"Go outside more" is absolutely a nonstarter in the places that most need this: northern latitudes in the winter.

Not only is "outside" often so cold that you need to cover enough of your skin that the sunlight would barely help, the weaker sun we get in winter just doesn't produce enough vitamin D in our bodies. That's the problem.

(Edit: "northern latitudes" can probably be replaced with "more extreme latitudes," as I believe there are some places that are far enough south that they also experience this, just at the opposite end of the year)