Linear regression analysis was repeated after stratification by age-group (18- 34, 35–74, and ≥75 years old) and by self-reported diabetes status (yes versus no). Interactions between vitamin D and age, BMI, or diabetes status were also included in the analysis. Multivariate linear regression assessed the relation of A1C with 25(OH)D after accounting for age, race/ethnicity, sex, BMI, self-reported diabetes, physical activity, any dietary supplement use, and parathyroid hormone (PTH). We used the medical examination clinic sampling weights for our analysis. Accordingly, results were weighted to reflect the actual U.S. population with oversampling of young people, African-Americans, and Hispanics. The NHANES data collection employs a complex, multistage, stratified probability sampling design to select subjects representing the civilian noninstitutionalized U.S. Of the 11,183 subjects, 9,773 aged 18 years and above had no missing data for all covariates for statistical analysis. All analyses were performed using SAS version 9.2 software (SAS Institute, Cary, NC) and accounted for sample weights for complex sampling methods of datasets.
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