Benefits
Tartar and plaque buildup on teeth
In a randomized crossover trial in 61 adults prone to tartar (55 completed), 500 mg a day of the dried powder for 6 months cut tartar by 52% versus control tablets and lowered plaque and gum bleeding, while gum inflammation scores did not differ. The trial was supported in part by the company that sells the powder and has not been repeated.
Later food intake after a seaweed bread meal
In a small single-blind crossover pilot in 12 overweight men, a breakfast with bread containing 4% knotted wrack was followed by 16.4% lower energy intake at a meal 4 hours later. Blood sugar and cholesterol after the meal did not differ. This was a single meal, not a weight change over time.
After-meal blood sugar and insulin responses
These are blood markers. With knotted wrack alone, 500 or 1,000 mg of an extract taken with white bread did not change blood sugar across 16 healthy adults, and breads baked with the seaweed did not lower it in 10 volunteers. A blend with bladderwrack lowered the insulin response to a carbohydrate load without changing blood sugar.
DNA damage and inflammation markers
In an 8-week placebo-controlled crossover trial in 80 overweight adults, a daily capsule with 100 mg of polyphenols did not change its main DNA damage measure overall, nor C-reactive protein, antioxidant status or inflammatory cytokines. A small drop in DNA damage appeared only in an unplanned analysis of the obese subgroup.
Natural source of iodine
In UK women with low iodine intake, 0.5 g a day of one powder (712 µg iodine per gram) for 2 weeks raised median urinary iodine from 78 to 140 µg/L, but TSH also rose, with two women above the normal range. Seaweed iodine was absorbed less completely than potassium iodide, about 33% versus 59%.
Mechanism of action
Carbohydrate-digesting enzyme inhibition in the test tube
Knotted wrack extracts inhibit maltase and sucrase, enzymes that release glucose from starch and sugar, and one comparison found only the knotted wrack extract inhibited sucrase. Baking the seaweed into bread weakened the effect in a digestion model, and human single-meal tests have not shown a consistent drop in blood sugar.
Phlorotannins are broken down mainly in the colon
After a capsule of knotted wrack phlorotannins, most metabolites appeared in urine and blood 6 to 24 hours later, pointing to breakdown by gut bacteria in the large intestine, with large differences between people. An 8-week trial found the same family of metabolites, which may serve as markers of intake.
Polysaccharides and iodine in the seaweed
Fucoidan, mannitol, alginate and laminaran together made up 347 to 528 mg per gram of dry Arctic knotted wrack, and polyphenol content differed by harvest area. The seaweed also concentrates iodine from seawater, which the thyroid uses to make its hormones, so whole powders can supply large amounts.
Clinical trials
Randomized, double-blind, placebo-controlled crossover trial; a daily 400 mg capsule holding 100 mg Ascophyllum nodosum polyphenols versus a maltodextrin capsule for 8 weeks; registered primary outcome lymphocyte DNA damage (NCT02295878); funded by the EU Seventh Framework Programme and a Spanish ministry grant; two authors were affiliated with CyberColloids Ltd (Baldrick et al. 2018, Am J Clin Nutr)
80 adults aged 30 to 65 with a BMI of 25 or more.
DNA damage did not fall in the whole group; a modest decrease appeared only in the obese subset, an analysis the authors describe as exploratory, not preplanned and underpowered. C-reactive protein, antioxidant status and inflammatory cytokines did not change. Phlorotannin metabolites were identified in urine and plasma.
Randomized crossover trial; two capsules a day giving 500 mg dried Ascophyllum nodosum powder (ProDen PlaqueOff) versus two negative control tablets, 6 months each with a 1 month washout and professional cleaning at the start of each period; supported by the Västerbotten County Council and SDC Swedencare, which sells the powder (van Dijken et al. 2015, Clin Oral Investig)
61 adults with moderate to heavy tartar formation; 55 completed.
Tartar (supragingival calculus) was 52% lower after the seaweed period than the control period (p < 0.0001), and 52 of 55 participants formed less. Plaque (p = 0.008) and gingival bleeding (p = 0.02) were also lower, but gingivitis scores did not differ (p = 0.13). One trial, partly supported by the seller, not yet replicated.
Iodine bioavailability test against potassium iodide, then a 2-week before-and-after study of 0.5 g a day of an encapsulated Ascophyllum nodosum powder holding 712 µg iodine per gram (Combet et al. 2014, Br J Nutr)
Healthy non-pregnant UK women of childbearing age with low dairy and seafood intake (22 in the absorption test, 42 in the 2-week study).
Seaweed iodine was about 33% bioavailable versus 59% for potassium iodide. Median urinary iodine rose from 78 to 140 µg/L. TSH rose from 1.5 to 2.1 mIU/L, with two women above the normal range but normal free thyroxine.
Acute, randomized, three-arm crossover trial designed as double-blind, though participants could taste the higher dose and were ultimately not considered blinded; 500 mg or 1,000 mg of an Ascophyllum nodosum extract with white bread (50 g carbohydrate) versus bread alone; primary outcome peak incremental glucose (NCT05460884); EU Horizon 2020 funding; Marigot Ltd supplied the extract and employed three of five authors (Konic Ristic et al. 2023, Mar Drugs)
16 healthy adults with normal blood sugar.
Across all participants neither dose changed glucose or insulin responses versus control. In a subgroup of 10 whose glucose rose more than 1 mmol/L after plain bread, 1,000 mg lowered peak glucose. No adverse effects were reported. A single-meal blood marker result.
Single-blind crossover pilot; a breakfast with wholemeal bread containing 4% Ascophyllum nodosum versus the same bread without seaweed (Hall et al. 2012, Appetite)
12 healthy overweight men, mean age 40 and mean BMI 30.8.
Energy intake at a test meal 4 hours later was 16.4% lower after the seaweed bread. Blood glucose and cholesterol responses did not differ between arms. Single meal; no longer-term intake or weight outcome was tested.
Double-blind, randomized, placebo-controlled crossover trial; 500 mg of InSea2, a commercial blend of Ascophyllum nodosum and Fucus vesiculosus, 30 minutes before 50 g carbohydrate from bread (Paradis et al. 2011, Appl Physiol Nutr Metab)
23 healthy adults (11 men, 12 women) aged 19 to 59.
Compared with placebo, the insulin incremental area under the curve was 12.1% lower (p = 0.04) and the Cederholm insulin sensitivity index 7.9% higher (p < 0.05); the glucose response did not differ (p = 0.24). No adverse events. A single-meal test of a two-seaweed blend, not of knotted wrack alone.
Randomized, double-blind, placebo-controlled parallel trial; 500 mg a day of InSea2 (Ascophyllum nodosum and Fucus vesiculosus) 30 minutes before the main meal, with advice for moderate weight loss, for 12 weeks; primary outcomes named in the paper blood glucose and body weight, while the registry (NCT03075943) also lists insulin, C-peptide and body composition as primary; funded by the Quebec agriculture ministry and innoVactiv, which supplied the capsules (Vodouhè et al. 2022, Mar Drugs)
56 overweight or obese adults with prediabetes who completed the trial.
No effect on body weight or blood glucose, the primary outcomes. Late C-peptide during a 2-hour glucose tolerance test (p = 0.002), heart rate (p = 0.01) and the rise in interleukin-6 (p = 0.02) were lower than placebo. A blend, not knotted wrack alone.
Randomized, double-blind, placebo-controlled trial of Gdue, a tablet combining InSea2 (Ascophyllum nodosum and Fucus vesiculosus, 95 to 5) with chromium picolinate, added to existing diabetes medicines for 6 months; the authors report no conflicts of interest (Derosa et al. 2019, Diabetes Metab Syndr Obes)
175 Caucasian adults with type 2 diabetes; 164 completed.
HbA1c fell versus placebo (p < 0.05), with fasting glucose 18.0% and post-meal glucose 11.1% lower than placebo; lipids did not change. Chromium picolinate was part of the product, so the result cannot be credited to the seaweed, and knotted wrack alone was not tested.