Iodine content in milk from German cows and in human milk: new monitoring study

Objective: Milk can provide more than 1/3 of the iodine content in human diet. Moreover, iodine supply in cow's milk in Germany has improved throughout the last decade. Since 1982, analyses of iodine content of cow's and human milk have been undertaken regularly in the State of Thuringia (East Germany). Data show increasing iodine concentration in milk over the past few years. Nonetheless, dietary supply of iodine via milk needs to be continuously monitored. Material and methods: To investigate the latest trend of iodine content in milk over time, 135 samples of cow's milk and 65 samples of human milk were analyzed by ICP-MS after digestion with tetramethyl-ammonium hydroxide. Samples of cow's milk (conventionally and organically produced) were purchased from the same supermarkets in Thuringia in March and November every year between 2007 and 2011. Human milk samples were obtained on a voluntary basis from breast-feeding women in Thuringia during the same time period. Results: Samples of cow's milk from 2007 to 2011 showed a mean iodine concentration of 122.0 +/- 36.8 mu g/l. There was no significant change during these 5 years (p > 0.05). In the same period, iodine content of conventionally produced milk was on average 51 mu g/l higher compared to organically produced milk (p < 0.001). For all human milk samples, the mean iodine content was 170 96 mu g/l, with a range of 45.6 - 478.4 mu g/l. In breast milk, not only were high variations present between samples of milk from the individual women, but iodine content also varied over lactation time. Conclusions: The current results demonstrate that in East Germany the average iodine content in cow's milk has remained stable at an appropriate level during the last 5 years. Although iodine concentrations in human milk likewise reveal relatively constant values on average, individual variations can be substantial.