Your body makes Vitamin D from exposure to sunlight.
Dark-skinned people in the far north (and nursing home patients who don't get outside) can expect Vitamin D deficiency unless they take supplements.
“Dark-skinned people in the far north (and nursing home patients who don’t get outside) can expect Vitamin D deficiency unless they take supplements.”
Testing indicates that the deficiency is almost as bad among fair-skinned people in the sunny climates. Apparently the body’s mechanism for converting sunlight to D isn’t particularly efficient, and requires a greater amount of skin exposure than one might think (and there are likely individual metabolic differences at play as well) - especially with respect to attaining “therapeutic levels” of D.