Vitamin D is not really a vitamin. When scientists worked out what it actually does in the body, they found it behaves like a hormone, a messenger that talks to cells all over you. By then the name had stuck, so vitamin D it stayed.
That is why it matters so much. It is not doing one job in one place. Receptors for it sit in your bones, your muscles, your immune cells and your brain.
When your levels are where they should be, it helps your body take up calcium and keep your bones and teeth strong, it supports your muscles, and it supports the immune system that gets you through winter.
When your levels are low, most people do not feel ill. They feel flat. Tired in a way that sleep does not fix. Achy through the back and hips. Low in mood, especially in winter. Catching every cold that goes around, and taking longer than everyone else to shake it.
We live in one of the sunniest countries on earth and I still see low levels constantly, because most of us work indoors, cover up, wear sunscreen, and darker skin needs far more sun to make the same amount.
One tablet a day, with food, because vitamin D needs a little fat to be absorbed properly.
And if you want to know where you actually stand, ask your doctor for a blood test. It is the only way to be sure, and it is worth knowing.