1. Research Center of Clinical Epidemiology, Peking University Third Hospital, Beijing, China
2. Department of Nutrition and Food Hygiene, School of Public Health, Peking University, Beijing, China
3. Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
4. UNICEF China, Beijing, China
5. Department of Maternal and Child Health, School of Public Health, Peking University, Beijing, China