The association of circulating 25-hydroxyvitamin D and calcium with cognitive performance in adolescents: cross-sectional study using data from the third National Health and Nutrition Examination Survey.

Studies in middle- and older-aged adults have shown positive associations between serum levels of 25-hydroxyvitamin D [25(OH)D] and cognitive function. Higher serum calcium levels have been associated with greater cognitive decline in older adults. There have been relatively few studies of the associations of 25(OH)D and calcium with cognitive function in adolescents. We investigated the cross sectional association between circulating levels of 25(OH)D and pH-normalised calcium with different domains of cognitive function (block design and digit span components from Wechsler Intelligence Scale for Children-revised and reading and mathematics subtests of Wide-range Achievement Test-revised) in adolescents (age 12-16.9 years) of the third National Health and Nutrition Examination Survey (NHANES III). Serum 25(OH)D was positively associated with performance in all four cognitive tests (e.g. 1 standard deviation (SD) change in digit span score per 1SD in 25(OH)D was 0.10 [95% confidence interval 0.03, 0.16]), but the association was attenuated to the null after adjusting for ethnicity/race and language used in test (1SD change in digit span per 1SD in 25(OH)D after adjusting for self-reported race/ethnicity and language used in test was 0.01[-0.06, 0.09]). pH-normalised calcium levels were not associated with any of the cognitive domains and adjustment for serum calcium levels did not alter the association between 25(OH)D and cognitive function. Thus, we found no evidence that the serum levels of 25(OH)D or calcium were associated with cognitive function in adolescents. A positive association of 25(OH)D with cognitive function was completely explained by confounding due to race/ethnicity.