Last verified: today
Kelp
Also known as: Fucus vesiculosus, bladderwrack, Saccharina latissima, brown seaweed, Laminaria
Evidence under review. — Not yet rated
Iodine-rich seaweed that affects thyroid function; limited evidence for blood pressure or hormone effects.
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What it does
Kelp is a brown seaweed naturally high in iodine, along with bioactive compounds like fucoidan and polyphenols. Because of its iodine content, kelp supplementation measurably raises TSH...
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Evidence quality
Evidence base hasn't been formally rated yet. See research below.
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Clinical dose
No established dose from provided studies
What the Science Says
Kelp is a brown seaweed naturally high in iodine, along with bioactive compounds like fucoidan and polyphenols. Because of its iodine content, kelp supplementation measurably raises TSH (thyroid-stimulating hormone) levels in healthy adults, even at low doses over 4 weeks. A meta-analysis of edible algae (including kelp) found modest blood pressure reductions of about 2 mmHg systolic when consuming over 3 g/day for at least 12 weeks, particularly in people with elevated baseline blood pressure.
What It Doesn't Do
Won't boost your metabolism or help you lose weight — studies show no change in basal metabolic rate. Not a proven cancer preventive in humans. The hormone and menstrual cycle effects come from a 3-person case report — that's not real evidence. Don't expect dramatic thyroid 'support' — it disrupts thyroid function more than it helps it. No proven detox effect.
Evidence-Based Benefits
Raises TSH levels in healthy adults, even at low doses, within 4 weeks of daily supplementation.
Moderate EvidenceEffective at: 2–4 kelp capsules/day (dose-dependent effect observed)
Significantly increases urinary iodine within hours of consumption, reflecting high iodine bioavailability.
Moderate EvidenceEffective at: Single meal with seaweed or daily supplement
Edible algae including kelp modestly lower systolic and diastolic blood pressure in adults with elevated levels.
Weak EvidenceEffective at: >3 g/day whole algae for at least 12 weeks
May lengthen menstrual cycles and reduce estrogen levels in pre-menopausal women — but evidence is very preliminary.
Weak EvidenceEffective at: 700–1400 mg/day bladderwrack
Absorption & Bioavailability
Unknown for most bioactives. Iodine from kelp is well-absorbed, as shown by large spikes in urinary iodine excretion within hours of consumption. Absorption of fucoidan and polyphenols in humans is not established from the provided studies.
Red Flags to Watch For
- Kelp significantly raises TSH levels even at low doses — people with thyroid conditions (hypothyroidism, hyperthyroidism, Hashimoto's) should avoid it without medical supervision
- Iodine content varies wildly between kelp products and species — a single meal with freshly harvested seaweed can spike urinary iodine by 400%
- Kelp bioaccumulates environmental contaminants including cesium and chlorinated paraffins — product sourcing and testing matters
- The menstrual cycle and hormone effects are based on only 3 women in a case report — not reliable evidence for any claim
- High-dose kelp supplementation caused persistent TSH suppression even 2 weeks after stopping — effects may outlast use
Products Containing Kelp
See how Kelp is used in these analyzed products:
Research Sources
- PubMed
- NIH DSLD
This information is for educational purposes only and is not medical advice. Always consult a healthcare professional before starting any supplement regimen. Last updated: 2026-05-25