Your speculation has merit but the body is very adaptive. Fair skinned people will acquire very significant sun tans at the equator. The sun tan would adjust to the appropriate level of vitamin d creation.
Doctors address vitamin D deficiencies:
--snip
Alice Carter, a 63-year-old from Milwaukee, is lying on a bed with chest pain in the ER at Sinai Medical Center.
After Whitcomb determines the Milwaukee woman is having a bout of angina, he begins telling her about vitamin D.
He seems pleasantly surprised when she tells him that she started taking 1,000 IU a day about a year ago, but he says she should up the dose to 2,000.
"Your skin pigment protects you from sunburn, but it also means you need to get three to four times as much sunlight to make the same amount of vitamin D," he tells Carter, who is African-American.
He tells her that many African-Americans he has tested have vitamin D levels of less than 10, but blacks living near the equator in Africa have levels near 60, he says.
"So are you saying I need to move down South?" she jokes.
Whitcomb says that won't be necessary if she increases her summertime dose to 2,000 IU and her wintertime dose to 4,000.
"I'll take the 2,000," she says.
--end snip