Friday Favorites: The Healthiest Natural Source of Iodine

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Dairy milk supplies between a quarter and a half of the daily iodine requirement in the United States, though milk itself has little native iodine. The milk iodine content is mainly determined by factors like the application of iodine-containing teat disinfectants. The iodine residues in milk appear to originate mainly from the contamination of the teat surface. The teats are sprayed or dipped with betadine-type disinfectants, and the iodine just kind of leaches into the milk.

Too bad most of the plant-based milks on the market aren’t enriched with iodine, too. Fortified soy milk is probably the healthiest of the plant milks, but even if it was enriched with iodine, what about the effects of soy on thyroid function? It’s funny that when I searched the medical literature on soy and thyroid, this study popped up. A cost-effective way to train residents to do thyroid biopsies. Just stick the ultrasound probe right on top and go to town. It turns out that on ultrasound your thyroid gland looks a lot like tofu.

Anyways, the idea that soy may influence thyroid function originated over eight decades ago when marked thyroid enlargement was seen in rats fed raw soybeans, though the observation that people living in Asian countries have consumed soy foods for centuries with no perceptible thyrotoxic effects certainly suggests their safety. The bottom line is there does not seem to be a problem with people who have normal thyroid function. However, soy foods may inhibit the oral absorption of Synthroid, thyroid hormone replacement drugs, but so do all foods. That’s why we tell patients to take it on an empty stomach. But you also have to be getting enough iodine, so it may be particularly important for soy food consumers to make sure their intake of iodine is adequate.

What’s the best way to get iodine? For those who use table salt, make sure it’s iodized. Currently, only half of table salt sold contains iodine, and the salt used in processed foods is typically not iodized. Of course, ideally, we shouldn’t add any salt at all. Dietary salt is a public health hazard. Think this title is a little over the top? Dietary salt is the #1 dietary risk factor for death on the planet Earth, wiping out more than three million people a year—twice as bad as not eating your vegetables.

What’s the best source of iodine, then? Sea vegetables! You can get a little iodine here and there from a whole variety of foods, but the most concentrated source by far, with up to nearly 2,000 percent of your daily allowance in just a single gram (which is like the weight of a paperclip) of seaweed.

Given that iodine is extensively stored in the thyroid, it can be safely consumed intermittently, meaning you don’t have to get it every day, which makes seaweed use in a range of foods attractive, and occasional seaweed intake enough to ensure iodine sufficiency. However, some seaweed should be used with caution due to its overly high iodine content, like kelp. Too much iodine can cause hyperthyroidism, a hyperactive thyroid gland. A woman presented with a racing heartbeat, insomnia, anxiety, and weight loss thanks to taking just two tablets a day containing kelp.

In my last video, I noted how the average urinary iodine level of vegans was less than the ideal levels, but there was one kelp-eating vegan with a urinary concentration over 9,000 mcg/liter. Adequate intake is when you’re peeing out 100-199 mcg/liter. Excessive iodine intake is when you break 300 mcg/liter, and 9,437 mcg/liter is way too much.

The recommended average daily intake is 150 mcg/day for non-pregnant, non-breastfeeding adults, and we may want to stay below 600 mcg/day on a day-to-day basis, whereas a tablespoon of kelp may contain 2,000 mcg. I’d stay away from kelp because it has too much, and stay away from hijiki because it contains too much arsenic. Here’s how much of common seaweed preparations should give you an approximate daily allowance: two nori sheets, which you can literally just nibble on them as snacks like I do; one teaspoon of dulse flakes, which you can just sprinkle on anything; one teaspoon of dried arame, which is great to add to soups; or one tablespoon of seaweed salad.

If iodine is concentrated in marine foods, this raises the question of how early hominins living in continental areas could have met their iodine requirements. Here’s what bonobos do, perhaps our closest relatives. During swamp visits, they all forage aquatic herbs.

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