Through regression analysis, researchers estimated the risks of fatal stroke based on low vitamin D blood levels <15 ng/ml and race, eliminating the effects of socioeconomic status (including age, sex, income and education) and stroke health factors (including diabetes, high blood pressure, cholesterol, body mass index, smoking, physical activity and alcohol use).

Blacks may have a natural resistance to the negative effects of low vitamin D levels, which might also explain the lower frequency of bone fractures despite the higher prevalence of vitamin D deficiency among blacks, Michos said.

"Since stroke is the No. 3 cause of death in the United States, it's important for us to consider low vitamin D as a possible risk factor for stroke at least among whites," Michos said.

Clinical trials are needed to determine whether treating vitamin D deficiency will help lower stroke risks, she said.

Researchers only had access to death records and thus couldn't account for stroke survivors, which potentially limits the study's reach. Since blood levels of vitamin D were only measured at the beginning of the study, this may not have been an accurate reflection of the participants' lifetime vitamin D status, Michos said.

Source: American Heart Association