According to the findings, only Caucasian kids living in the northern US actually achieved the minimum daily dose of vitamin D (and this only occurred during the summer if they didn't wear sunscreen).
How the researchers came to this conclusion is that they calculated the average production of vitamin D from sun exposure based on sex, age, skin type, clothing, season, and latitude (45 degrees N for the northern kids, 35 degrees N for southern kids).
The analysis suggests that kids living in northern latitudes have a better chance of meeting the minimum vitamin D requirements from sun exposure -- perhaps since there is less emphasis on sun avoidance and sunscreen use at northern latitudes. As you go south, awareness of sun exposure increases and kids are more likely to wear sunscreen more often.
These results contradict the statement made by the American Academy of Dermatology, which said people will continue to make "ample" vitamin D (meaning only 1000 IU/day) because most get plenty of "casual" UV sun exposure.
This would clearly suggest that nutritional supplementation or food fortification is increasingly important.
Source: Solar UV Doses of Young Americans and Vitamin D3 Production
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