Have you ever wondered where dietary iodine comes from? Most people are familiar with iodized salt and shellfish containing high levels of iodine, but few realize a vast assortment of food and drinks contain this essential nutrient.
What Food Products Contain the Highest Levels of Iodine?
It may come as a surprise that most dietary iodine comes from dairy products such as milk, cheese and yogurt. Iodine is used to prevent bacteria growth in cattle feed and it can also be used as a sanitizer when milking cows. Because cows typically graze in fields during the summer and feed inside during the winter, iodine levels in dairy products vary quite considerably and can be higher during the winter . On average, one cup of milk provides 56 µg iodine .
Marine products such as seafood and seaweed can be very rich in iodine; the concentration of iodine in the ocean is around 60 µg/L . Marine fish iodine content ranges from 2 µg/100 g to more than 1000 µg/100 g wet tissue, with cod containing the highest amount . Seaweed iodine content can range from 16 µg/g in a sheet of nori to over 8000 µg/g in kelp granules, based on dry weight . There are many things to consider when estimating iodine content of seaweeds, including species, dry or wet weight, time of year, water quality, and cooking style.
Bread and cereal used to contain high levels of iodine when it was used as a conditioner; bread iodine levels averaged 150 µg a slice . However, many bakeries now use bromine instead of iodine as a dough conditioner .
Another great source of iodine is eggs. Iodine content varies depending on the location and feed; iodine content should be about 25 µg/egg . Egg yolks contain a much higher amount of iodine (around 5x) than do egg whites .
What Diet Trends Are Becoming More Common and How Does that Affect Our Iodine Intake?
Iodine intake may possibly be on the decline in the US due to government recommendations and health trends that are growing in popularity.
The Departments of Agriculture and Health and Human Services are asking over half of the US population (those who are at risk of higher blood pressure, i.e., people who are over 50 or African American) to reduce their recommended daily intake (RDI) of 2.3 g sodium a day to around 1.5 g a day . Iodized salt in the US contains 77 µg iodine/g. Assuming the use of iodized salt and daily consumption of the RDI, that is a loss of 62 µg of iodine a day, which has a significant impact on iodine levels in the body.
Less milk is consumed today than it was 50 years ago. The consumption of milk in gallons per person per year dropped from 30 in 1968 to 19 in 2008 . Since milk is one of the largest sources of dietary iodine, this decline lowers the overall dietary iodine intake in the US. Also, a health trend to lower cholesterol has resulted in an increase in consumption of egg whites instead of whole eggs, thereby eliminating the high-iodine yolk.
If the body does not get enough iodine, thyroid hormone synthesis is impaired and thyroid-related problems can occur (hypothyroidism, goiter).
Why Do We Need Iodine in Our Diet?
Iodine is required primarily for thyroid hormone synthesis and is the main constituent of the thyroid hormones thyroxine [T4] and triiodothyronine [T3]. If the body does not get enough iodine, thyroid hormone synthesis is impaired and thyroid-related problems can occur (hypothyroidism, goiter).
How Does the Body Utilize Dietary Iodine?
When the body digests food products, iodine absorption occurs in the stomach and small intestine for 1-2 hours, and the circulating iodine is taken up by the thyroid and other cells in the body . Around 97% of the dietary iodine not taken up by the thyroid or other cells is excreted in the urine, with the remainder in sweat, tears, saliva, and feces . Absorption of dietary iodine by the thyroid can be twice as high in people who are iodine deficient .
How Do I Know If I Am Getting Enough Iodine in My Diet?
The recommended daily intake for adults (non-pregnant) is 150 µg of iodine a day. If you focus on consuming one or two products such as milk or whole eggs daily, eat seafood/seaweed a couple of times a week, or simply take an iodine supplement, reaching 150 µg a day will not be an issue. By testing iodine levels in urine, you can approximate how much iodine is consumed daily. ZRT Laboratory has developed a simple dried urine test for iodine as well as selenium, an essential mineral for thyroid hormone synthesis.
- Handout: Iodine Content in Foods
- Data Sheet: Iodine Testing in Dried Urine
- Start Testing Your Iodine Levels Today
 Zanzonico PB, Becker D. Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout. Health Phys 2000 ; 78 : 660 – 7.