Zi Cai
StarPyropia yezoensis (Ueda) M.S.Hwang & H.G.Choi
Synonyms: Porphyra yezoensis Ueda, Neopyropia yezoensis (Ueda) L.-E.Yang & J.Brodie, Porphyra haitanensis T.J.Chang & B.F.Zheng, Porphyra tenera Kjellman
Traditionally used for
- Cough & breathing
- Digestion
- Urinary & fluids
Cautions & contraindications
- Kidney conditions
☯ TCM Properties
Resolves Phlegm, softens hardness; Dispels Heat; Promotes diuresis
Traditional Chinese Uses
Zi Cai (紫菜) is laver — the dried red alga Pyropia yezoensis and related Bangiaceae (Herba Porphyrae), the seaweed eaten as nori. Sweet and salty with a cold nature, entering the Lung channel, its salty softening quality resolves Phlegm and softens hardness, while its cold nature clears Heat and it also promotes urination.
Its classic use is for Phlegm-and-hardness accumulations of the neck — goiter and scrofula (thyroid and lymph-node swellings) — often paired with other seaweeds such as Kun Bu and Hai Zao. It is also used for Phlegm-Heat cough, and its diuretic action addresses edema and damp accumulation, including beriberi swelling. It is commonly taken as food or in decoction. As a cold, salty substance it is used cautiously in Spleen-Stomach deficiency-cold with poor appetite and loose stools.
Relationships
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Botanical Description
Zi Cai is the dried foliose thallus of laver, a marine red alga of Pyropia (formerly Porphyra, Bangiaceae). Several species are officially used, principally Pyropia yezoensis, P. haitanensis and Porphyra tenera, cultivated along the coasts of China, Korea and Japan. The gametophyte blade is thin and membranous, only one to two cell layers thick, ovate to lanceolate with frilled or entire margins, 10-30 cm long, and purplish-red to brownish-black, drying to near-black in pressed sheets. It anchors to intertidal rock by a small holdfast. Its life cycle alternates the foliose gametophyte with a filamentous conchocelis phase boring into mollusc shells. Being an alga, it is not a vascular plant; the whole thallus is the drug.
Cultivated and wild on rocky intertidal and shallow subtidal substrates along temperate to warm-temperate coasts.
Active Constituents
Porphyran
Sulfated galactan polysaccharideConcentration: the dominant polysaccharide of the thallus; reported at up to about 48% of dry weight in Pyropia species
A linear sulfated galactan built from D-galactose, 3,6-anhydro-L-galactose, 6-O-methyl-D-galactose and L-galactose-6-sulfate. Human digestive enzymes cannot break it down, so it behaves as a soluble, gel-forming fibre; it is degraded instead by porphyranases that gut Bacteroides in nori-eating populations acquired by horizontal gene transfer from marine bacteria. It is the constituent behind the drug's traditional softening-and-dispersing action and behind its prebiotic effects in animal models.
Iodine
Inorganic halogen (essential trace element)Concentration: red laver carries less iodine than kelps such as Laminaria/kombu, but EFSA's European survey still identified laver as one of only two seaweed groups whose consumers-only mean iodine intake exceeded 20 micrograms per kilogram of body weight per day
The single most clinically important constituent of this drug. An iodine load can precipitate hypothyroidism through the Wolff-Chaikoff effect in susceptible people, or iodine-induced thyrotoxicosis in those with autonomous nodules, and it destabilises both thyroid replacement and antithyroid therapy. Content varies widely with harvest site, season and processing, so a dose cannot be assumed from a nominal figure.
Cadmium, lead, total and inorganic arsenic
Toxic trace elements (accumulated contaminants, not natural actives)Concentration: highly variable and site-dependent. EFSA found that seaweed consumption alone can give adult consumers cadmium exposure within the range previously estimated for the whole diet, and 10-30% of whole-diet exposure for inorganic arsenic and lead; South Korea sets a maximum of 0.3 mg/kg cadmium for laver including seasoned laver.
Red algae concentrate divalent metals and arsenic from seawater. Cadmium is the element of most concern in laver: it is nephrotoxic, has a decades-long biological half-life and accumulates in renal cortex. Most of the arsenic in nori is organic rather than the more toxic inorganic form, and inorganic arsenic in nori has frequently been below the limit of determination, but neither the metal nor the arsenic burden is a property of the species so much as of the growing water.
Vitamin B12 (cobalamin, including adenosylcobalamin and methylcobalamin)
Cobalamin (corrinoid)Concentration: purple laver carries true, biologically active cobalamin rather than inactive analogues; 5 g of roasted nori was calculated to supply about 2.4 micrograms per day in a controlled human trial
Unusually among plant and algal foods, laver contains genuine cobalamin, with adenosyl- and methylcobalamin making up a substantial share of the total rather than the inactive corrinoid analogues found in spirulina. In a randomised dose-response trial in unsupplemented vegetarians, 5 g of roasted nori daily significantly improved serum B12, holotranscobalamin and homocysteine over four weeks.
Mycosporine-like amino acids (porphyra-334, shinorine, palythine)
Mycosporine-like amino acidsConcentration: characteristic UV-absorbing metabolites of Porphyra/Pyropia; the three named above are the major MAAs reported in extracts of this alga
Small water-soluble UV-absorbing compounds that protect the alga from solar radiation. In mouse models of UV-induced photoaging, MAA fractions from this alga reduced the loss of endogenous antioxidant enzymes. They underpin the alga's use in topical preparations rather than any internal action.
Phycoerythrin and phycocyanin
PhycobiliproteinsConcentration: the major light-harvesting proteins of the thallus and the source of its purple-red colour
Water-soluble chromoproteins responsible for the colour that gives Zi Cai its name. They are heat- and light-labile, which is why the colour changes on roasting, and they contribute to the antioxidant activity measured in extracts.
Taurine
Sulfonic amino acidConcentration: reported as a characteristic free amino acid of fresh Pyropia yezoensis, alongside choline and inositol
A free amino acid abundant in red algae. It contributes to the savoury taste of the dried sheet and is one of the constituents cited in the alga's reported hepatoprotective and lipid-modulating activity in animal studies.
Eicosapentaenoic acid (EPA)
Omega-3 polyunsaturated fatty acidConcentration: the principal fatty acid of the small lipid fraction of fresh Pyropia yezoensis
The lipid content of laver is low, so the absolute EPA delivered by a normal culinary or medicinal dose is small. It is nevertheless the dominant polyunsaturated fatty acid present and is cited in work on the liposoluble fraction of this alga.
⚠ Drug Interactions
Levothyroxine and antithyroid drugs (carbimazole, methimazole, propylthiouracil)
Laver is an iodine-bearing seaweed and repeated dosing adds an uncontrolled iodine load on top of a titrated thyroid regimen. In a non-randomised pre-post clinical study of 49 habitual seaweed consumers in Norway, median estimated iodine intake was 658 micrograms per day, above the tolerable upper intake level, and both urinary iodine and serum TSH fell significantly after six weeks off seaweed, with the largest TSH fall in those with the highest baseline intakes. That study covered a mixture of seaweed species eaten habitually in Norway, not Pyropia yezoensis specifically; EFSA's European assessment separately identified laver, along with kombu, as the seaweed groups driving intakes above 20 micrograms per kilogram body weight per day.
Clinical note: Ask about laver and other seaweed intake before adjusting a thyroid dose. Keep intake constant rather than intermittent, and recheck TSH about six weeks after a course of Zi Cai is started or stopped.
Radioiodine (iodine-131) therapy and diagnostic thyroid uptake scanning
Radioiodine uptake depends on the thyroid's iodine pool being depleted; a low-iodine diet is standard preparation, and seaweed is among the first foods excluded. Zi Cai delivers dietary iodine directly and its use in the preparatory window works against the whole procedure.
Clinical note: Stop Zi Cai and all seaweed for at least one to two weeks before radioiodine scanning or therapy, in line with the low-iodine diet the treating unit specifies.
Amiodarone
Amiodarone is around 37% iodine by weight and releases substantial free iodide during metabolism; amiodarone-induced thyroid dysfunction is common. Adding a dietary iodine source such as laver increases the total iodine burden in a patient whose thyroid is already being challenged. No study has combined the two directly.
Clinical note: Avoid medicinal doses of Zi Cai in patients on amiodarone, and mention seaweed intake when thyroid function tests drift.
Lithium
Lithium inhibits thyroid hormone release and is a recognised cause of goitre and hypothyroidism; excess iodine independently inhibits release through the Wolff-Chaikoff effect. The two share a target, so their combination is theoretically additive. No case series links laver to this specifically.
Clinical note: Monitor TSH as usual on lithium, and consider seaweed intake as a contributor if thyroid function deteriorates.
Nephrotoxic drugs and other dietary cadmium sources
EFSA's assessment found that seaweed consumption alone can contribute cadmium exposure comparable to the whole-diet estimates made previously, and cadmium accumulates in the renal cortex with a half-life measured in decades. In a patient with existing tubular injury, or one on drugs that damage the proximal tubule, a chronic seaweed cadmium load is an avoidable additional insult. This is an inference from exposure data, not a reported interaction.
Clinical note: Avoid long-term daily medicinal doses in patients with chronic kidney disease; source from suppliers who test for cadmium where prolonged use is intended.
Evidence Tier
Moderate evidence · 3 studiesRecorded studies by study design, strongest design at the top. This is a study-design tier only, not a GRADE rating: it does not weigh risk of bias, consistency or precision.
Systematic review / meta-analysis
0
Randomized controlled trial
1
1 verified · 0 unverified
Other clinical trial
0
Observational / case report
1
0 verified · 1 unverified
In vitro / animal
1
0 verified · 1 unverified
Other / unclassified
0
Verified: design read from PubMed for a DOI that resolves to the cited paper Unverified: taken from the study's recorded description
Clinical Studies
Effect of roasted purple laver (nori) on vitamin B12 nutritional status of vegetarians: a dose-response trial
An open-label, parallel, dose-response randomised controlled trial in 30 unsupplemented vegetarians assigned to no nori, 5 g of roasted nori daily, or 8 g daily for four weeks. Nori consumption significantly improved serum vitamin B12, holotranscobalamin, homocysteine and the combined 4cB12 score; the effect plateaued at the 5 g dose, which raised serum B12 by 59 pmol/L over control. The trial used commercial roasted purple laver identified only to genus as Neopyropia sp., the current name for Pyropia, so it is genus-level rather than species-confirmed evidence for P. yezoensis.
Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomized pre-post clinical study
Forty-nine habitual seaweed consumers in Norway were assessed before and after a six-week cessation of seaweed. Median urinary iodine fell from 270 to 87 micrograms per litre and estimated iodine intake from 658 to 189 micrograms per day, moving from above the tolerable upper intake level to well below it. Serum TSH fell significantly after cessation, most in those with the highest baseline intakes. The participants ate a mixture of seaweed species available in Norway rather than Pyropia yezoensis, so this shows the class effect of habitual seaweed intake on thyroid function rather than an effect specific to laver.
Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota
Characterised the first porphyranases, from the marine bacterium Zobellia galactanivorans, acting on porphyran from red algae of the genus Porphyra, and showed that the genes for these porphyranases and associated agarases had been transferred to the human gut bacterium Bacteroides plebeius isolated from Japanese individuals. Comparative gut metagenomes found these enzymes frequent in the Japanese population and absent from North American data. It explains why the porphyran in Zi Cai is fermentable for some populations and simply an inert bulk fibre for others.
References
- Sharma A, Yoon NY, Lee HJ. Therapeutic Potential of Neopyropia yezoensis: An Updated Review . Marine Drugs (2025) [DOI]
- European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA. Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population . EFSA Journal (2023) [DOI]
This information is for educational purposes only and is not intended to replace professional medical advice. Always consult a qualified healthcare provider before using any herbal remedy, especially if you are pregnant, nursing, or taking medications.
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