Iodine concentrations in cow's milk in Central and Northern Bohemia

Iodine deficiency and related risks of medical and/or developmental disorders in humans are a worldwide problem. In the last years, Czech endocrinologists and paediatricians have observed a significant increase in the occurrence of goitre in children and adolescents caused by a low dietary intake of iodine. Given the low consumption of seafood, milk and dairy products are the main sources of iodine in theCzechRepublic. Iodine contents of milk of dairy cows of different breeds from seven farms located in Central andNorthern Bohemia were studied over a period of 17 months (April 2000 through August 2001). Iodine content was determined by high-performance liquid chromatography with electrochemical detection (HPLC-ECD) based on IDF Standard 167 (1994), following alkaline mineralization of the sample. An analysis of the certified reference material CRM 063 was used to examine the accuracy of determination. Two-way analysis of variance (ANOVA) with and without replications, including Scheffe's comparison at a 5% level of significance, was used for statistical evaluation of experimental data. The iodine content of milk ranged from 147 to 605 µg/kg (mean 251 110 µg/kg) during the winter season, and from 35 to 484 µg/kg (mean 212 104 µg/kg) during the summer season. The mean iodine concentration in all samples of milk during the studied period was 225 109 μg/kg.The wide range of iodine concentrations is connected with differences in iodine saturation of dairy cows and it is a result of multiple factors. Iodine reserves in the soil play an important role and influence the content of iodine in feeds for dairy cows and thereby the iodine levels in milk. The presence of goitrogenic substances in feed for dairy cows is another important factor. The iodine content was found to be statistically significantly lower in the milk from dairy cows fed a diet enriched with rapeseed cakes compared to that of dairy cows on a diet without rapeseed cake. Dairy cows on farm F6, which were fed maize silage without added rapeseed cake during winter, had a statistically higher iodine content in milk during the winter season compared to summer, when they were fed fresh fodder. Fresh fodder is supposed to contain goitrogenic substances which significantly reduce iodine levels in milk. This opinion is supported by experimental findings that the date of milk sample collection has no statistically significant influence on milk iodine levels if the dairy cows are fed a diet containing added rapeseed cake (farms F1, F2, F3, and F4) throughout the year. Compared to the other farms, statistically significantly higher milk iodine content was found on farm F5. The mean iodine content in milk from farm F5 was 425 74 μg/kg during the studied period. The main cause is probably that the diet contained no rapeseed cake.