The body naturally produces it from sunlight.
Carlos Camargo from Harvard Medical School in Massachusetts and other researchers did the study.
It followed more than nine hundred children in New Zealand until they were five years old.
Dr. Camargo said the problem of vitamin D deficiency is not limited to countries with the least sun.
There are low levels of vitamin D in people living in areas where there is a lot of sun.
This is because people are spending more time indoors.
For VOA Special English I'm Alex Villarreal.
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