Suboptimal vitamin D status in Korean adolescents: a nationwide study on its prevalence, risk factors including cotinine-verified smoking status and association with atopic dermatitis and asthma

OBJECTIVE To determine the prevalence and risk factors for suboptimal vitamin D status in Korean adolescents and to assess its relationship with atopic dermatitis (AD) and asthma at a national level. DESIGN This is a cross-sectional study with data from the Korean National Health and Nutrition Examination Survey. Information regarding socioeconomic characteristics, clinical data and environmental factors was collected. Blood and urine samples were taken for vitamin D and cotinine, respectively. Descriptive and multivariable logistic regression was performed on the data. SETTING South Korea (nationwide). PARTICIPANTS 2515 individuals aged 10-18 years who participated in the Korean National Health and Nutrition Examination Survey from 2008 to 2011. MAIN OUTCOME MEASURES Vitamin D status was determined through measurement of serum 25-hydroxyvitamin D (25OHD). Smoking status was classified based on the urine cotinine level. Physician diagnosed with AD and asthma were assessed using a questionnaire. RESULTS Overall, 73.3% of the subjects were vitamin D deficient (25OHD <20 ng/mL) and 24.4% of the subjects were vitamin D insufficient (25OHD, 20-29.9 ng/mL). Older age (p<0.001), female gender (p<0.001), urban residence (p=0.019), higher body mass index (p=0.003) and sampling in winter months (November-March) (p<0.001) were independently associated with low serum 25OHD levels. With cotinine verification, 18.2% of the participants were classified as active smokers, and 43.4% were classified as passive smokers. After adjusting for potential confounders, serum 25OHD status showed no association with AD or asthma. CONCLUSION Vitamin D deficiency is highly prevalent in Korean adolescents. Cotinine-verified prevalence of smoking was also high, but its relationship with vitamin D deficiency was not confirmed in our study. Above all, our results provide epidemiological evidence against the association of vitamin D status with AD and asthma at the national level among Korean adolescents.