Iodine as an essential trace element for human, both excessive and insufficient intake would be harmful for human health. As susceptible population, children will be more sensitive to adverse effects caused by inadequate intake of iodine since they are at the peak of growth and development. Meanwhile, there are few direct evidence to establish dietary intakes levels of iodine for 3-6 years old pre-school children.

This study plan to conduct iodine balance experiment by providing devised diet consist of different iodine contents, in order to simulate iodine equilibrium of iodine deficiency to iodine excess in children and find the lowest iodine requirement of iodine for pre-school children, then aim to provide data to establish recommend nutrient intake of iodine for 3-6-year-old pre-school children. On the other hand, this study will make use of natural geographic environment with high water iodine in China. The investigators will conduct big-scale epidemiologic study in areas with high water iodine levels in Shandong and areas with adequate iodine in Tianjin, to enroll pre-school children with various iodine intake levels. The investigators will collect 24-hour urine samples as well as dietary records to evaluate iodine intake levels, and detect thyroid volume and thyroglobulin to evaluate the effects of various iodine intake on pre-school children, aiming to find the lowest safe intake level of iodine and then establish the tolerable upper intake level (UL) of iodine for 3-6-year-old pre-school children. This study aims to define the RNI and UL of iodine for 3-6-year-old pre-school children, to provide data for revision of Chinese dietary intake levels of iodine, and to guide children safe and scientifical iodine fortification.

Inclusion Criteria:

• for children with mild malnutrition: hemoglobin <110g / L, or weight for age \

3-6 years old pre-school children are the research subjects. They were recruited in Tianjin and Shandong.