a Naturopath / ER Doc I know says 1-2 hours a day in sunlight is more than enough to cause your body to create adequate Vit D. Outside of that would need a blood draw to find if deficient. D helps support the immune system, which when you look at the "lockdown" part of this past year and change, a lot of people stayed inside becoming deficient just from that….leading to weakened immune systems…. simple math once you think about things.