about the beginning of December. This year I have not had to use a sunbed even once. Now, about the middle of spring, my skin is about the same as it would have been in Cape Town at this stage of the season. I have three small lesions on my back and a few tiny pink spots elsewhere. Few people looking at me on the beach would realise that I have a skin condition.
While this is not a scientific finding by any means, it is a good indication to me that I have benefitted by using a Vitamin D supplement. The drawback (there is always a flipside to the coin) is that I have to work a bit on my tan this spring, which I have not had to do before. The other flipside (this coin has three sides) is that I am exposing myself less to the risks of skin cancer without the sunbed.