Seaweed, Iodine, and Reclaiming the Critical Minerals Missing From Our Food

This newsletter exists to help others, and as a result, I frequently receive correspondences from individuals with pressing questions I want to answer. Unfortunately, due to how much work goes into producing the material here, I simply no longer have the time to respond to most of those correspondences.

The best solution I’ve been able to come up with is to have monthly open threads where anyone can ask whatever is needed (e.g., any lingering questions from the previous month) so I could make a point to always reply to them (as having them all in a single place makes it easier to get to them and also possible for others with the same question to see that answer). Thinbi Liposomal NAD+ ... Buy New $30.64 (as of 06:46 UTC - Details)

Each time I do a monthly thread, I try to tag it to a topic I’ve wanted to write about but I do not quite feel is enough for its own article. In last week’s article, since many asked, I shared some of my key perspectives on supplements, including that ideally for the body, nutrients (particularly minerals) should be gotten from your diet rather than in concentrated supplements. However, that can be tricky, especially if you do not live in an area where it’s possible to procure high quality produce grown locally in healthy soil. Fortunately, one food group bypasses the soil problem entirely and is readily available to us.

Seaweed

“Seaweed” is a catch-all for thousands of species of marine algae, grouped by pigment into browns (kombu, wakame), reds (nori, dulse, sea moss), and greens (sea lettuce). Lacking true roots, they absorb everything they contain directly from the surrounding seawater, concentrating minerals far beyond what land plants typically reach, something coastal cultures around the world each seem to have discovered on their own. For example:

• In Monte Verde, Chile, archaeologists found nine species of seaweed alongside medicinal plants in a settlement dated to roughly 14,000 years ago, which suggests they were used for both food and medicine.

• By the first centuries AD, Chinese medical texts were specifically documenting Sargassum seaweed as a treatment for goiter, a medicinal use still incorporated today.

• Greco-Roman remedies included seaweed in the treatment of gout, joint ailments, and venomous bites.

This recognition by ancient cultures across the world that had no communication with each other suggests seaweed provided unique health benefits that were tangible enough the simplest medical systems could recognize them. Modern nutritional science, in turn, shows they were onto something. One of the more remarkable demonstrations of this is in the gut: the intestinal bacteria of Japanese individuals carry genes for digesting the polysaccharides in nori that are absent in North Americans, and those genes were acquired directly from the marine bacteria that live on seaweed.¹

Note: another extraordinary aspect of seaweed is how quickly it grows (e.g., the widely consumed species that make laver and nori are ready for their first harvest within 30-90 days of seeding, and then can have numerous successive harvests every 10-20 days¹), along with the fact it can be grown across vast stretches of coastal water where nothing else is being farmed. As such, at a time when the nutrient content of our food supply keeps declining, one of the few foods able to make up the difference also happens to be one of the easiest and cheapest to produce at scale, both for people and for livestock.

Seaweed’s Nutritional Content

Vitamin World Vitamin ... Buy New $14.14 (as of 01:11 UTC - Details) Sea vegetables carry an unusually broad spectrum of minerals and trace elements the body needs for basic physiological function, often at levels that dwarf what you’d find in land plants. Most contain vitamins A, C, and a range of B vitamins, and a handful are among the only plant-based foods on earth that provide meaningful amounts of B12 (e.g., a 2024 study found seaweed significantly improved B12 levels in vegetarians).
Note: high B12 seaweed contains the best forms of B12¹ and is also a rich source of (healthy) folate.¹

Protein content swings anywhere from roughly 10% to over 40% of dry weight depending on species, and seaweed protein is complete, so all nine of the essential amino acids are present (the nine the body cannot synthesize on its own). Seaweeds are also low in fat and remarkably high in fiber and among the richest natural dietary sources of iodine.

Most importantly, since seaweed absorbs minerals directly from seawater rather than soil, mineral content in seaweed runs up to 100x higher per gram than land vegetables across the board (e.g., one study found nori has 3 times more bioavailable iron than spinach, gram for gram — and sea lettuce, 5 times more).

Note: land vegetables today measurably contain less calcium, iron, and other minerals than they did in 1950. While this is commonly attributed to the soil becoming demineralized (as the primary input given to soil is nitrogen, phosphorus, and potassium), decades of selective breeding to meet other priorities also plays a key role¹—both of which are not applicable to seaweed.

Conversely, this also means that if seaweed is grown in contaminated water, it will pick up the contaminants present there, which has led to extensive debate on which brands and forms of seaweed are the best to eat, particularly since many of the ancient societies which cultivated seaweed did so at a time when there was much less lead in the oceans. This, I believe, argues for two key principles—getting seaweed from clean sources and not consuming them in excess but rather treating them as a complementary part of a complete diet.

Note: one of the most common concerns with seaweed is its arsenic content. Data in this area is often misleading, as testing rarely distinguishes organic arsenic (which in seaweed exists mostly as arsenosugars and is far less toxic) from the significantly more toxic inorganic form. The main exception is hijiki, which contains a substantial amount of inorganic arsenic, so many guidelines (appropriately) caution against consuming it. For other seaweeds, preparation also makes a large difference: one study found that washing, soaking, cooking, and boiling reduced total arsenic content by up to 58.8%, 91.5%, 50%, and 60% respectively.

Homienly Wheat Straw D... Buy New $24.99 (as of 06:51 UTC - Details) The key nutritional components of commonly consumed forms of seaweed are as follows:^1,2,3,4,5,6

• Nori — Comparable to beef liver on B12, tomatoes and lettuce on vitamin C, and bananas in fiber content.¹
• Wakame — A meaningful source of iodine and folate (which are needed for healthy fetal development), that also has a high fucoxanthin content that’s shown promise for metabolic health.¹
• Kombu (Kelp) — Among the strongest iodine accumulators of any living organism on Earth, with potassium and calcium levels that dwarf most conventional foods.¹ Is also a natural source of fucoidan, a bioactive polysaccharide studied for its wide variety of therapeutic properties (particularly reducing pain and inflammation).¹
• Dulse — Exceptionally high in potassium and B12 for a plant, with EPA (a key fat in fish oil) making up to half its total fatty acid content.^1,2 Particularly protein-rich when harvested in winter and early spring.^1,2
• Irish moss — High in protein (35% dry weight), with a favorable mineral profile and floridoside, the compound behind its use in natural moisturizers.^1,2
• Sea lettuce — B12 high enough on its own to exceed the full adult daily RDA, plus calcium covering over a third of an adult male’s daily need. Unusually low iodine for seaweed.¹
• Caribbean sea moss — A diverse source of minerals and trace elements, with notable protein and EPA content.^1,2

The Global Iodine-Deficiency Predicament

Seaweed is one of the richest natural sources on the planet of iodine (which the thyroid requires to function). Just a few grams a day can meet or exceed your entire daily requirement without supplements, fortified salt, or anything processed. Fire-Maple Petrel G3 U... Buy New $25.59 (as of 06:51 UTC - Details)

How many grams depends entirely on the species. Dried nori averages around 16 mcg of iodine per gram and wakame around 42, whereas kombu averages roughly 2,350 (and some kelp flakes exceed 8,000).^1,2 As such, reaching the 150 mcg RDA takes about 9 grams of nori but less than a tenth of a gram of kombu, and a single gram of kombu delivers twice the 1,100 mcg US upper limit.

Note: because iodine is critical for health, to ensure the population received it, it is added to (iodized) salt. In contrast, most of the natural sources of salt—which I detailed here and believe are superior for your health compared to table salt due to their robust mineral content—do not contain significant amounts of iodine (which makes alternative dietary sources like seaweed important for individuals consuming them).

However, despite decades of widespread iodized salt initiatives, roughly 1.9 billion people (31% of the global population) still have inadequate iodine intake, and diagnosed deficiency cases rose about 23% between 1990 and 2021. Moreover, iodized salt itself isn’t the dependable dietary source most people assume: a 2008 study of US salt samples found iodine content ranging from 12.7 to 129 mg/kg against a labeled 45 mg/kg, with over half the samples falling below the FDA’s own recommended range.
Note: since other halogens (e.g. fluorine and bromine) compete with iodine, many have argued their widespread presence (e.g., via water fluoridation, perchlorate contamination of groundwater and food,¹ or the switch from iodate to bromate as a dough conditioner) is responsible for the widespread functional iodine deficiency seen here. Supporting this, adequate iodine intake has been found to mitigate fluoride’s effects on the thyroid.¹ Fortunately, California recently outlawed bromated flour, so this may at last be changing.

In the United States, the decline is visible in the national survey data. Median urinary iodine fell from 320 mcg/L in 1971–1974 to 145 mcg/L by 1988–1994 (with pregnant women since dropping further to 125 mcg/L),¹ and more than 15% of American women of childbearing age now fall below 50 mcg/L (the level the WHO uses to define moderate deficiency in a population).¹ A newer contributor is the move away from dairy, which at roughly 85 mcg per cup¹ is the main source of iodine in most Western diets, as a 2017 analysis of 47 plant-based milks found most contained about 2% of the iodine in cow’s milk.¹

Japan’s historically low goiter rate — as low as one case per million people — is attributed to heavy seaweed consumption. Compare that to the US today, where goiter affects roughly 1.3–2.5% of working-age adults, or 13,000 to 25,000 cases per million — a rate 13,000 to 25,000 times higher than Japan’s historic low, even after decades of salt iodization. At the same time, excessive iodine should also be avoided (e.g., subclinical and overt hypothyroidism is more common in iodine-sufficient, seaweed-heavy regions and in individuals with chronic iodine deficiency, a sudden increase can create hyperthyroidism), which is why intake and preparation methods are essential. Fire-Maple Greenpeak 1... Buy New $19.95 (as of 06:51 UTC - Details)

Traditional cultures that ate the most seaweed also utilized practices that kept the dose in a sustainable range. The best example is kombu. Since its iodine is water-soluble, boiling kombu for 15 minutes removes up to 99% of it,¹ so kombu that is boiled and then eaten (as many traditional preparations are) carries a fraction of what the dried strip did. Conversely, that iodine ends up in the water, making dashi (the broth made by simmering kombu and then discarding it) a concentrated source, with kelp-based soups measured at anywhere from 165 to 7,750 mcg per serving.¹ The Japanese also eat kombu sparingly (roughly half a gram per person per day) and typically alongside soy and cruciferous vegetables, both of which reduce how much iodine the thyroid takes up.¹ None of this context made it into the Western conversation about seaweed, where kombu is often eaten whole or swallowed as kelp tablets.

Note: Indigenous Mesoamerican cultures processed corn with limewater (nixtamalization) before eating it, which makes niacin bioavailable. Skipping this step results in pellagra, a niacin-deficiency disease that killed thousands across the American South and Europe into the 20th century.

Human trials, in turn, have tested whole seaweed providing roughly 475–1,320 mcg of iodine per day for up to eight weeks without causing thyroid abnormalities, exceeding the European upper limit in some cases while remaining below Japan’s 3,000 mcg upper limit. This hence indicates that, while high, the iodine levels in seaweed are not a risk if it is consumed appropriately. GasOne 15,000 BTU Prop... Best Price: $55.19 Buy New $59.99 (as of 06:51 UTC - Details)

Isolated iodide, however, has not shown the same margin. In a four-week randomized trial of healthy adults, subclinical hypothyroidism began appearing at 400 mcg per day of supplemental iodide (in 5% of participants) and reached 47% at 2,000 mcg,¹ which is why the American Thyroid Association advises against iodine or kelp supplements above 500 mcg per day.¹ The people who need to be most careful, though, are those with existing thyroid disease (e.g., Hashimoto’s, Graves’, or thyroid nodules), in whom excess iodine can tip the gland into either hypothyroidism or hyperthyroidism.¹ Similarly, high seaweed intake can cause or worsen symptoms in individuals with thyroid autoimmunity, and the transient hypothyroidism and iodine-induced goiter seen in Japan typically reverse once seaweed is restricted.¹ Likewise, daily kombu was found to suppress thyroid function in healthy Japanese adults until it was stopped.¹ If you have a thyroid condition or take thyroid medication, I would hence stay with the low-iodine seaweeds (e.g., nori and sea lettuce) unless someone is monitoring your thyroid.

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