Iodine is a fascinating mineral, and it's somewhat surprising how relatively little research has been done on the role of iodine in human health, in comparison to the amount of research that has been done on the role of iodine in the environment. Although we usually keep our website nutrient profiles focused closely on food and health, we would like to provide you with a little bit of information about iodine outside of a strict health context. This broader context may actually end up shedding some light on future research about iodine and health.
Scientists already understand how iodine plays a multi-faceted role in the environment and serves as a key mineral in oceans, soil, and atmosphere. The stability of the earth's ozone layer, for example, is known to have a relationship to iodine levels in the atmosphere. Atmospheric iodine, in turn, is known to be related to iodine balance in the oceans and in the soil. Scientists also know that iodine concentrations in the atmosphere influence a wide range of oxygen-based reactions. Iodine has also been studied within the context of soil and plant health. Based on research in this area, we know that in some plants—for example, lettuce—iodine can help to offset stresses that get placed on the plants to due excess presence of salt in the soil (salinity stress).
Unfortunately, these roles of iodine in oxygen-based reactions and in balancing mineral levels (especially when sodium, potassium, and chlorine are involved) have not studied in human health as extensively as they have been studied in health of the environment, and we will need to wait on future studies for a clear understanding of these potential iodine roles in human health. However, one area of human health where we do not need to wait for more information is the area of thyroid gland function. The role of iodine in thyroid gland function has been studied in depth, and we will be telling you much more about this role later in this profile.
Approximately 30% of our WHFoods contain some iodine, and you will find most food groups represented in this overall list of iodine-containing foods. In the vegetables group, for example, you will find sweet potatoes, onions, and spinach. In the fruit group, you will find strawberries, banana, and cantaloupe. In the grains you will find barley, and in the nuts you will find peanuts. (Although we list peanuts under "Nuts & Seeds" on our website, they are technically classified as legumes). Yet despite the presence of iodine in this diverse group of foods, it is difficult to get our recommended daily amount of 150 micrograms unless you consume foods from two food groups not listed above. These two groups are Seafood and Eggs & Dairy. In fact, only 11 of our WHFoods rank as good, very good, or excellent sources of iodine, and these two groups (Seafood and Eggs & Dairy) account for over 90% (10/11) of all of our iodine-rich foods. We'll be providing you with much more information about this iodine-food relationship in our Summary of Food Sources section.
At one point in time, iodine deficiency was fairly common in most of the northern part of the United States. This "goiter belt" included New England, the upper Great Lakes region, and the Pacific Northwest. The combination of iodine-depleted soils and lack of access to (or lack of acceptance of) seafoods left up to 30% of the population with severe iodine deficiency and its telltale goiter. ("Goiter"—a word that comes from the Latin "guttur" meaning "throat"—is a non-technical term that refers to enlargement of the thyroid grand. While iodine deficiency is one of the reasons that the thyroid gland can become enlarged, it does not always become enlarged following iodine deficiency and it can become enlarged for other reasons not involving dietary iodine.)
Starting in the early 20th century, an Ohio doctor aptly named David Marine began experimenting with adding iodine to local diets by way of iodized salt. (We say "aptly" here since seafoods are such an important dietary source of iodine.) By the 1920s, widespread consumption of this fortified salt in the U.S. had largely eliminated widespread iodine deficiency. We will discuss this issue of iodized table salt—and its potential role in your meal plan—later in this article.
As described earlier, even though researchers know a good bit about iodine in relationship to the environment, less is known about other health support roles for iodine in the body. However, one area in which we are not lacking for information is the role of iodine in thyroid health.
Iodine is a key component of the hormones made in the thyroid gland. These hormones are absolutely critical to human health, helping to control energy production and utilization in nearly every cell of the body.
The balance of iodine in the thyroid gland is tricky, and both too much and too little iodine can slow down the production of hormones. This is not a situation where more is always better. Our WHFoods recommended daily intake level of 150 micrograms is a level that makes the most sense to us as a general public health recommendation for preserving balanced production of thyroid hormones. For most people, we would predict that falling far below this level or greatly exceeding it would potentially increase the risk of imbalanced thyroid hormone production.
When it comes to iodine, one food stands so far above the rest that the chart at the top of the page almost looks like a misprint. Some sea vegetables contain as much as 500% of the Daily Value (DV) per serving.
This is only true, however, for some sea vegetables. A good rule of thumb is that the brown sea vegetable species—for instance, kelp and wakame—are richer in iodine than the red forms. Still, we view sea vegetables (regardless of variety) as a potentially concentrated source of iodine, and anyone consuming 1 tablespoon or more of these foods on a daily basis mighty want to evaluate their total daily intake of iodine to make sure it does not exceed the Tolerable Upper Limit (UL) established by the National Academy of Sciences for adults 19 and older of 1,100 micrograms. For more on iodine and how to pick sea vegetables, read our profile on these interesting and useful foods.
Sea vegetables like kelp and wakame are not the only sea-based foods rich in iodine, however. All six of our seafoods rank as good, very good, or excellent sources of this mineral. In the excellent category you will find scallops and cod. In the very good category you will find shrimp. And in the good category you will find tuna, salmon, and sardines. Both cod and scallops will provide you with nearly 90% of the daily recommended amount for iodine in a single 4-ounce serving. We would also like to note that many fish not profiled on our website contain significant amounts of iodine, generally falling into the range of 25-140 micrograms per 4-ounce serving. As a general rule (that does have numerous exceptions, however), shellfish are more concentrated in iodine than finfish.
Two of our dairy foods—cow's milk and yogurt—as well as eggs rank as very good sources of iodine. You can get about 20% of your daily iodine from 4 ounces of cow's milk, 1 egg, or 1/2 cup of yogurt.
The WHFoods that we have described above account for all but one of our ranked food sources of iodine. The only source we haven't mentioned are strawberries, which rank as a very good source and provide about 13 micrograms per cup. (This ranking is largely due to the fact that strawberries are a high-water and low-calorie fruit, providing only 46 calories per cup.) While you would not want to rely on strawberries for your iodine intake, it would not be unreasonably to expect strawberries to provide about 10% of the iodine you need on any given day when you choose to eat them.
As described earlier, you can find foods in the vegetable group (like sweet potatoes, onions, and spinach), other foods in the fruit group (like bananas and cantaloupe), and foods in the grain group (barley) as well as the nuts group (peanuts, which are technically classified as legumes rather than nuts) that provide iodine. Like strawberries, however, you would not want to rely heavily on these foods to provide you with your daily iodine requirement. Conservatively speaking, including these non-seafood, non-dairy foods in your day's food would most likely provide you with about 5-30% of the iodine you need.
As you can see from the above food summary, people who enjoy eating seafood on a near-daily basis have a good chance of meeting their daily iodine needs because they will often be able to get 50% of more of those needs from the seafood alone. A single serving of dairy foods on the same day might move this percentage up closer to 75%, and other foods would be able to make up the remainder.
For people who completely avoid seafood in their meal plan, iodine needs become a little bit trickier to meet. One meal plan addition worthy of consideration here would be to choose sea vegetables as a recipe component. Since 1 tablespoon of a sea vegetable like dulse can provide five times the daily iodine requirement all by itself, you could enjoy a recipe with this amount of sea vegetable and meet your iodine requirement over a five-day time period. Our 5-Minute Miso Soup with Dulse recipe will provide you with exactly that amount per serving. Dried kelp flakes or other forms of dried sea vegetables can be sprinkled on top of many dishes, and it is important to remember that it only takes one-fifth of a tablespoon—just a little bit more than half a teaspoon—to meet your recommended daily iodine level.
Of course, another alternative available to everyone is iodized salt. Iodized salt is a fortified form of table salt that has been processed to contain significant amounts of iodine. The general government standard for fortification of salt with iodine is 76-77 micrograms of iodine per gram of salt. However, many iodized salts don't actually end up containing this much iodine. An average marketplace range seems to be closer to 45-50 micrograms per gram. Still, at 6 grams per teaspoon, this level of 45-50 micrograms would mean that 1 teaspoon of iodized salt would be likely to contain at least 270-300 micrograms of iodine and 1/4 teaspoon would be likely to contain at least 67-75 micrograms. So it is easy to see how 1/4 teaspoon of iodized salt could provide about half of a person's daily recommended iodine. (This same 1/4 teaspoon would provide about 580 milligrams of sodium, or about 12% of the Daily Value.)
As a general rule, we always prefer whole, natural foods as a source of all nutrients, and there simply isn't any form of iodized salt that is whole and natural. By definition, iodized salt is a processed, fortified ingredient. It is possible, of course, to purchase iodized sea salt, but even in this situation, the sea salt has been fortified with iodine during processing. (While iodine is naturally present in sea salts along with other minerals, it is not present in amounts that would qualify the salt to be labeled as "iodized.")
In addition to our preference for whole, natural foods as a source of all nutrients, we also emphasize the pleasures of herbs, spices, and natural flavors found in fresh foods. The idea of substituting salt for the true pleasures of good cooking does not make sense to us. (That's why you will find many of our recipes to be devoid of table salt as an ingredient, and our ingredients followed by the option, "salt and pepper to taste.")
At the same time, we are not aware of any special problems related to the process of fortifying salt with iodine. In addition, we realize that many people rely on small amounts of iodized salt to boost up an otherwise deficient iodine intake level. Especially for persons who avoid seafood and dairy in their meal plans, iodized salt might make a logical addition to meet daily iodine needs. Obviously, the decision about whether to include iodized salt in a meal plan is a personal decision. From our perspective, it could be a very sensible choice, depending on all of the circumstances involved. We would, however, caution anyone who has been placed on a salt-restricted diet, or who suspects that they might fall into the minority of U.S. adults who are salt-sensitive in terms of blood pressure regulation, to talk over their best options for meeting daily iodine needs with their healthcare provider.
We would like to add one final note here on the relationship between salt and iodine intake. Processed foods in the U.S. have a well-deserved reputation for being overly high in salt. This trend has not been limited to fast foods or foods at a corner grocery. Many popular canned soups, frozen vegetables, and other widely enjoyed pre-packaged foods contain large amounts of salt. However, the salt added to processed foods is typically not iodized salt that has been fortified with iodine. For this reason, it simply is not correct to assume that consumption of a processed, high-sodium food is likely to provide you with the iodine you need, even if you venture out into processed, prepackaged foods as a regular part of your diet.
|World's Healthiest Foods ranked as quality sources of|
|Sea Vegetables||1 TBS||10.8||750.00||500||829.5||excellent|
|Yogurt||1 cup||149.4||71.05||47||5.7||very good|
|Shrimp||4 oz||134.9||46.00||31||4.1||very good|
|Cow's milk||4 oz||74.4||28.06||19||4.5||very good|
|Eggs||1 each||77.5||27.00||18||4.2||very good|
|Strawberries||1 cup||46.1||12.96||9||3.4||very good|
Density>=7.6 AND DRI/DV>=10%
Density>=3.4 AND DRI/DV>=5%
Density>=1.5 AND DRI/DV>=2.5%
Iodine is stable to storage and many types of processing. For example, we don't see loss of iodine in sea vegetables, even if stored for long periods of time. You do not need to choose or store your iodine-rich foods in a special way to protect against loss. We do recommend, however, that you take a quick glance at the sea vegetables profile to help you select the best types for your nutritional and culinary needs.
As you cook iodine-rich foods, you will extract a significant portion of the iodine into the cooking water. This can be a good thing for nutrition, for example when you are making soup stock. Boiling sea vegetables for 15 minutes can extract from half to almost all its iodine content into the stock, making this soup now a great source of iodine nutrition.
Although becoming less popular, the use of iodine-containing dough conditioners to help to strengthen integrity of bread still remains a common practice. The iodine used in this processwill be listed on the food label as calcium iodate.
The risk of iodine deficiency is substantial in the United States and has been on the rise. The average urinary iodine level—a good measure of recent dietary iodine intake—has dropped by more than half since the 1970s. Of course, we are talking about a very broad population-level trend here, in a country where processed, prepackaged foods play a major role in the average U.S. adult meal plan.
The reason we see iodine levels dropping in the population is two-fold. One is that within the world of commercial baking, many bread manufacturers have moved away from iodine-containing compounds to keep dough fresh. But a bigger change is that the average U.S. household is doing less and less home cooking and resorting more and more often to prepackaged foods, ready-to-eat foods, and restaurant eating (including fast food eating). As mentioned earlier, even though many prepackaged foods are high in sodium, the salt added to these foods has not necessarily been fortified with iodine.
Still, there is a good bit of unpredictability in the iodine content of prepackaged and ready-to-eat foods. Some food preparers use salt that is iodized, including some fast food restaurants. But because the level of iodine in "away-from-home" foods can be so unpredictable, we recommend that you focus on obtaining iodine from whole, natural foods. The food sources of iodine section above should help you figure out what combination of whole, natural foods will work best for you.
Perhaps most concerning is the recent finding that the average pregnant woman in the United States has substandard iodine nutrition. Iodine—as a constituent of thyroid hormone—is critical to the developing nervous system, and low iodine levels in children are associated with impaired development.
Counter-balancing this concern is evidence from the most recent National Health and Nutrition Examination Survey which shows that this drop in iodine intake has stabilized. So while we need to do a better job of making sure at-risk people—especially pregnant women—get enough iodine, at least the public health problem is not continuing to get worse.
Iodine content of soils varies by region. At one point in time, when food didn't travel very far to get to the plate, the low-iodine region of the Great Lakes region was referred to as the "goiter belt" due to problematic iodine nutrition. Because food travels much more in our modern supply chain, regional differences in soil iodine content don't play as prominent a role as in earlier periods of U.S. history.
Because fish and dairy foods are among our richest sources of iodine, vegans (individuals who eat no animal foods products whatsoever) appear to be at increased risk of iodine deficiency. A 2011 study found that the average U.S. vegan had a urinary iodine level that would be considered deficient. Even among this at-risk group, however, we did not see thyroid disease related to the low-iodine diets. Note that vegetarians who include milk and eggs in their diets end up with iodine levels very similar to the entire population.
There are compounds called thiocyanates in some commonly consumed foods. At high concentrations, these chemicals can interfere with the uptake of iodine into the thyroid gland, making a person seem like they have iodine deficiency, when they may not. The common thiocyanate containing foods include cassava, soy, and Brassica family vegetables. Tobacco smoke also contains thiocyanates.
The most commonly reported version of thiocyanate-related disease is seen in areas of Africa where cassava root is an important dietary staple. In the U.S., we occasionally see this issue related to soy-based infant formulas, but even then almost exclusively in infants born with thyroid disease
Contrary to what we've read elsewhere on the Internet, we believe that at the amounts we consume these foods regularly, there is not compelling evidence of significant risk. For example, a 2011 study found no association between reported intake or soy or blood concentrations of soy nutrients and problems with thyroid function in pregnant women, a group otherwise at high risk for thyroid disease. For more on this topic, click through to this article about thiocyanate containing foods and thyroid disease.
The most likely thyroid disruptors in the environment are not in foods, but in medications or man-made toxins. Lithium (used to treat bipolar disorder) and phenylbutazone (used as an anti-inflammatory) are examples of drugs that can impair iodine nutrition.
Perchlorate, a chemical used in rocket fuel, is an environmental toxin found in water supplies in the U.S. at varying concentrations. It can also impair uptake of iodine into the thyroid. According to the Natural Resources Defense Council, if you have perchlorate in your water, you'll need a reverse osmosis filter to effectively remove it.
We also know that deficiency of iron makes the thyroid dysfunction seen in iodine deficiency worse. At this point in time, we don't have a clear explanation why. We do know, however, that this is a big public health problem worldwide, especially in the developing world.
There is an acute toxicity that can occur from excessive iodine intake that leads to mouth pain, nausea, and vomiting. This almost never occurs from dietary iodine alone, and if it did, it would require that another serious medical condition (e.g., kidney failure) be present.
The Tolerable Upper Intake Limit (UL) of 1,100 mcg / day is set by the National Academy of Sciences in its Dietary Reference Intakes (DRIs) to prevent more chronic and subtle health problems related to iodine overconsumption. Oddly, diets high in iodine are associated with increased size and decreased function of the thyroid gland, the very same symptoms we see with too little iodine. We would note here that the UL of 1,100 micrograms applies to adults ages 19 and older. For teens 14-18 years of age, the UL is set lower at 900 micrograms, and for teens 9-13 years, at 600 micrograms. For children 4-8 years of age, the iodine UL is 300 micrograms, and for children 1-3 years, it is 200 micrograms. You can review the full range of DRIs for iodine in our Public Health Recommendations section.
Luckily, diets that routinely go above the UL for iodine appear to be rare in the US, as well as throughout the world. The easiest way to get to iodine excess would be heavy consumption of sea vegetables, which can contain up to four times the UL in a single one-quarter ounce serving. For best thyroid health, we would consider the most iodine-rich sea vegetables a "sometimes" food rather than a daily indulgence.
Heavy use of iodized salt could also be a contributor toward excess iodine consumption. Iodine can be added to salt at amounts up to 77 mcg per gram. If a person was consuming 5,000 mg of sodium from iodized salt—a fairly standard sodium intake for 25% of adult U.S. males—that person might be getting just shy of 1,000 mcg of iodine per day. Of course, this example makes the unlikely assumption that all the salt in a person's diet had been fortified with iodine. As described earlier, this situation would be unlikely, since most processed, high-sodium foods have not been processed using iodized salt.
In 2001, the National Academy of Sciences established a set of Dietary Reference Intakes (DRIs) for iodine. This set of recommendations included Recommended Dietary Allowances (RDAs) for all individuals over 1 year of age, and Adequate Intakes (AIs) for infants under 1 year. These DRI recommendations are as follows.
The DRI report also established a Tolerable Upper Intake Levels (ULs) for iodine. These ULs vary with age. For adults 19 years and older, the UL is set at 1,100 micrograms per day. For teens 14-18 years of age, the UL is set lower at 900 micrograms, and for teens 9-13 years, at 600 micrograms. For children 4-8 years of age, the iodine UL is 300 micrograms, and for children 1-3 years, it is 200 micrograms. The iodine ULs are intended as average maximum limits. Occasionally going above the UL is not generally believed to be health concern.
The Daily Value (DV) for iodine is 150 mcg. This is the value that you will see on food and supplement labels, and the value that we have chosen as our WHFoods daily recommended amount.