Ascophyllum nodosum

Ascophyllum nodosum

1. Identity and Taxonomy

Ascophyllum nodosum (Linnaeus) Le Jolis is a large, perennial brown macroalga. Commonly known as rockweed, it is a brown alga belonging to the family Fucaceae of the order Fucales (class Phaeophyceae, phylum Ochrophyta). Other vernacular names in use include knotted wrack, egg wrack, knotted kelp, and Norwegian kelp. The genus is monotypic — that is, A. nodosum is the sole species within Ascophyllum. Its common name "knotted wrack" derives from its distinctive appearance, with long fronds dotted with nodular swellings resembling small bubbles.

The genus is basal in Fucaceae and is found from Arctic Canada, Greenland, Iceland, and northern Norway to the southern reaches of Portugal and Long Island (USA). It is often found along the rocky coasts of the North Atlantic, as well as on the northeastern coast of the United States and the east coast of Canada, where it forms dense colonies in the intertidal zone. A. nodosum is a common fucoid alga found on sheltered, intertidal, and rocky Arctic shores.

Common Forms and Preparations

The harvested biomass is processed into a range of commercial forms for human dietary supplementation, animal nutrition, agriculture, and cosmetics. Ascophyllum is used in the production of alginate, fertilizers, animal nutrition, and cosmetics. For human use, preparations include:

• Dried whole-thallus powder — used directly in capsule or tablet form, or added to foods.
• Standardized polyphenol-rich extracts — concentrated fractions enriched in phlorotannins, produced by solvent (e.g., ethanol:water) extraction systems. One food-grade extraction approach used an ethanol:water (60:40 vol/vol) extraction system specifically developed for use with fresh or frozen Ascophyllum nodosum.
• Liquid extracts and tinctures — for flexible dosing in nutritional applications.
• Veterinary and pet dental products — powders, chews, and dry food inclusions, evaluated in several controlled veterinary trials.

Ascophyllum is used in the production of alginate, fertilizers, animal nutrition, and cosmetics, with approximately 50 commercial companies manufacturing A. nodosum extracts for use in agriculture and horticulture.

2. Historical and Traditional Use

Ascophyllum has been traditionally used in coastal European and North American cultures for its nutritional and medicinal properties. It has a long history of use as a soil fertilizer and animal feed, valued for its ability to enhance plant growth and improve livestock health. In more recent decades, its nutritional and therapeutic value for humans has been recognized, especially in Scandinavian and Celtic diets.

Historically, coastal populations used dried kelp as a mineral-rich food supplement and as a remedy for goiter and other iodine-deficiency conditions. The seaweed's role as a remedy for goiter reflects an empirical recognition of its high iodine content, centuries before the biochemical basis was understood.

Fucus vesiculosus and Ascophyllum nodosum extracts have been traditionally used for the treatment of obesity and several gastrointestinal diseases. Over the centuries, this seaweed has evolved from a simple food source to a valued component in dietary supplements for its health benefits.

For centuries, seaweed has inspired botanical, industrial, and pharmaceutical interest. In ancient times, algae have been used as fertilizers. The use of A. nodosum specifically as a human food or medicinal agent is historically centered on the North Atlantic coastal communities of Ireland, Scotland, Iceland, Scandinavia, and maritime Canada, where its accessibility and nutrient density made it a dietary staple.

During the late 1990s, it was observed that a diet containing a brown marine alga (Ascophyllum nodosum) resulted in decreased deposit formation in heavy-calculus-producing patients — an observation that, combined with veterinary observations, subsequently prompted formal clinical investigation of its oral health effects.

3. Chemical Composition and Key Constituents

Macronutrient Profile

A. nodosum is a nutritionally rich source of protein (5–10%), lipids (2–7%), carbohydrates (40–70%), ash (15–25%), and other compounds. It also contains vitamins and minerals, including sodium (3–4%), potassium (2–3%), magnesium (0.5–0.9%), and iodine (700–1200 mg/kg dry weight). The main fatty acid in Ascophyllum nodosum is oleic acid. It also contains saturated fatty acids and polyunsaturated fatty acids omega-3 and omega-6.

Polysaccharides

A. nodosum is rich in unique polysaccharides, including alginic acid (15–30%), fucoidans (4–10%), mannitol (5–10%), and laminaran (0–10%). It is also rich in the fucan ascophyllan. Fucoidans and fucans are rich in fucose, a carbohydrate. The polysaccharide composition varies significantly by geographic origin and reproductive stage: the total content of dominating carbohydrates (fucoidan, mannitol, alginate, and laminaran) ranged from 347 mg/g dry weight in Norwegian Sea samples to 528 mg/g dry weight in Barents Sea samples.

Phlorotannins (Polyphenols)

The tannins present in Ascophyllum nodosum are called phlorotannins, which are found exclusively in brown algae. These compounds are the primary bioactive phenolics of interest for human health applications. A. nodosum stands out in the algae kingdom due to its impressive content of total polyphenols and flavonoids.

Phlorotannin content is subject to notable seasonal variation: research has demonstrated a clear seasonal variation in terms of phenolic content, with June and July being the highest (36.4 and 37 mg/g, respectively) and May the lowest (21.8 mg/g). The antioxidant activities, in terms of DPPH free radical scavenging activity, correlated with the phenolic contents observed (r = 0.81), with July being the highest (58%) and April the lowest (26%).

Carotenoids, Simple Phenolics, and Other Compounds

A. nodosum, similarly to other brown seaweeds, contains different carotenoids including fucoxanthin, fucoxanthinol, zeaxanthin, violaxanthin, α- and β-carotene, and others; chlorophyll; anthocyanin; simple phenolics like catechin and epigallocatechin; hydroxybenzoic acid; coumaric acid; cinnamic acid; rosmarinic acid; and caffeic acid; as well as amino acids.

The main inorganic compounds of A. nodosum are Na⁺, Mg²⁺, K⁺, Cl⁻, SO₄²⁻, and the most significant organic substances are phenols, fucoidans, lipids, proteins, alginic acid, vitamins, hormones, and enzymes.

4. Mechanisms of Action

Inhibition of Carbohydrate-Digesting Enzymes

The most extensively characterized mechanism relevant to human health is the inhibition of the digestive enzymes α-amylase and α-glucosidase by phlorotannins. The phlorotannins contained in algae exert a noncompetitive and reversible blocking action of the α-amylase and α-glucosidase enzymes in the intestine, causing a slowdown in the absorption and digestion of carbohydrates and demonstrating an antihyperglycemic action in vivo, particularly on postprandial hyperglycemia.

In vitro work has quantified this inhibitory capacity: the 80°C hot-water extract had the highest α-glucosidase and α-amylase inhibitory activity, with IC₅₀ values of 0.24 and 1.34 μg phenolics, respectively, compared to IC₅₀ values of the reference inhibitor acarbose of 0.37 and 0.68 μg. These results show that fresh A. nodosum has strong α-glucosidase and mild α-amylase inhibitory activities that correlated with phenolic contents. Researchers have also found that phlorotannin-enriched A. nodosum extracts were reported as among the strongest α-amylase inhibitors in comparisons between brown algal species.

The results across studies suggest that polyphenols from Ascophyllum nodosum seaweed hold significant potential as enzyme inhibitors, although the inhibitory activity may vary depending on the extraction conditions and the specific enzyme involved.

Inhibition of Intestinal Glucose Transport

Beyond enzyme inhibition, fucoidan isolated from A. nodosum has been identified as an inhibitor of the intestinal sodium-glucose cotransporter 1 (SGLT1), the primary active transporter of glucose across the intestinal brush border. This mechanism has been demonstrated in preclinical (animal) studies and represents a distinct, complementary pathway for reducing postprandial glucose absorption.

Antioxidant Activity

Special attention has been given to the antioxidant activity of phlorotannins. The inhibitory capacities of polyphenols, specifically phlorotannins from Ascophyllum nodosum, against digestive enzymes such as α-amylase and α-glucosidase have been explored alongside their antioxidant properties. Phlorotannin extracts demonstrate significant free radical scavenging capacity, and this activity co-varies with phenolic content across seasons and geographic origin.

Lipase Inhibition

The polyphenols contained in Ascophyllum nodosum extract can inhibit digestive enzymes such as lipase and alpha-amylase, and are therefore capable of inhibiting the absorption of starch and lipids. Inhibition of pancreatic lipase — which is required for fat digestion — has been proposed as a mechanism relevant to body weight and lipid metabolism.

Effects on Oral Microenvironment

The mechanism by which A. nodosum affects oral health has not been fully elucidated. Unlike brushing or chews, Ascophyllum nodosum does not work by physically removing plaque or tartar. Instead, it is often described as a systemic dental ingredient, because it is consumed, digested, and then thought to influence the oral environment through compounds that reach the mouth via saliva. Ascophyllum nodosum contains numerous biologically active ingredients, including micro-elements, vitamins, and several other compounds; however, the exact mechanism of its beneficial action remains unclear. The very first metabolomic data suggest that it could change the composition of dog saliva.

5. Scientific Evidence by Health Area

5.1 Blood Glucose and Metabolic Health

This is the most extensively clinically studied area for A. nodosum in humans, though results are mixed.

Key randomized controlled trials (RCTs):

A double-blind, randomized, placebo-controlled crossover study examined the impact of brown seaweed on post-load plasma glucose and insulin concentrations, enrolling 23 participants (11 men, 12 women) aged 19–59 years. The single ingestion of 500 mg of brown seaweed had no significant effect on the glucose response (p = 0.24, adjusted for baseline). Glucose and insulin responses were similar between men and women. Consumption of the seaweed capsules was not associated with any adverse event. These data suggest that brown seaweed may alter insulin homeostasis in response to carbohydrate ingestion.

A second RCT specifically studied the A. nodosum extract in normoglycemic healthy subjects: the effects on postprandial glucose and insulin responses to white bread were investigated in an acute, randomized, double-blind, three-arm, crossover, controlled trial in healthy, normoglycemic subjects. Sixteen subjects were administered either control white bread (50 g total digestible carbohydrates) or white bread with 500 mg or 1000 mg of BSW extract. Biochemical parameters were measured in venous blood over 3 hours. Analysis of the responses of all subjects to either 500 mg or 1000 mg of BSW extract versus control revealed no significant effects of treatments. The variation in response to the control was used to classify individuals into glycaemic responders and non-responders, suggesting that individual baseline glycaemic variability may be an important moderator of response.

Preclinical (mouse) evidence: a phytocomplex from Fucus vesiculosus and Ascophyllum nodosum only delayed and reduced the peak of blood glucose (p < 0.05) in mice fed with a normal diet without changing the area under the blood glucose curve. In the NASH model, however, the phytocomplex was able to reduce both the postprandial glycaemic peak and the AUC.

An observational study (not an RCT) assessed a nutraceutical combining A. nodosum and Fucus vesiculosus in patients with one or more components of the metabolic syndrome: a longitudinal, retrospective, observational study was conducted among primary care physicians, nutritionists, and specialists from various disciplines. The impact of 180 days of administration was assessed on body weight, waist circumference, fasting blood glucose, HbA1c, lipid profile, and blood pressure levels. The likelihood of experiencing a first major cardiovascular event over ten years was estimated using Italian risk charts.

A review of clinical trials in this area concluded: none of the clinical trials included in the review reported any major adverse effects in response to the administration of seaweed extracts. Administration of A. nodosum and F. vesiculosus was well tolerated and there were no signs of organ toxicity or negative effects on physiological function.

Overall evidence strength: The available RCTs in healthy subjects have generally failed to demonstrate statistically significant effects on postprandial blood glucose in normoglycemic populations. In vitro and animal data supporting enzyme inhibition are robust, but translation to human clinical outcomes remains inconsistent. Evidence in prediabetic or metabolic syndrome populations is preliminary and largely drawn from observational or mixed-product study designs. The body of evidence is currently characterized as preliminary to mixed.

5.2 Oral Health (Calculus, Plaque, and Gingivitis)

Oral health is one of the areas with the most direct and consistent human clinical evidence for A. nodosum.

Key RCTs in humans:

A randomized controlled crossover study evaluated the effect of daily intake of Ascophyllum nodosum on supragingival calculus, plaque formation, and gingival health over a 6-month period. Sixty-one adults with moderate to heavy calculus formation participated. In a randomized order over two 6-month periods, they swallowed two capsules daily, comprising a total of 500 mg dried marine alga powder (Ascophyllum nodosum, ProDen PlaqueOff®) or two negative control tablets. Plaque (p = 0.008) and gingival bleeding (p = 0.02) were also significantly less in the alga group. However, no significant difference was found between groups on all calculus measures. The study further reported that the alga intake had a systemic effect on oral health, and that the structure of calculus that did form became more porous, making it easier to remove.

Veterinary controlled trials: The majority of controlled dental studies using A. nodosum have been conducted in dogs and cats. A placebo-controlled, double-blind, randomized study (designed according to evidence-based medicine standards) determined the effect of 90-day administration of edible treats containing A. nodosum on plaque and dental calculus accumulation in dogs, as well as on other parameters including plaque index, calculus index, oral health index, gingival bleeding index, and volatile sulfur compound concentration. Sixty client-owned dogs underwent professional dental cleaning and were randomly subdivided into two groups receiving daily edible treats containing A. nodosum, or placebo, adjusted to their bodyweight. Clinical trials in dogs and cats revealed that Ascophyllum nodosum exerts the strongest preventive action as powder, followed by dental bites and dry pet food. Based on available clinical data it is recommended to administer Ascophyllum nodosum to dogs and cats after an oral prophylactic procedure to reduce the recurrence of plaque and calculus formation.

Overall evidence strength: For oral health in humans, the evidence from one RCT is supportive of effects on plaque and gingival bleeding, though calculus reduction results were not uniformly significant. Veterinary evidence (dogs, cats) is more extensive and consistently positive for preventive oral hygiene support. The mechanism remains incompletely characterized.

5.3 Antioxidant Activity and Oxidative Stress

One well-designed RCT has directly assessed the in vivo antioxidant effects of an A. nodosum phlorotannin extract in humans: the aim was to investigate the bioavailability and effect of a brown seaweed (Ascophyllum nodosum) (poly)phenol extract on DNA damage, oxidative stress, and inflammation in vivo. A randomized, double-blind, placebo-controlled crossover trial was conducted in 80 participants aged 30–65 years with a BMI ≥ 25. The participants consumed either a 400-mg capsule containing 100 mg seaweed (poly)phenol and 300 mg maltodextrin or a 400-mg maltodextrin placebo control capsule daily for an 8-week period.

Bioactivity was assessed with a panel of blood-based markers including lymphocyte DNA damage, plasma oxidant capacity, C-reactive protein (CRP), and inflammatory cytokines. Untargeted metabolomics analysis of urine and plasma samples was used to explore the bioavailability of seaweed phenolics.

Results: Consumption of the seaweed (poly)phenols resulted in a modest decrease in DNA damage but only in a subset of the total population who were obese. There were no significant changes in CRP, antioxidant status, or inflammatory cytokines. Phlorotannin metabolites identified as potential biomarkers of seaweed consumption included pyrogallol/phloroglucinol-sulfate, hydroxytrifurahol A-glucuronide, dioxinodehydroeckol-glucuronide, diphlorethol sulfates, C-O-C dimer of phloroglucinol sulfate, and C-O-C dimer of phloroglucinol. To the best of the authors' knowledge, this represented the first comprehensive study investigating the bioactivity and bioavailability of seaweed (poly)phenolics in human participants.

Separately, in 43 healthy subjects, one trial did not find significant differences between treatment and placebo groups regarding soluble markers of inflammation (TNF-α, IL-6, and CRP) after 6 weeks of treatment with 1800 mg A. nodosum and 350 μg iodine.

Overall evidence strength: Human RCT evidence suggests modest, population-subgroup-specific reductions in DNA damage (in obese participants), with no significant broader antioxidant or anti-inflammatory effects at the doses tested. In vitro evidence for antioxidant activity is robust, but human translation is limited. Evidence is preliminary.

5.4 Iodine Nutrition and Thyroid Function

Preliminary clinical research conducted in women with iodine insufficiency shows that taking Ascophyllum nodosum capsules at a dose of 500 mg per day for 14 days nearly tripled urinary iodine excretion and increased thyroid-stimulating hormone (TSH) levels by 36% compared with baseline. The validity of these results is limited by the absence of a control group.

Regarding bioavailability: in humans, urinary excretion of iodine following Ascophyllum nodosum ingestion was reported as only 33%, compared to 59% excretion with a potassium iodide control. The reduced iodine bioavailability was attributed to reduced release of iodine from the algal matrix.

Overall evidence strength: The iodine-delivery effect is biologically well-supported, though the bioavailability of algal iodine is lower than that from potassium iodide. The clinical significance for thyroid hormone production has been studied only in small, uncontrolled settings. Evidence is preliminary.

5.5 Lipid Metabolism and Body Weight

Preclinical data in rodent models have shown effects on lipid metabolism. One animal study reported that A. nodosum and F. vesiculosus extracts improved lipid metabolism and reduced inflammation in high-energy diet-induced hyperlipidemia rats. Some in vivo studies have also shown that Ascophyllum nodosum can decrease weight. The proposed mechanism for weight-related effects involves inhibition of pancreatic lipase (reducing fat absorption) and alpha-amylase/glucosidase (reducing carbohydrate absorption), alongside alginate-mediated effects on satiety.

Human RCT data in this area are sparse. The observational metabolic syndrome study referenced above included body weight and waist circumference as endpoints, but its non-randomized, retrospective design limits causal inference.

Overall evidence strength: Evidence for weight and lipid effects in humans is not yet established through well-designed RCTs. Mechanistic rationale from in vitro and animal studies is sound, but human clinical evidence is insufficient to draw firm conclusions.

5.6 Inflammation

Clinical evidence for anti-inflammatory effects in humans is limited and largely negative at the doses tested. One randomized, double-blind, placebo-controlled crossover trial did not find significant differences between treatment and placebo groups regarding soluble markers of inflammation (TNF-α, IL-6, and CRP) after 6 weeks of treatment with 1800 mg A. nodosum and 350 μg iodine in 43 healthy subjects. The RCT assessing DNA damage likewise found no significant changes in CRP, antioxidant status, or inflammatory cytokines.

Overall evidence strength: Human clinical evidence for anti-inflammatory effects is negative or absent at the doses tested to date. In vitro activity is documented, but has not translated to significant effects in controlled human trials.

6. Body Systems and Health Areas Associated with A. nodosum

• Endocrine system / Thyroid: A. nodosum is a dietary source of iodine, which is required for thyroid hormone synthesis. It has been traditionally used for iodine-deficiency goiter.
• Metabolic / Glycemic: Phlorotannins inhibit intestinal carbohydrate-digesting enzymes (α-amylase, α-glucosidase) and may influence postprandial insulin homeostasis.
• Oral / Dental: Oral supplementation has demonstrated reductions in plaque and gingival bleeding in a human RCT, and consistent preventive effects in veterinary trials.
• Antioxidant / DNA integrity: Phlorotannins are bioavailable and associated with modest reductions in lymphocyte DNA damage in obese individuals.
• Digestive / Lipid metabolism: Alginates and phlorotannins may modulate fat absorption via lipase inhibition; polysaccharides may influence gut microbiota.
• Cardiovascular (indirect): Metabolic syndrome components (blood glucose, lipids, blood pressure, body weight) have been assessed as secondary endpoints in observational studies.

7. Dosage Forms and Reported Dosages

Dosages reported in clinical studies vary by preparation and indication:

• Ascophyllum nodosum has most often been used by adults as a dried powder at doses of 500 mg by mouth daily for up to 6 months.
• Dried Ascophyllum nodosum powder has been used with apparent safety at doses up to 500 mg per day for periods of up to 6 months.
• The calculus/plaque RCT used two capsules daily comprising a total of 500 mg dried marine alga powder (Ascophyllum nodosum, ProDen PlaqueOff®).
• The postprandial glycaemic RCT tested 500 mg and 1000 mg of brown seaweed extract administered with a standardized carbohydrate meal.
• The DNA damage/antioxidant RCT used a 400-mg capsule containing 100 mg seaweed (poly)phenol and 300 mg maltodextrin, taken daily for an 8-week period.
• One inflammation trial used 1800 mg of A. nodosum combined with 350 μg iodine for 6 weeks in healthy subjects.
• Preliminary iodine research used 500 mg per day for 14 days in women with iodine insufficiency.
• One nutraceutical product containing A. nodosum and F. vesiculosus was reported to reach intestinal concentrations 25–50 times higher than in vitro inhibitory concentrations at an oral dose of 250–500 mg.

8. Safety Considerations and Interactions

General Tolerability

When taken by mouth, Ascophyllum nodosum is possibly safe when used for up to 6 months. It is usually well-tolerated. None of the clinical trials included in published reviews reported any major adverse effects in response to administration of seaweed extracts. Administration of A. nodosum and F. vesiculosus was well tolerated and there were no signs of organ toxicity or negative effects on physiological function.

The currently available α-glucosidase inhibitors including acarbose, miglitol, and voglibose produce gastrointestinal side effects such as flatulence and diarrhoea due to the fermentation of undigested carbohydrates in the intestine. Notably, one brown seaweed trial showed that a relatively small dose of α-amylase and α-glucosidase inhibitors from a brown seaweed extract was not accompanied by gastrointestinal intolerance or discomfort.

Heavy Metal Accumulation

Seaweeds such as Ascophyllum nodosum can accumulate high levels of heavy metals such as arsenic. Arsenic in the form of arsenate anion can be accumulated by seaweeds from the water; even though it gets metabolized into arsenosugars and arsenolipids (organoarsenic compounds), it can still be present in seaweed biomass. However, previous studies have revealed that the concentration of inorganic arsenic (iAs) in A. nodosum is less than 1% of the total arsenic content. The organic form is considered to be less toxic than the inorganic form, since inorganic arsenic is acknowledged as carcinogenic for humans. Metal concentrations vary significantly by harvesting location: basic concentrations in dried algal tissue from Lofoten, Norway, were considered to be 75 mg Zn/kg DW, 5.5 mg Cu/kg DW, <3 mg Pb/kg DW, and <0.7 mg Cd/kg DW; however, concentrations from the Trondheimsfjorden were very high for Zn (375–700 mg/kg) and Cu (18–60 mg/kg).

Iodine-Related Risks

Regular intake of iodine-rich seaweeds such as kelps (Laminaria/Ascophyllum/fucoids) has the potential for exposure to excess iodine with possible adverse effects on thyroid function, particularly in those with pre-existing thyroid disorder, pregnant women, and neonates. The relationship between iodine intake and thyroid disorders is U-shaped, and both iodine deficiency and excess may cause thyroid dysfunction. Although iodine excess is generally well tolerated by most individuals, increased iodine intake over time or abrupt changes in iodine intake can cause the thyroid gland to become overactive, resulting in hyperthyroidism. Other studies, as summarized in a systematic review, have found that long-term iodine excess is associated with subclinical hypothyroidism in different population groups, including adults, children, and pregnant women.

Ascophyllum nodosum contains iodine. Long-term use or high doses of iodine might make certain thyroid disorders worse. Additionally, people with autoimmune thyroid disease might be especially sensitive to the harmful effects of iodine.

Pregnancy and Lactation

There is not enough reliable information to know if Ascophyllum nodosum is safe to use when pregnant or breastfeeding.

Drug Interactions

• Antithyroid drugs: Concurrent use of antithyroid drugs and iodine may increase the antithyroid effect and cause hypothyroidism. Ascophyllum nodosum contains significant amounts of iodine.
• Amiodarone: Ascophyllum nodosum and amiodarone both contain iodine. Taking Ascophyllum nodosum with amiodarone might increase the levels of iodine in the blood. Too much iodine in the blood can cause side effects that affect the thyroid.
• Other iodine-containing supplements: Ascophyllum nodosum contains iodine. Taking it with other products that contain iodine might cause iodine levels to go too high, causing side effects such as changes in thyroid function. Examples of supplements that contain iodine include seaweed products such as dulse, laminaria, and sea moss.
• Anticoagulants (Warfarin): Seaweed contains vitamin K, and a published case report and pharmacological commentary (Bartle WR, Madorin P, Ferland G. Am J Health Syst Pharm. 2001;58(23):2300) have identified the potential for seaweed to interact with warfarin through its vitamin K content. This interaction has been documented in published literature cross-referenced in drug information resources.

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