Determinants of plasma 25-hydroxyvitamin D and development of prediction models in three US cohorts

Epidemiological and other evidence suggests that vitamin D may be protective against several chronic diseases. Assessing vitamin D status in epidemiological studies, however, is challenging given finite resources and limitations of commonly used approaches. Using multivariable linear regression, we derived predicted 25-hydroxyvitamin D (25(OH)D) scores based on known determinants of circulating 25(OH)D, including age, race, UV-B radiation flux at residence, dietary and supplementary vitamin D intakes, BMI, physical activity, alcohol intake, post-menopausal hormone use (women only) and season of blood draw, in three nationwide cohorts: the Nurses' Health Study, Nurses' Health Study II and the Health Professionals Follow-up Study. The model r 2 for each cohort ranged from 0·25 to 0·33. We validated the prediction models in independent samples of participants from these studies. Mean measured 25(OH)D levels rose with increasing decile of predicted 25(OH)D score, such that the differences in mean measured 25(OH)D between the extreme deciles of predicted 25(OH)D were in the range 8·7-12·3 ng/ml. Substituting predicted 25(OH)D scores for measured 25(OH)D in a previously published case-control analysis of colorectal cancer yielded similar effect estimates with OR of approximately 0·8 for a 10 ng/ml difference in either plasma or predicted 25(OH)D. We conclude that these data provide reasonable evidence that a predicted 25(OH)D score is an acceptable marker for ranking individuals by long-term vitamin D status and may be particularly useful in research settings where biomarkers are not available for the majority of a study population.