But vitamin D almost certainly does help with certain conditions. For example, I have psoriasis and can develop awful (painful, itchy, ugly) plaques on my knees and elbows, as well as painful skin nodules on the palmar surfaces of my hands and feet and dandruff. I had seen a dermatologist who prescribed some cream that sort of worked, and she recommended I expose the surfaces to 15 mins of sunlight a day. Those kind of worked, but the plaques and dandruff remained. I started supplementing Vitamin D on my own (my PCP recommended it since I'm slightly deficient), and the plaques disappeared within a few weeks. The nodules only rarely appear when I'm having a stressful time. If I forget to take vitamin D for a few days (or, in the case of the pandemic, when I run out of it and can't get a timely resupply), voila, the plaques reappear. Now that isn't a peer reviewed, double-blind controlled study or anything, but the signal is extremely strong that vit. D supplementation fixes the bulk of my psoriasis symptoms. Skepticism is warranted about all vitamin claims, but my priors for vitamin D are that it may actually be effective even when the peer reviewed research isn't out. My only caution to people is that since it's fat soluble, people should try to determine the right dose before gobbling it down.