The proportion is even higher among Hispanics (69%) and African Americans (82%), the investigators said.
Previous studies have reported links between vitamin D and cognitive decline, but they were conducted in primarily white cohorts. "Therefore, we investigated the associations between hypovitaminosis D and cognitive decline in an ethnically diverse cohort of older adults," Miller and colleagues wrote.
The 382 study participants were 41% white, 30% African American, 25% Hispanic, and 4% other. Their mean age was 75.5. More than half (62%) were women.
Serum levels of 25-hydroxyvitamin D (25-OHD) were measured at baseline. Study participants were categorized as vitamin D deficient (less than 12 ng/mL), insufficient (12-20 ng/mL), or adequate (20-50 ng/mL), according to Institute of Medicine guidelines.
Neuropsychological tests were administered at baseline and at approximate yearly intervals for an average follow-up of 4.8 years. At baseline, 17.5% of participants had dementia, 32.7% had mild cognitive impairment, and 49.5% were cognitively normal. Key results included the following:
The average baseline vitamin D level for the entire cohort was 19.2 ng/mL, which is just below adequate.
Approximately 61% of the entire cohort was either deficient (26%) or insufficient (35%) in vitamin D.
The mean vitamin D level among whites was on the low side of adequate (21.7 ng/mL). Mean levels were insufficient among African Americans (17.9 ng/mL) and Hispanics (17.2 ng/mL).
Mean vitamin D levels were significantly lower in the dementia group compared to the normal group (16 ng/mL versus 20 ng/mL; P=0.006).
Compared to study participants with adequate vitamin D, those who were deficient experienced significantly faster rates of decline in episodic memory (β= -0.04, [SE=0.02],P=0.049) and executive function (β= -0.05, [SE=0.02], P=.01).
Similarly, vitamin D insufficient individuals also experienced faster rates of decline in episodic memory (β= -0.06, [SE=0.02], P < .001) and executive function (β= -0.04, [SE=0.02], P=0.008).
Vitamin D status was not significantly associated with decline in semantic memory or visuospatial ability.
When analyzing rates of cognitive decline among study participants, the investigators controlled for factors including age, sex, education, ethnicity, vascular risk, and APOE4genotype.
"Our results extend current knowledge regarding the effect of hypovitaminosis D on cognitive function and underscore the relevance of identifying high-risk populations with low [vitamin] D status," Miller and colleagues said. "Our data suggest that hypovitaminosis D may be another risk factor for dementia among individuals of nonwhite race/ethnicity."