Vitamin D insufficiency was associated with faster decline in cognitive functions among a group of ethnically diverse older adults, according to an article published online by JAMA Neurology.
In addition to promoting calcium absorption and bone health, vitamin D may influence all organ systems. Both the vitamin D receptor and the enzyme that converts 25-hydroxyvitamin D (25-OHD) to the active form of the vitamin are expressed in all human organs, including the brain. Thus, research has increasingly examined the association between vitamin D status and a variety of health outcomes, including dementia and age-associated cognitive decline.
Joshua W. Miller, Ph.D., of Rutgers University, New Brunswick, N.J., and coauthors from the University of California, Davis, examined baseline vitamin D status and change in subdomains of cognitive function as measured on assessment scales in an ethnically diverse group of 382 older adults.
Serum (blood) 25-OHD was measured and vitamin D status was defined as follows: deficient was less than 12 ng/mL; insufficient was 12 to less than 20 ng/mL; adequate was 20 to less than 50 ng/mL; and high was 50 ng/mL or higher.
Study participants were an average age of 75.5 years and nearly 62 percent were female, while 41.4 percent of the group was white, 29.6 percent were African American and 25.1 percent were Hispanic. At study enrollment, 17.5 percent of the participants had dementia, 32.7 percent had mild cognitive impairment and 49.5 percent were cognitively normal.
The authors report:
- The average 25-OHD level among participants was 19.2 ng/mL, with 26.2 percent of participants being vitamin D deficient and 35.1 percent vitamin D insufficient.
- Average 25-OHD levels were lower for African American and Hispanic participants compared with their white counterparts (17.9, 17.2 and 21.7 ng/mL, respectively).
- Average 25-OHD levels were lower in the dementia group compared with mild cognitive impairment and cognitively normal groups (16.2, 20.0 and 19.7 ng/mL, respectively.
- During an average follow-up of 4.8 years, rates of decline in episodic memory and executive function among vitamin D deficient and vitamin D insufficient participants were greater than those with adequate vitamin D status after adjusting for a variety of patient factors.
- Vitamin D status was not significantly associated with decline in semantic memory or visuospatial ability.
The authors note limitations to their study including that they did not directly measure dairy intake, sun exposure or exercise, each of which can influence vitamin D levels.
"Our data support the common occurrence of VitD [vitamin D] insufficiency among older individuals. In addition, these data show that African American and Hispanic individuals are more likely to have VitD insufficiency or deficiency. Independent of race or ethnicity, baseline cognitive ability, and a host of other risk factors, VitD insufficiency was associated with significantly faster declines in both episodic memory and executive function performance, which may correspond to elevated risk for incident AD [Alzheimer disease] dementia. Given that VitD insufficiency is medically correctable, well-designed clinical trials that emphasize enrollment of individuals of nonwhite race/ethnicity with hypovitaminosis D could be useful for testing the effect of VitD replacement on dementia prevention," the study concludes.