Few would dispute the fact that iodine is an important component in a healthy diet. Even if you don’t know much about mammalian metabolism, you would at least be vaguely aware of our need to consume a certain amount of iodine daily. For adults that’s 150 mcg/day. Pregnant and breast-feeding women require at least half that amount again to ensure the health of their child. Children 5 years and under need 90 mcg/day and children from 6 – 12 years mcg/day. The body can’t manufacture iodine so it must come from our diet.
The big problem we have today is that many of the more traditional sources of iodine, such as dairy, certain types of seafood, and many fruits and vegetables, no longer contain the amount of iodine they used to, or are no longer safe to eat in sufficient quantities to provide enough daily iodine. There are various reasons for this.
Soil levels of iodine in some parts of the world are becoming depleted, so crops and animals raised on these soils do not have the iodine levels they once did. A lot of seafood, a very valuable source of iodine, is now contaminated with dangerous levels of heavy metals, notably mercury. Therefore, recommended daily limits on seafood consumption to keep mercury consumption below safe limits is now commonplace. The trade off though may be that these quantities are insufficient to meet daily iodine requirements.
Iodine In Milk Isn’t What It Used To Be
Then we come to dairy products. Although considered one of our primary sources of iodine, milk does not naturally contain large amounts of iodine. Much of its iodine content has traditionally come from iodine in the disinfecting solutions used to sanitize and sterilize dairy equipment, and to clean teats and udders on dairy cows prior to milking. The iodine on their skin is absorbed by the cows and adds an appreciable amount of iodine to their milk.
However, in many developed countries, such as Australia, the use of iodine-based disinfectants in the dairy industry has been phased out and the result is a significant drop in the levels of iodine in milk and milk products. To the point where recent studies indicate that iodine consumption in some Australian states is low to borderline. To help address this situation the Australian government mandated in 2009 that all salt used in packaged bread must be iodized salt and the labeling modified to indicate this. So, if you buy a loaf of packaged bread in Australia now you will find that it lists iodized salt as one of the ingredients. Indeed, iodized salt, either in food products or as table salt, is now one of the primary sources of dietary iodine in many countries.
Another way iodine levels have been increased in dairy products is through the addition of iodine to commercial dairy cattle feeds. In some countries, notably the US, this method has been used for some time to maintain iodine content in milk. However, amid increasing concerns about too much dietary iodine, the amount of iodine allowed to be fed to dairy cattle was restricted in 1986 to a daily maximum of 10 mg per cow. This led to a significant drop in milk iodine content. Despite this, dairy products along with iodized salt continue to be a primary source of dietary iodine in the US.
A Pinch Of Salt – iodized Salt That Is
One of the primary sources today of dietary iodine today is iodized salt. Like milk and grains, salt does not naturally contain much iodine, not even sea salt. It has been added during processing.
Iodized salt has become one of the most unobtrusive and easily implemented solutions to iodine deficiency in many countries like the US, Canada, Australia and New Zealand. In Australia, a government mandate in 2009 made the use of iodized salt in packaged bread mandatory. This initiative was intended to reverse an alarming trend of low to borderline iodine intake found across most Australian states with the exception of Queensland and Western Australia, where levels of dietary iodine are adequate. The drop in iodine intake is partly a result of the phasing out of iodine-based disinfectants in the dairy industry in Australia.
Likewise, across North America it’s been found that around two thirds of households get most of their iodine via iodized salt. In Europe though iodized salt is not as readily available or as commonly used, due to the increasing awareness of the health risks associated with excessive salt intake. Especially when considering the amount of salt to be found in most processed and fast foods, which is not iodized incidentally. So across Europe and in the UK, dairy remains one on the highest sources of iodine.
Interestingly, the amount of iodine in milk across Europe fluctuates according to season. In winter, iodine levels are higher than they are in summer. This is because dairy cattle are fed indoors over winter with mineral-fortified feed, which contains iodine. Therefore one needs to be careful when looking at figures for iodine content in milk and milk products because these will naturally be higher if winter milk has been tested. Good tests will take a sample from summer and winter milk then use the average of the two results.
Get Your Daily Bread, And Too Much Iodine
Another source of iodine has traditionally been bread. The iodine was added via iodate conditioners used to extend shelf life and preserve freshness. During the 1960’s tests showed that a single slice of bread could contain as much as 150 mg a slice, the entire recommended daily dose of iodine for an adult! 
Increasing concerns about this excessively high iodine content led many commercial bakeries to stop using iodate conditioners. This contributed to the dropping levels of dietary iodine in the diets of most Americans.
However, recent studies done on a number of brands of bread in Boston in the US found that at least 3 contained high amounts of iodine (up to 587 mg a slice), pointing to the possible use of iodate conditioners. So consuming one slice of this bread provides nearly 4 times the recommended daily intake for an adult. Other brands were found to not contain the amount of iodine that was specified on the packaging.
One of the conclusions therefore to come out of this research was that a lot of work needs to be done to ensure that food labeling accurately reflects the iodine content of foods.