Contents
What it is
Sea mustard, or wakame, is a brown alga whose fronds are split into feather-like lobes. It has long been eaten in Korea and Japan and is a different species from kelp — where kelp is a thick, stiff blade, wakame is thin and soft. It is sold dried or salted, and it swells several times over when soaked, which is worth remembering when judging quantities. In Korea it is customarily simmered into a soup after childbirth and on birthdays. Its iodine content is high, so anyone with a thyroid condition should watch the amount.
Scientific name Undaria pinnatifida
The Korean custom of feeding new mothers seaweed soup has a nutritional basis. Wakame is rich in iodine and calcium, and iodine requirements rise sharply after birth for thyroid hormone production and milk supply.
The same fact cuts the other way. Average iodine intake in Korea is already among the highest in the world, and for anyone with thyroid disease, excess seaweed can be a problem.
Key benefits
- Nutrition data
Iodine is the raw material for thyroid hormone
Thyroid hormone cannot be made without iodine. During pregnancy and lactation, requirements rise to about 1.5 times normal, and adequacy is tied directly to fetal brain development. That is the background to seaweed soup as postpartum food.
- Nutrition data
It supplies calcium and magnesium together
150 mg of calcium and 107 mg of magnesium per 100 g fresh. In diets low in dairy, seaweed acts as a genuine calcium source.
- Nutrition data
Alginate acts as soluble fibre
The viscous alginate in wakame forms a gel in the gut that slows absorption of sugars and fats. Some studies find a flatter post-meal glucose rise.
- Nutrition data
Extremely low in calories
Rehydrated wakame is mostly water, so it contributes almost no energy for its volume. It is an easy way to add bulk to soups and salads.
How to eat it well
- Soak dried wakame for about ten minutes, no longer. Over-soaking turns it soft and leaches water-soluble components.
- For soup, sauté it in sesame oil first, then add water — better for absorbing fat-soluble compounds.
- Salt the soup at the end. Wakame retains salt of its own, so seasoning early makes it too salty.
- Two or three times a week beats daily, given the accumulation of iodine.
- Dressing it with vinegar, as in a cold salad, favours calcium absorption.
How to read the grades
Each benefit carries a label showing how far research on people supports it. Grades differ between items even within the same ingredient.
- Nutrition data
- A fact verifiable from food composition databases or health authorities. This is a different kind of statement from a clinical effect.
- Strong evidence
- Consistently confirmed across randomised controlled trials or meta-analyses.
- Moderate evidence
- Human studies exist, but effect sizes are small or results are mixed.
- Early research
- Cell or animal work, or small preliminary studies.
- Not established
- Commonly claimed, but not confirmed in studies on people.
What to watch out for
- Very high in iodine. With hyperthyroidism (Graves') or Hashimoto's thyroiditis, discuss frequency with your clinician.
- Before thyroid testing or radioactive iodine treatment, seaweed must be restricted; it affects accuracy.
- Seaweeds can accumulate heavy metals such as arsenic, so sourcing matters. Hijiki in particular is high in inorganic arsenic.
- On warfarin, account for wakame's vitamin K.
- If you restrict potassium for kidney disease, check your intake.
Nutrition
Approximate values per 100 g of fresh wakame. Dried wakame expands more than tenfold when soaked.
- Calories
- 45 kcal2%
- Carbohydrates
- 9.1 g3%
- Fiber
- 0.5 g2%
- Protein
- 3.0 g6%
- Iodine
- ~1,600 µg999%+
- Calcium
- 150 mg12%
- Magnesium
- 107 mg25%
- Folate
- 196 µg49%
Percentages are of the US FDA Daily Value for adults and children 4 and over. National reference intakes differ by sex and age.
Calculate this alongside other ingredients →
Key nutrients
Dishes with this ingredient
Compare with
Sources
The databases and health authorities this article's nutrition figures and health statements draw on.
