Variation in milk-iodine concentration around the world: a systematic review and meta-analysis of the difference between season and dairy-production system

Iodine is essential for thyroid hormone production. Milk and dairy products are important sources of iodine in many countries. We aimed to review systematically the variation in milk-iodine concentration between countries, seasons and farming practice. We searched online food composition tables and published literature for data since 2006. Milk-iodine concentration was available for 34 countries (from 66 sources) and ranged from 5.5 to 49.9 mu g/100 g (median 17.3 mu g/100 g). Meta-analyses identified that iodine concentration is significantly higher in: (i) winter than summer milk (mean difference 5.97 mu g/100 g; p = 0.001), and (ii) in conventional than in organic milk (mean difference 6.00 mu g/100 g; p < 0.0001). Sub-group analysis showed that the difference between organic and conventional milk was only significant in summer (p p = 0.0003). The seasonal variation in milk-iodine concentration may affect iodine intake and status so should be considered in dietary surveys, and when assessing population iodine status.