I thought that was only true in non-tropical places due to the sun being weaker and the lifestyle during colder months. In addition, many Dominicans get lighter, in more than a few cases considerably lighter, from the color they had in the DR when they move to places like NY. Traveling back to the DR the person will be lighter than people remember them for a few days and then in a matter of days their color begins to get darker. These things are much more visible on men than on women.
Then their are many Dominican men who will have a considerable darker color in the parts exposed to the sun (arms, face, neck, upper torso close to the neck, etc) but the parts not often exposed to the elements will be considerably lighter in color. Another common occurrence among older very light or white Dominicans is for dark spots along the arms, hands, sometimes on the face, neck, sometimes on the lower legs. This is due to a lifetime of exposure to the sun.
Bottom line is that in addition that darker skin is more efficient in producing Vitamin D in tropical regions die to high sun rays, the evidence that many people are exposed to the sun (and consequently getting a natural source of Vitamin D) is staring at everyones face.
Usually darker people are told to take Vitamin D supplements in colder places like NY, Seattle, all of Canada, etc because most will have a deficiency due to darker skin requiring a stronger sun to produce adequate levels of Vitamin D. Lighter and white people in those conditions usually don't require a Vitamin D supplement because lighter skin is able to produce Vitamin D from a weaker sun. Not the case in the DR because the sun is strong everywhere. Actually, that's one of the reason that lighter and white people are at a greater risk of developing skin cancer in tropical regions vs darker people who are naturally able to take a stronger sun for much longer before their skin begins to act up.
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