Determinants of Iodine Insufficiency in Taiwanese Adults: Evidence from NAHSIT 2017-2020 on Eating-out Behaviours and Iodine-Rich Food Consumption

Iodine deficiency remains a public health concern despite salt iodization programs, and mild deficiency continues to be reported in Taiwan, where over 80% of households use iodized salt. This study analyzed 4,663 adults aged ≥19 years from the 2017–2020 Nutrition and Health Survey in Taiwan (NAHSIT) to examine determinants of urinary iodine concentration (UIC) in relation to dietary behaviours, with iodine insufficiency defined as UIC <100 µg/L. Adults reporting ≥15 weekly eating-out occasions had significantly higher median UIC compared with those eating out ≤7 times per week (109.9 vs. 97.9 µg/L, p < 0.05), particularly among younger adults. Frequent consumption of eggs, dairy products, and fish was consistently associated with higher UIC and lower odds of insufficiency, whereas no significant associations were observed for other aquatic products, and seaweed intake could not be assessed separately due to food grouping limitations. These findings demonstrate that while eating out may contribute to iodine intake in younger populations, regular consumption of eggs, dairy, and fish is the most consistent dietary determinant of sufficiency. From a policy perspective, results highlight the need to strengthen labeling, promote iodine-rich foods, and evaluate iodine sources in restaurant-prepared meals to complement salt iodization strategies in modern dietary environments.