The Undisputed Winner: Seaweed
Seaweed is, by far, the most concentrated natural source of iodine. Its ability to absorb and store minerals from the ocean gives it an exceptional nutrient density. However, the iodine content can vary dramatically depending on the species and where it is harvested. This makes it a powerful but potentially variable dietary component that requires careful portioning.
The Power of Kombu Kelp
Among all varieties of seaweed, kombu (a form of kelp) consistently contains the highest levels of iodine. Just one gram of dried kombu can contain nearly 2,984 micrograms (mcg) of iodine, which is many times the recommended daily intake for adults. Due to this extreme concentration, it is often used sparingly, such as in small pieces for flavoring soups and broths like dashi in Japanese cuisine.
Other Seaweed Varieties: Wakame and Nori
Other common seaweeds offer more moderate, but still very high, levels of iodine:
- Wakame: Often found in miso soup, wakame has a milder flavor and lower iodine concentration than kombu. Its content can still be very high and varies based on its origin.
- Nori: The red seaweed used to wrap sushi rolls has a significantly lower iodine level than brown seaweeds like kelp, making it a safer option for more frequent consumption.
Top Seafood Sources of Iodine
Beyond seaweed, many types of seafood are also excellent sources of iodine, absorbing the mineral from the seawater they inhabit. The content can differ based on whether the fish is wild-caught or farm-raised.
Cod and Other Lean Fish
Lean fish are particularly good sources of iodine. For example, a three-ounce serving of baked cod provides around 158 mcg of iodine, meeting or exceeding the daily requirement for most adults. Other lean fish, like pollock, also contain substantial amounts. The iodine content in cod can even vary based on the specific body of water it comes from.
Shellfish Options
Shellfish also offer a rich supply of iodine. Three ounces of cooked oysters can provide about 93 mcg, while scallops offer an impressive 135 mcg per serving, which is 90% of the daily recommended intake. Shrimp is another source, though it contains slightly less iodine per serving.
Dairy Products: A Common Source
For many in industrialized nations, dairy products are a major source of iodine. The iodine content in milk and dairy is influenced by factors such as the cow's feed, the season, and the use of iodophor sanitizing agents on milking equipment.
- Milk: A single cup of nonfat milk can contain around 84 mcg of iodine.
- Yogurt: Plain Greek yogurt provides an even more concentrated dose, with three-quarters of a cup containing about 87 mcg.
- Cheese: The iodine content in cheese varies by type, but it remains a good contributor.
The Role of Iodized Salt
Iodized table salt has been instrumental in reducing global iodine deficiency. A quarter-teaspoon of iodized salt contains approximately 78 mcg of iodine, making it a reliable and accessible source for many people. It is important to note that specialty salts, such as Himalayan or sea salt, are typically not iodized and therefore contain negligible amounts of iodine.
Less Obvious Iodine Sources
While less concentrated than marine and dairy products, some other foods also contribute to iodine intake:
- Eggs: One large hard-boiled egg contains about 31 mcg of iodine, primarily in the yolk.
- Enriched Bread: Some commercially prepared breads use iodate dough conditioners, which can significantly increase their iodine content, though manufacturers are not required to list it on nutrition labels.
- Plant-Based Foods: Certain plant foods like potatoes and lima beans contain small amounts of iodine, but this is dependent on the iodine content of the soil they are grown in.
Comparison of High-Iodine Foods
| Food Source | Serving Size | Approximate Iodine Content (mcg) |
|---|---|---|
| Kombu Kelp | 1 gram | 2,984 |
| Cod (baked) | 3 ounces | 158 |
| Scallops | 3 ounces | 135 |
| Greek Yogurt (plain, nonfat) | ¾ cup | 87 |
| Milk (nonfat) | 1 cup | 84 |
| Iodized Salt | ¼ teaspoon | 78 |
| Wakame Seaweed | 1 gram | 66 (average) |
| Oysters (cooked) | 3 ounces | 93 |
| Egg (large) | 1 hard-boiled | 31 |
| Nori Seaweed | 1 gram | 16–43 (varies) |
Why Adequate Iodine Intake Is Critical for Health
Iodine's primary function is to support the thyroid gland in producing essential hormones that regulate metabolism, growth, and development. A deficiency can lead to numerous health problems, including an enlarged thyroid gland (goiter), hypothyroidism, and developmental issues, particularly for pregnant women and infants. In infants, severe deficiency can result in cretinism, causing irreversible intellectual disability and growth problems. Ensuring a balanced intake is therefore vital for preventing these disorders.
Conclusion: Balancing Your Iodine Intake
While kombu kelp holds the record for the highest iodine content, it must be consumed in moderation due to the risk of excessive intake. A balanced approach to getting enough iodine is ideal, incorporating a variety of sources. Seafood, dairy products, eggs, and using iodized salt in moderation are safer, more consistent ways to meet your daily needs. For those with dietary restrictions like vegans, fortified foods or a supplement may be necessary to ensure adequate levels. To explore the scientific data in more detail, refer to the NIH Office of Dietary Supplements Iodine Fact Sheet.
