Extraordinarily high in iodine — which is both the benefit and the risk.
Seaweed is the richest dietary source of iodine there is, by a wide margin. That makes it
genuinely useful in iodine-deficient areas and genuinely risky in
excess.
Kelp/kombu is the most concentrated; nori is far lower. A single gram of dried
kelp can contain many times the daily iodine requirement, and sustained excess iodine causes
thyroid problems in both directions — over- and underactive.
Seaweed is also one
of the few plant foods that can contain some vitamin B12, though the form and reliability are
debated.
Per 100g raw. Iodine content varies enormously between species — kelp is far higher than nori.
Based on the nutrients and fibre this food supplies. These are supporting foods, not treatments.
Nori is the safe everyday one. Kelp and kombu are far higher in iodine and
should be occasional, not daily.
Do not take kelp supplements casually.
This is where iodine excess most often comes from.
Rinse salted seaweed
before use — the sodium content can be high.
Kombu in dashi is a
brief infusion, not eaten whole, which keeps the iodine load lower.
Iodine is required to make thyroid hormone, and both deficiency and excess cause thyroid
disease. The therapeutic window is narrower than for most nutrients.
Seaweed's iodine
content varies by orders of magnitude between species and even between harvests, which makes
it difficult to dose deliberately. That variability is the main argument for treating high-iodine
seaweeds as an occasional food rather than a daily supplement.
Thyroid conditions: seaweed's iodine content can worsen both overactive
and underactive thyroid disease. Talk to your doctor before eating it
regularly.
Pregnancy: iodine requirements rise, but excess is also
harmful. Do not use kelp supplements without medical advice.
Kidney disease: potassium may need limiting. Ask your doctor for your
numbers.
IBS: several vegetables are high-FODMAP and commonly trigger
symptoms — tolerance is personal.
This is information, not a prescription. If you are managing a diagnosed condition or taking medication, talk to your doctor before making a large change to your diet.
Nutrition figures on this page come from standard food composition data. These links let you check the numbers and read the underlying research yourself rather than taking our word for it.