See Optimal Serum Concentration of 25-Hydroxyvitamin D for Bone Health in Older Korean Adults PubMed See also VitaminDWiki 9X increase in Koreans seeking help with vitamin D deficiency in just 4 years – Jan 2015 Vitamin D levels continue to fall in Korea– June 2018 South Koreans - half take supplements, Vitamin D levels dropping, COVID-19