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The modifying effect of season on the association between vitamin D supplement use (– – ▴ – –, supplement use = no; —●—, supplement use = yes) and vitamin D status (measured as serum 25-hydroxyvitamin D (25(OH)D) concentration (ng/ml)) was significant (P < 0·001), after adjustments were made for other variables: adolescents aged 12–18 years (n 1510), Korea National Health and Nutrition Examination Survey (KNHANES) 2008–2009
