CHICAGO -- Vitamin D deficiency does not appear to confer an increased risk of fatal stroke among blacks -- in contrast to findings among white populations, according to an analysis of U.S. population data.
Analysis of National Health and Nutrition Examination Survey (NHANES) data encompassing almost 8,000 people, showed that over a median of 14 years, whites with levels of 25-hydroxyvitamin D level below 15 ng/mL had double the risk of dying from stroke compared with those who had higher levels (HR 2.13, 95% CI 1.01 to 4.50), reported Erin Michos, MD, of Johns Hopkins University in Baltimore.
However, no such association was found in black NHANES participants (HR 0.93, 95% CI 0.49 to 1.80) -- despite their lower overall vitamin D levels and greater fatal stroke risk compared with whites, she told the American Heart Association annual meeting here.
Michos said the finding was surprising because it was thought that vitamin D deficiency might explain the higher stroke rate observed in blacks.
She speculated that African Americans might have developed an adaptive resistance to the adverse effects of low vitamin D levels -- an idea supported by their lower rates of fractures and osteoporosis compared with whites.
Sources of vitamin D include exposure to ultraviolet B rays in sunlight, as well as fatty fish, egg yolks, and fortified foods such as milk products and breakfast cereals.
Clinical trials are needed to definitively determine whether treating vitamin D deficiency with supplementation will prevent strokes, Michos said, noting that the ongoing Vitamin D and Omega-3 Trial (VITAL) will hopefully provide some answers about the potential benefits of vitamin D for preventing a number of health outcomes.
But, she said that evidence from observational studies is compelling enough to recommend screening for and treating low vitamin D levels now -- and that a dose of 1,000 to 2,000 IU a day is probably safe and beneficial for most adults.
The Institute of Medicine currently recommends 200 to 600 IU a day depending on age, but Michos said that many researchers and clinicians feel that that dose is probably too low to correct a deficiency.
The current analysis by Michos and colleagues looked at NHANES III data that included a nationally representative sample of 7,981 black and white participants, who were at least 30 years old and free of coronary heart disease and stroke at baseline from 1988 to 1994.
Through a median follow-up of 14.1 years, there were 176 stroke deaths -- 116 in whites and 60 in blacks.
As expected, blacks had lower mean vitamin D levels (19.4 versus 30.8 ng/mL) and a higher prevalence of vitamin deficiency (32.3% versus 6.6%).
Also consistent with previous studies was the higher fatal stroke rate in blacks (1.47% versus 1.24%), which remained significant even after adjusting for demographics, cardiovascular risk factors, and vitamin D levels (HR 1.65, 95% CI 1.01 to 2.69).
In multivariate models adjusted for age, sex, season, income, education, body mass index, smoking, physical activity, alcohol consumption, C-reactive protein, diabetes, hypertension, and hypercholesterolemia, vitamin D deficiency was associated with a higher fatal stroke risk in whites, but not blacks.
Michos noted that the study was limited by the use of vitamin D levels measured once at baseline, which precludes the assessment of lifetime patterns of vitamin D status and exposure.
In addition, the study may have been underpowered because the number of fatal strokes among blacks was low.
Although only fatal strokes were examined in the current study, Michos said future studies will explore the relationship between low vitamin D levels and nonfatal stroke and transient ischemic attack.
Primary source: American Heart Association
Source reference: Michos E, et al "Vitamin D deficiency is associated with increased risk of fatal stroke among whites but not blacks: the NHANES III linked mortality files" AHA 2010; Abstract 9478.