It really depends on what counts as insufficient. There is enough debate in the scientific literature on what constitutes a "sufficient" level of Vit D for most healthy adults. If we set the sufficiency level of 25(OH)D of 20 ng/mL (50 nmol/L), most of the world's adult population would be in the healthy range. These levels can be achieved in a healthy Caucasian adult in a northern latitude in summer (eg, Edinburgh, Scotland) with as little as, say, two sessions of 10 minutes each in a week, exposing just the arms and legs.

Serum levels of Vitamin D also do not plummet with the winter months - it's more of a slow dip, and the seasonal low levels do not seem to negatively effect most adults.