Sunshine and milk!!
I live in Florida, drink milk everyday, plus I was taking a calcium plus vitamin D supplement everyday...still had really low vitamin D. So MD put me on 50,000 units per week for a couple months, and now I take 2000 units per day.
the Vitamin D in milk is trivial. The article says 2000 units per day.
Sunshine will do the job, but the problem is it needs to be every day. From November till say March and periods of low sun or cold there is no or inadequate vitamin production