Milk and dairy products as sources of iodine – which factors influence the content?
Milk and dairy products are potentially good sources of iodine but their content varies widely. FSVO research projects show which factors influence the iodine concentration and how they can contribute to the public’s iodine intake in a more targeted way.
What determines the iodine concentration in milk?
Iodine is an essential trace element for both humans and for cows and their calves. Milk and dairy products generally contain iodine, but the iodine content can vary widely. Milk and dairy products are therefore not considered reliable sources of iodine.
The iodine concentration in cow’s milk depends on a number of factors. They include the type of farm, season, feed, use of disinfectants containing iodine and substances in fodder crops that inhibit the absorption of iodine.
Research projects funded by the FSVO studied the factors that influence the iodine content in milk and dairy products and the mechanisms responsible. If we know the main influencing factors, targeted measures can be taken to improve the public’s iodine intake.
Research projects on milk and dairy products
These research projects provide the scientific evidence to use milk and dairy products in dietary iodine intake in a more targeted way.
The study collected data on iodine concentrations in conventional and organically-produced UHT milk over the space of a year, and compared it with earlier levels. The researchers also looked at the influence of UHT treatment on iodine content.
Swiss UHT drinking milk now contains around twice as much iodine as it did 25 years ago. It is assumed that this increase is due to higher iodine supplementation in dairy cows’ feed. The seasonal differences were marked: in summer the iodine content was low and in winter it was high. Organic milk contains around 35% less iodine on average, which presumably is systemic. For example, iodised licking stones are prohibited in organic dairy farming.
The method used in Switzerland to produce UHT milk doesn’t influence the iodine content.
This study examined how different iodine concentrations in feed impact the iodine content of milk. To this end, five groups of five cows each were given graduated iodine dosages. The study also looked at iodine losses in the manufacture of cheese and yoghurt from milk with varying iodine concentrations.
The results revealed a clear linear relationship between the iodine concentration in feed and that in milk. This evidence allows the iodine content of milk and dairy products to be increased in a targeted way through feed supplements. This would allow milk to become a relevant and reliable source of iodine. However, in the manufacture of cheese, a large proportion of the iodine is lost in whey.
This study looked at potential determinants of fluctuations in milk iodine concentrations, including organic and conventionally produced milk.
The researchers analysed feed including water and farm characteristics, feeding practices and disinfection practices when milking. In addition, the study estimated cows’ iodine intake in the summer and winter. Mineral mixtures are the biggest contributor to iodine concentration in milk, but their influence varies by season.
In summary, the iodine concentration in milk varied significantly between farms due to different farm types and practices that result in differences in dairy cow exposure – both via ingestion and skin application.
In the manufacture of cheese, the majority of the iodine from milk remains in the whey. Nevertheless, cheese can be a key contributor to iodine intake. Immersion in iodised brine allows iodine to penetrate the entire cheese wheel and distribute uniformly, even in rindless cheeses.
Since 2008, predominantly non-iodised salt has been used in Switzerland. It was assumed that iodine only remained in the rind. This study disproves this assumption and shows that iodine diffuses into the interior of the cheese. If iodised salt was used again, cheese could make a measurable contribution to iodine intake, with a potential increase of over 10% of the recommended daily dosage.
The research findings come from sub-projects carried out as part of a thesis at the Laboratory of Human Nutrition at ETH Zurich. Agroscope Posieux and the Animal Nutrition Group at ETH Zurich supported the feeding trials involving dairy cows. The studies on the seasonality of milk iodine content and on the diffusion of iodine in ripening cheese wheels were conducted at the FSVO laboratories in collaboration with Agroscope Liebefeld.
