It's another case of a logical hypothesis colliding with brutal facts.
Researchers at Johns Hopkins University thought low levels of vitamin D levels could help explain why AfricanAmericans are more likely to suffer strokes. After all, low vitamin D is linked to strokes in whites. And blacks are more likely to have severely low blood levels of vitamin D than whites.
But when the Hopkins researchers looked closely at a sample of nearly 8,000 people of both races followed for 14 years, they found vitamin D didn't explain anything about blacks' 60 percent higher risk of death from strokes.
"Don't blame vitamin D deficits for the higher number of strokes in blacks," says a statement by Dr. Erin Michos, who presented the results at the American Heart Association's annual scientific concerence in Chicago. "Something else is surely behind this problem."
And that something else isn't the well-known higher rates of diabetes and hypertension among blacks. Those aren't enough to explain their higher risk of stroke.
She thinks blacks may have adapted to vitamin D deficiency, so lower blood levels don't affect them as much.
Blacks' skin pigmentation prevents them from absorbing as much ultraviolet light, which is necessary for humans to make vitamin D in the skin.
Vitamin D is important for healthy bones. One other hint that blacks may not suffer the same ill effects from low vitamin D levels is that they actually have higher bone density and lower fracture rates than whites, even though their rates of severe vitamin D deficiency in the Hopkins study were five times higher than whites.
It all goes to show that scientists have a lot more to learn about vitamin D. In the meantime, people shouldn't leap to conclusions about the need for high doses of vitamin D supplements to correct low blood levels.
Later this month the National Institute of Medicine is scheduled to release an exhaustive report on vitamin D and calcium that will revise the Dietary Reference Intakes that people need to stay healthy.