Seaweed and Sprouts

TL;DR. Two small categories with outsized effects. Seaweed is the only significant plant source of iodine, and the doses are wildly uneven: nori is sensible, kombu can deliver a hundred times the daily upper limit in a single serving, and thyroid dysfunction from kelp supplements is documented. Sprouts concentrate nutrients and vitamin C and are the single highest-risk raw produce category for food poisoning, because the germination conditions are ideal for bacteria. Broccoli sprouts are the richest practical source of sulforaphane precursor there is.

Seaweed

What it is

Not plants: marine algae, in three groups distinguished by pigment.

Seaweed farming is one of the fastest-growing forms of aquaculture, at around 35 million tonnes a year, overwhelmingly in China, Indonesia, South Korea, and Japan. It requires no land, no fresh water, no fertiliser, and no feed, absorbs nitrogen and carbon from the water, and provides habitat. As food production goes, its environmental profile is close to unbeatable.

What is inside it

Per 10 g dried (a realistic portion, since nobody eats 100 g of dried seaweed):

Compare with the adult RDA of 150 µg and the EU upper limit of 600 µg a day.

That table is the single most important thing in this section. A standard sushi nori sheet (about 3 g) delivers perhaps 500 to 1,300 µg, which is above the RDA and in the manageable range for occasional consumption. A piece of kombu used to make dashi can deliver tens of thousands of micrograms, hundreds of times the upper limit, though much stays in the stock. Kelp supplements are the worst case, and documented cases of both hyperthyroidism and hypothyroidism from them exist.

Beyond iodine, seaweed provides: glutamate (kombu is where Kikunae Ikeda isolated it in 1908 and named umami, which is the origin of MSG), alginates and carrageenan (soluble fibres used industrially as thickeners), fucoidans (sulphated polysaccharides with laboratory activity and little human evidence), and modest minerals.

Vitamin B12 in seaweed is a trap. Nori contains compounds that assay as B12, and some studies suggest a portion is genuinely bioactive while much is pseudo-B12 analogues that occupy the transport machinery without functioning, potentially making things worse. Spirulina is the clearer case: its B12 is overwhelmingly pseudo-B12 and it is not a reliable source. Anyone eating a vegan diet must use a supplement or fortified food, not algae (Chapter 15).

Cautions

• Iodine excess causes thyroid dysfunction in both directions and is genuinely common with regular kombu or kelp supplement use. Japanese populations have adapted to higher habitual intakes; that does not transfer to someone starting kelp capsules.
• Hijiki contains high levels of inorganic arsenic. The UK Food Standards Agency, and food agencies in Canada, New Zealand, and elsewhere, advise avoiding it. Other seaweeds are far lower.
• Heavy metals accumulate from the water: cadmium and lead vary by species and source.
• Sodium is high in most seasoned seaweed snacks.
• Anticoagulants: high vitamin K in some seaweeds.

How to use it

Nori as a wrapper, a snack, or crumbled as seasoning. Wakame in miso soup and salads, where it expands enormously on soaking. Kombu to make dashi and to add to cooking beans, where its glutamates improve flavour and its alginates are traditionally said to soften them. Dulse fried crisp. Agar as a vegan gelatine.

Sprouts and microgreens

What they are

Sprouts are germinated seeds eaten whole, root and all, after 2 to 7 days, grown in water, without light or soil.

Microgreens are seedlings grown in a medium and harvested at the first true leaves, 7 to 21 days, with only the stem and leaves eaten.

The distinction matters entirely for food safety: sprouts are grown warm and wet in the dark and eaten root and all; microgreens are grown in light and cut above the growing medium. Microgreens carry far less risk.

Why sprouting changes a seed

Germination is metabolically dramatic. The seed mobilises its reserves, and measurable changes follow:

• Vitamin C appears where the dry seed had almost none, sometimes rising several-fold. This is why sprouted pulses were used against scurvy on long voyages.
• Phytate falls by 30 to 70 percent, because the seed activates its own phytase, so iron and zinc absorption improves (Chapter 18).
• Enzyme inhibitors and some antinutrients decline.
• Starch is partly converted to simpler sugars, making them easier to digest.
• Some vitamins, notably folate and some B vitamins, increase.

Broccoli sprouts are the standout case. They contain 10 to 100 times the glucoraphanin of mature broccoli, which makes them the most concentrated practical source of the sulforaphane precursor there is, and they are the material used in most human sulforaphane research (Chapter 39). They take four to five days in a jar on a windowsill.

The food safety problem, which is serious

Sprouts are the highest-risk raw produce category there is. The reasons compound:

1. Seeds can carry pathogens internally, from contamination in the field, and no surface wash removes them.
2. Germination conditions (warm, humid, nutrient-rich, 20 to 25 °C for days) are close to ideal bacterial culture conditions. A few cells can multiply to millions.
3. They are eaten raw, so there is no kill step.

The outbreak record is substantial: the 2011 German E. coli O104:H4 outbreak traced to fenugreek sprouts caused over 3,900 illnesses, around 850 cases of haemolytic uraemic syndrome, and more than 50 deaths. Salmonella outbreaks from alfalfa and mung bean sprouts recur regularly in the US, UK, and Australia.

Consequently:

• Pregnant, immunocompromised, elderly, and very young people should not eat raw sprouts. This is standard advice from the FDA, FSA, and equivalents, and it is one of the more consistently ignored pieces of food safety guidance.
• Everyone else should buy fresh, refrigerated sprouts, keep them cold, and cook them if in doubt. Cooking eliminates the risk entirely, and stir-fried beansprouts are excellent.
• Home sprouting does not remove the risk, because the problem starts with the seed. Buy seed explicitly sold for sprouting, which is tested; rinse twice daily with clean water; keep equipment scrupulously clean; and do not let sprouts sit warm.

Which sprouts and what they give you

Microgreens have been measured as containing several-fold higher concentrations of vitamins C, E, K, and carotenoids than the mature leaves of the same plant, per gram. Per portion the difference is smaller, because you eat a few grams rather than a hundred. They are a garnish with real nutritional density, not a vegetable substitute.

The bottom line

• Seaweed is the only significant plant iodine source, and the dose range is extreme. Nori is sensible; kombu and kelp supplements can deliver hundreds of times the upper limit and cause thyroid disease. Avoid hijiki entirely for its inorganic arsenic.
• Neither seaweed nor spirulina is a reliable B12 source. Much of what assays as B12 is inactive analogue. Vegans need a supplement.
• Seaweed farming has one of the best environmental profiles of any food production: no land, no fresh water, no fertiliser, no feed.
• Sprouting raises vitamin C, cuts phytate substantially, and makes seeds more digestible. Broccoli sprouts are the richest practical sulforaphane source there is.
• Raw sprouts are the highest-risk produce category for food poisoning, because the seeds can be internally contaminated and germination conditions are ideal for bacteria. Vulnerable groups should cook them or avoid them, and cooking removes the risk entirely.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Seaweed iodine content ranges follow analyses summarised by Zava and Zava, Thyroid Research, 2011, and UK FSA surveys, which show enormous variation between species and batches. Kelp supplement thyroid dysfunction follows published case reports and endocrinology reviews. Hijiki inorganic arsenic and the resulting FSA, Health Canada, and New Zealand advisories are published by those agencies. Glutamate isolation from kombu by Kikunae Ikeda in 1908 is the origin of umami as a described taste. Pseudo-B12 in nori and spirulina follows Watanabe's work on corrinoid analogues, which is the basis of the recommendation that vegans use supplements. Sprouting effects on vitamin C, phytate, and digestibility follow Gibson's and Marton's reviews. Broccoli sprout glucoraphanin concentration is Fahey, Zhang, and Talalay, PNAS, 1997. The 2011 German E. coli O104:H4 fenugreek sprout outbreak figures are from the Robert Koch Institute and EFSA final reports. Sprout risk guidance follows FDA and FSA advice to vulnerable groups. Alfalfa L-canavanine and lupus-like effects follow primate studies and case reports. Microgreen nutrient density follows Xiao et al., Journal of Agricultural and Food Chemistry, 2012.

Open questions. How much of the B12-like activity measured in nori is genuinely bioavailable is disputed, and the safest reading remains that it should not be relied on. Long-term effects of habitual high-iodine seaweed intake outside Japan, where populations are adapted, are not well characterised.

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