This article reviews and discusses why North Americans don't have to worry about Vitamin D intake;
"IT'S time to stop worrying about your vitamin D intake if you live in the US or Canada, according to a report published on Tuesday. The report, put together by the US Institute of Medicine, was commissioned by the countries' governments partly to clarify the supposed benefits of huge doses of vitamin D.
The report confirmed the importance of calcium and vitamin D in maintaining bone health, but said other claims, such as those suggesting massive doses of vitamin D can prevent cancer, cardiovascular disease and diabetes, remain unproven...."
http://www.newscientist.com/article/mg20827892.600-north-americans-neednt-worry-about-vitamin-d.html
Source: newscientist.com