If you don't have a primarily fish based diet or spend hours under direct overhead sunlight every day without a shirt on, you're probably deficient as well.
> Recommendations to Obtain Adequate Amount of Vitamin D.
> Sun exposure to the face, arms, hands, and legs without sunscreen for 5 to 30 minutes at least twice a week or more between the hours of 10 am and 4 pm
Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC8299926/
Now that's for most people: if you live in Alaska then during the winter maybe you need a litle more or something ... but the point is most people just need a little sun, < 30 minutes twice a week.
That study also claims 12-20ng/mL 25-hydroxyvitamin D is sufficient. My doctor says the post-COVID studies suggest 75ng/mL is closer to optimal, and <30ng/mL had been considered deficient for about a decade before COVID.
Personally I had no problem hitting 20ng/mL through sunlight alone and I'm pretty fair skinned but live somewhere sunny. But I haven't been able to get anywhere near 75ng/mL even when I've made an effort to increase vitamin D through supplements and exposure to the sun.
In Calgary, it's almost half the year that you basically cannot rely on the sun at all for vitamin D.
basically any patient he did blood work on, that was an office worker, was moderately or seriously deficient.