Why has the dose of Vitamin D become controversial?

Our initial understanding of vitamin D grew out of the fact that cod liver oil and sunlight could cure rickets, a disease causing bent and weakened bones. That's because  Vitamin D from the sun and fish oils helped to absorb calcium and magnesium which were needed to form teeth and strengthen bones. 

But recent research suggests that vitamin D can also help prevent high blood pressure, some autoimmune diseases, and even cancer.

It takes only 20 minutes a day of intense sunlight on the face to provide adequate levels of vitamin D for fair-skinned individuals to prevent rickets without considering their diet (if they don't wear sunscreen). The sun's rays act as an enzyme, making Vitamin D out of the body's cholesterol. 

But you have to be  light-skinned and exposed to the sun in the summer  to get this effect. 

Sunscreens with a SPF of 30 block 98 % of the sun's rays needed to create that Vitamin D.  The dark skin of African-Americans reduces this capacity naturally (by up to 90 percent). And during the winter, the angle of the sun's rays in areas north of Atlanta cut down the vitamin D that can come from the sun. So, if you have dark skin or are living in Chicago in winter with only your face exposed to the less-than-clear sky, further vitamin D is going to be necessary. 

The result is that many people get less Vitamin D than they need. The Centers for Disease Control (CDC) released a report in 2011 that showed up to 11 % of 1 to 8 year-olds have low Vitamin D levels. Twenty-two percent (22 %) of older children and adolescents, and up to 28 % of adults have the same problem. 

Using a slightly different measurement, 56 % of teens in the Southeastern US had insufficiency (somewhat low levels) and 29 percent were deficient —and those statistics were worse for black youths, those who were overweight and those who live in the Northern US.

The concern isn't just that kids can get rickets, also that adults can get osteoporosis (a weakening of the bones) with advancing age. That's what we've known for decades. But we're learning that Vitamin D seems to have an important role in immune function, high blood pressure and preventing diabetes. But the dose we need to strengthen our bones isn't necessarily the same dose that's needed for these other roles.  So the dosing for Vitamin D is being debated.