Vitamin D deficiency in winter related to bone loss of older people in northern China  
Zhou Bo, Wang Xiao-hong, Guo Lian-ying, Xu Chao, Zhang Zhuo, Kan Zhi-ying
Chinese Journal of Tissue Engineering Research . 2011, (26): 4907 -4910 .  DOI: 10.3969/j.issn.1673-8225.2011.26.039