IngredientsThe genus Rhodymenia is one of the most notable members of Rhodophyta, placed in the order Rhodymeniales, with approximately 183 described species. Belonging to the family Rhodymeniaceae, the genus consists of species largely found in cold temperate coastal waters. It is distributed throughout tropical and temperate waters globally, from the lower intertidal to depths of approximately 100 m. The genus is characterized by flattened blades, a non-lubricous thallus, and gonimoblast cells containing carposporangia.
The name Rhodymenia is derived from Greek, where "rhode" means rose and "menia" refers to a form of algae or seaweed — an etymology that reflects the rose-colored appearance often associated with these algae. Members of the Rhodymenia genus exhibit a thin, leafy structure, often with a ruffled or lobed appearance; textures range from smooth to flattened, and color ranges from deep red to shades of brown or green, depending on species and environmental conditions.
The characteristic red or blue color of red algae in general results from the masking of chlorophyll by phycobilin pigments (phycoerythrin and phycocyanin). In Rhodymenia, growth of the vegetative thallus is driven by the activity of a group of apical cells that divide both longitudinally and transversely.
Within the broad genus Rhodymenia, scientific and nutritional attention has focused heavily on a species once classified within it — Palmaria palmata (L.) Weber & Mohr — which was historically placed in the genus Rhodymenia and is still commonly encountered in the literature under its former name Rhodymenia palmata. This species, also called dulse, dillisk or dilsk (from Irish/Scottish Gaelic duileasc/duileasg), red dulse, sea lettuce flakes, or creathnach, is a red alga (Rhodophyta) previously referred to as Rhodymenia palmata.
Palmaria palmata was first described by Linnaeus in 1753 under the name Fucus palmatus. It was later designated as P. palmata by the German naturalists Friedrich Weber and Daniel M.H. Mohr in 1805. In 1830, Greville grouped many red membranaceous species including P. palmata in the genus Rhodomenia (currently Rhodymenia). The species was then known as Rhodymenia palmata (Linnaeus) Greville, until Guiry (1974) proposed reinstatement of the genus Palmaria after observing morphological, anatomical, and reproductive features.
Additional species of scientific interest include Rhodymenia pseudopalmata (a tropical Atlantic species studied for bioactive compounds), Rhodymenia intricata (studied for polysaccharides), and Rhodymenia pacifica Kylin, a Pacific species with flat, dichotomously branched thalli 3–13 cm long, with tips usually rounded and 3–10 mm wide. Palmaria mollis is a closely related species found in the Pacific, where it is known as Pacific dulse or red ribbon.
The erect frond of dulse grows attached by its discoid holdfast and a short inconspicuous stipe epiphytically onto the stipe of Laminaria or to rocks. The fronds are variable in shape and color from deep rose to reddish purple and are rather leathery in texture. The flat foliose blade gradually expands and divides into broad segments ranging in size to 50 cm long and 3–8 cm in width, which can bear flat, wedge-shaped proliferations from the edge.
Its current scientific name is rooted in the Latin word palma for hand or palm of the hand, repeated in both parts of the name to emphasize the palm-like shape of the plant. In English, P. palmata is usually known as dulse, which comes from the Gaelic word duileasc (or duileasg).
Palmaria palmata is a well-known and highly valued red macroalga distributed along the North Atlantic shores within a latitude range of approximately 40 to 80 °N. Dulse is an arctic to cool temperate North Atlantic species found on both the European and North American coasts. In Europe it is found as far north as Spitzbergen, a large island in the Arctic Sea off the north coast of Norway, and as far south as Portugal. In North America it is found as far north as Arctic Canada and south to Long Island, NY. It grows best between 6 °C to 15 °C and can survive freezing temperatures. The upper lethal temperature is about 20 °C, though strains found at the southern limits of distribution are more adapted to warmer temperatures.
Rhodymenia / dulse is encountered commercially in a variety of forms:
5th-century laws of archaic Ireland existed concerning the eating of duileasc, establishing that its cultural and dietary significance dates back at least 1,500 years in the Celtic world. The earliest recorded use of this species is on the island of Iona, Scotland, where Christian monks harvested it over 1,400 years ago. In Ireland especially, and in many parts of the British Isles, P. palmata is widely available and has a strong historical tradition as a component of the local diet.
In Iceland, where it is known as söl, it has been an important source of dietary fibre throughout the centuries. Palmaria palmata has been used as human food in Norway, for example, since the Viking age. Other vernacular names include "creannach" (Gaelic), "søl" or "söl" (Norwegian/Danish and Icelandic), "goëmon à vache" (French, translated from the Breton "bezhin saout" meaning "cattle's seaweed"), and "botelho-comprido" (Portuguese).
Palmaria palmata is a species of commercial importance with historical records of use as food dating back several centuries, through to the current harvesting of dulse by hand-picking on the foreshore in Western Europe as well as Canada (New Brunswick and Nova Scotia) and USA (Maine). Dulse is commonly used in Ireland, Iceland, Atlantic Canada, and the northeast United States as food and medicine.
Dulse can be eaten fresh or dried; in traditional dishes it is boiled with milk and rye flour or made into a relish and is commonly served with fish and butter. The gelatinous substance contained in dulse is a thickening agent and imparts a reddish color to the food with which it is mixed.
Beyond its role as a food, traditional communities recognized the seaweed's broad nutritional qualities. Historically, dulse was more than just a food source; it was a valued remedy in traditional medicine. In Iceland, it has been an important source of dietary fibre throughout the centuries. It was also used as fodder for animals in some regions. The species was long associated with supporting general health, thyroid function, and prevention of nutritional deficiencies in coastal communities where other foods were scarce, particularly during winter months.
The reported proximate values for Palmaria palmata (syn. Rhodymenia palmata) have a relatively wide spread, ranging from 73–89% moisture and, on a dry weight basis, 12–37% ash, 8–35% crude protein, 38–74% carbohydrate, and 0.2–3.8% lipid. Some of the variation can be attributed to seasonal and nutritional conditions. These figures are drawn from a comprehensive chemical review compiled in the scientific literature.
Palmaria palmata is potentially a high-protein food source, and its protein quality rates well with vegetables of good nutritional value. The proteins extracted from dulse are mainly composed of phycoerythrin (PE), followed by phycocyanin (PC) and allophycocyanin (APC). These phycobiliproteins — water-soluble, light-harvesting pigment-proteins — represent the primary structural and bioactive protein fraction. Among studied Atlantic seaweeds, P. palmata exhibits the highest protein content compared to Ascophyllum nodosum and Chondrus crispus.
In Rhodymenia pseudopalmata, protein ranged from 4.9 to 10.5% DW under experimental conditions. Sixteen amino acids were detected; glutamic acid, proline, and methionine may act as osmolytes, while histidine, methionine, and tyrosine may be involved in antioxidant capacity.
The major polysaccharide of P. palmata is a β-(1→3) and β-(1→4)-linked xylan, which distinguishes dulse from the sulfated galactans (agars and carrageenans) typical of many other red algae. Red algae also contain high proportions of polysaccharides (38–74%), such as carrageenan, and a large variety of minerals (12–37%), such as iodine, potassium, sodium, calcium, magnesium, and iron.
Polysaccharide content in the genus varies with species and environmental conditions. In Rhodymenia pseudopalmata, carbohydrate content increased up to 54% DW under high photosynthetically active radiation and low salinity. For Rhodymenia intricata, a 2024 study found that a red alga named Rhodymenia intricata was explored, and extraction technology and antioxidant capacity of its polysaccharides were investigated. The crude polysaccharides were extracted using the ultrasound-assisted water extraction method, precipitated by alcohol, and purified using the trichloroacetic acid method. Scavenging rates of polysaccharides on hydroxyl, DPPH, and ABTS free radicals were determined prior to and following purification to evaluate antioxidant activity. Infrared spectrum analysis suggested that Rhodymenia intricata polysaccharide might be α-pyranose, and the Congo red test illustrated that the polysaccharide contained a triple helix structure.
Palmaria palmata has potassium, chlorine, and sodium as its major mineral constituents and, in comparison to terrestrial fruits and vegetables, is a good source of iron, magnesium, calcium, and iodine. The exceptionally high potassium content is clinically significant (see Safety section). The iodine content is meaningful and pharmacologically relevant, particularly for thyroid physiology.
Vitamin A (as carotene) and, in the fresh plant, vitamin C, are present in appreciable amounts. Despite a low proportion of lipids (0.2–3.8%), P. palmata can contain liposoluble vitamins such as β-carotene (provitamin A) and tocopherol (vitamin E), as well as omega-3 fatty acids. Seasonal and harvest month effects on these constituents have been documented: results suggest a more favorable composition in June specimens, with a higher content of most nutrients, minerals, and bioactive compounds, while October specimens are richer only in carbohydrates and carotenoids.
R-phycoerythrin (R-PE) is a principal bioactive pigment of Rhodymenia/Palmaria palmata. The proteins extracted from dulse are mainly composed of phycoerythrin (PE), followed by phycocyanin (PC) and allophycocyanin (APC). R-PE functions both as a photosynthetic antenna pigment and has demonstrated antioxidant activity in vitro. Research on a newly discovered peptide, SR18 (SLLYSDITRPGGNMYTTR), linked to the pigment allophycocyanin, has been identified by in silico analysis and found to have very strong antioxidant properties in vitro.
Other bioactive molecules present in P. palmata include the osmolyte floridoside, which has antioxidant and stress-protective roles, and polar lipids such as glycolipids and sulfolipids enriched with eicosapentaenoic acid (EPA), which have demonstrated anti-inflammatory and bioactive properties in vitro.
Palmaria palmata extract contains polyphenols, sulphated polysaccharides, R-phycoerythrin, and bioactive peptides that exert antioxidant activity via free-radical scavenging (DPPH• and ABTS+ inhibition). In Rhodymenia pseudopalmata, growth under high photosynthetically active radiation and low salinity (20 psu) produced the highest increase in total phenolic compounds. A strong correlation between phenol content and antioxidant activity evaluated by the DPPH assay has been reported for R. pseudopalmata.
Palmaria palmata is a natural source of desmosterol, a cholesterol precursor that has attracted attention in relation to membrane biology and lipid metabolism.
Multiple chemical classes in Rhodymenia/Palmaria palmata contribute to free-radical scavenging. Antioxidant activity proceeds via free-radical scavenging, including DPPH• and ABTS+ inhibition, mediated by polyphenols, sulphated polysaccharides, R-phycoerythrin, and bioactive peptides. In cultivated samples, the highest antioxidant activity is observed early in vegetative propagation, suggesting that physical wounding may increase the secretion of antioxidant compounds.
Angiotensin I converting enzyme (ACE) is physiologically important in the regulation of blood pressure by catalyzing the production of angiotensin II and the destruction of bradykinin. Specific inhibitors of the enzyme have been recognized as effective antihypertensive drugs. The proteins from dulse are mainly composed of phycoerythrin, phycocyanin, and allophycocyanin. The dulse proteins showed slight ACE inhibitory activity in their intact state, but this activity was extremely enhanced by thermolysin hydrolysis. Recombinant PE subunit peptides showed high ACE inhibitory activities (rPEα: 94.4%; rPEβ: 87.0%), leading to the conclusion that the original proteins of dulse ACE inhibitory peptides are phycobiliproteins.
Dulse is reported to contain anti-inflammatory and antioxidant compounds, and implication strategies to diminish neutrophil activation have the potential to prevent pathological states. One published study evaluated the ability of a phenolic dulse extract (DULEXT) to modulate the lipopolysaccharide (LPS)-mediated activation of primary human neutrophils. Measurement endpoints included intracellular reactive oxygen species (ROS) and nitric oxide (NO).
Sulfated polysaccharides from Rhodymenia species are of interest as potential antivirals. The antiviral activity observed in red algal polysaccharides suggests that sulphation, molecular weight, and carbohydrate nature of these polysaccharides may be involved in this activity. Biochemical analysis has suggested that enhanced antiviral activity may be related to the high degree of sulphation of polysaccharides, while low sulfate content is associated with lack of antiviral activity.
Fresh and dried thalli of Rhodymenia pseudopalmata were found to significantly inhibit the growth of the toxic dinoflagellate Ostreopsis ovata. The compounds responsible are currently being elucidated and have potential applications in the prevention of algal blooms and as antibacterials.
In vitro and animal evidence: P. palmata peptides exhibited ACE-1 inhibitory activity in several in vitro studies, as well as antihypertensive and renin-inhibitory effects in vivo in a trial with spontaneously hypertensive rats (SHRs).
Human/clinical evidence: The ACE inhibitory peptide evidence for dulse in humans is preliminary. The most directly relevant human clinical trial on Palmaria palmata and cardiovascular parameters is the Allsopp et al. (2016) randomized controlled trial (RCT). A randomised parallel placebo-controlled human intervention study was carried out to investigate the effect of consuming P. palmata (5 g/day) incorporated into a bread on serum markers of inflammation [C-reactive protein (CRP); cytokine analysis], with secondary analysis investigating changes in lipids (cholesterol, triglycerides), thyroid function (TSH), and antioxidant status (ferric reducing antioxidant power). The results were counter to the hypothesized benefits: consumption of P. palmata-enriched bread significantly increased serum CRP (+16.1%, P=0.011), triglycerides (+31.9%, P=0.001), and TSH (+17.2%, P=0.017) when compared to the control group. These findings highlight the complexity of translating in vitro and animal data to human outcomes, and represent a notable cautionary data point in the human clinical literature.
Evidence strength: Weak and mixed. In vitro ACE inhibitory data are promising but no positive human RCTs confirming antihypertensive effects of Rhodymenia/Palmaria palmata have been published as of the available literature.
In vitro evidence: In vitro studies have demonstrated cytotoxic activity against HeLa and HCT-116 colorectal cancer cells and measurable DPPH scavenging in ethanolic and screw-pressed juice fractions. A study investigating the nutritional composition and bioactive properties of Palmaria palmata (dulse), Ascophyllum nodosum, and Chondrus crispus assessed protein content, amino acid composition, mineral profile, fatty acids, polyphenols, total carotenoids, antioxidant activity, and cytotoxicity. For Rhodymenia intricata, a 2024 study demonstrated that crude polysaccharides were extracted using the ultrasound-assisted water extraction method, precipitated by alcohol, and purified; subsequent scavenging rates of polysaccharides on hydroxyl, DPPH, and ABTS free radicals were determined to evaluate antioxidant activity.
Human evidence: The Allsopp et al. (2016) RCT tested antioxidant status via ferric reducing antioxidant power (FRAP) but did not report significant antioxidant benefit in humans at 5 g/day in bread.
Evidence strength: Antioxidant activity is well-documented in vitro across multiple assays and species within the genus. Human clinical confirmation is lacking.
In vitro/ex vivo human cell evidence: Dulse is widely used as a source of fiber and protein, and is reported to contain anti-inflammatory and antioxidant compounds, though no study at the time of publication (2019) had investigated these effects in primary human neutrophils. The 2019 Marine Drugs study by Millan-Linares et al. evaluated a phenolic dulse extract in primary human neutrophils — representing ex vivo human cellular work rather than a fully controlled clinical trial.
Human clinical trial evidence: The Allsopp et al. (2016) RCT found no anti-inflammatory benefit and instead observed a statistically significant increase in CRP at 5 g/day, which was unexpected and not readily explained by the investigators. In vitro studies in the same trial investigated the inflammatory activity of P. palmata extracts (hot water, cold water, and ethanol extract), protein extracts, and associated protein hydrolysates using a Caco-2 inflammation cell model.
Evidence strength: Preliminary. In vitro anti-inflammatory signals are present but have not been confirmed — and in one RCT were contradicted — in humans.
In vitro evidence: Polysaccharides extracted from Rhodymenia pseudopalmata, Solieria filiformis, Hydropuntia cornea (Rhodophyta), and Sargassum fluitans (Phaeophyceae) were evaluated for cytotoxic and antiviral activities against Herpes simplex virus (HSV-Type 1). Chemical structures were characterized by FT-IR spectroscopy and 13C-NMR analyses. Polysaccharides from Sargassum fluitans (EC50 = 42.8 μg/mL) and Solieria filiformis (EC50 = 136.0 μg/mL) showed antiviral activity against HSV-1 in vitro without cytotoxicity at 1–200 μg/mL. Importantly, the polysaccharide extracted specifically from Rhodymenia pseudopalmata did not demonstrate measurable antiviral activity against HSV-1 in this study — an important negative finding within the genus.
Evidence strength: In vitro only. The antiviral data for the specific species R. pseudopalmata are negative for HSV-1; related species within the broader Rhodophyta show more encouraging results. No human trials exist.
Red algae have been reported to improve lipid and glucose metabolism in rats. A trial investigated the effects of Palmaria palmata, a red alga from northern Japan, on lipid metabolism in participants with hypercholesterolemia in a randomized double-blind placebo-controlled trial. As noted above, the Allsopp et al. 2016 RCT unexpectedly found a significant increase in triglycerides, which constitutes the principal human clinical evidence on lipid outcomes and stands in contrast to the favorable animal data.
Evidence strength: Preliminary and discordant. Animal models support potential lipid-lowering effects, while the available human RCT reported an adverse lipid signal at the dose studied (5 g/day).
Given its significant iodine content, Rhodymenia palmata / dulse has a documented pharmacological effect on thyroid parameters. In the Allsopp et al. (2016) RCT, consumption of P. palmata-enriched bread significantly increased thyroid-stimulating hormone (TSH) by +17.2% (P=0.017) compared to the control group. This suggests that the iodine delivered via dulse can exert measurable effects on the pituitary-thyroid axis, even at a relatively low dietary dose of 5 g/day. Reports of the iodine content of commercially available macroalgae have raised concerns from national food safety authorities regarding potentially negative health effects of excessive iodine intakes.
Evidence strength: Moderate human evidence (one RCT) demonstrating that dulse consumption at 5 g/day alters TSH levels, with the direction of effect suggesting a degree of iodine-mediated thyroid suppression. This is an important safety-relevant finding.
Palmaria palmata is potentially a high-protein food source, and its protein quality rates well with vegetables of good nutritional value. Red algae, including P. palmata, are known to be a valuable source of proteins, with protein contents ranging from 7 to 19% dry weight with approximately 30% of the essential amino acids counted in the total amino acid fraction. A published study in the Journal of Nutritional Biochemistry (Galland-Irmouli et al., 1999) examined the nutritional value of proteins from edible Palmaria palmata.
The dietary fiber content of dulse, particularly its xylan polysaccharide, is relevant to gastrointestinal health. Feeding trials in non-ruminants demonstrate that inclusion is feasible without adverse effects and may improve gut health and product quality; however, the evidence remains limited in scope and duration. No controlled human trials specifically assessing gut health outcomes from Rhodymenia species have been published in the peer-reviewed literature identified during this review.
The following dosages are reported directly from the scientific literature and should not be interpreted as recommendations:
A published case report represents the first documented report of hyperkalemia due to dulse consumption. Dulse is high in potassium, with concentrations upwards of 34 times greater than that found in bananas. A 66-year-old woman with diabetes and chronic renal disease presented to the emergency department with nausea, vomiting, and worsening malaise. ECG monitoring showed bradycardia and periods of asystole. Laboratory analysis revealed a serum potassium level of 8.6 mmol/L (normal range 3.5 to 4.9 mmol/L). The only recent change she could identify was consumption of approximately 200 g of dulse within the preceding 24 hours. A diagnosis of hyperkalemia was made, and the patient was treated successfully. Hyperkalemia is rare in individuals with normal renal function, due to the ability of the kidneys to adapt to increasing serum potassium concentrations. However, in patients with renal compromise, potassium homeostasis can become impaired.
Consumption of P. palmata-enriched bread (5 g/day) significantly increased TSH by +17.2% (P=0.017) in a randomised controlled trial. This is consistent with the known pharmacology of iodine: high iodine intake can suppress thyroid hormone synthesis via the Wolff-Chaikoff effect, increasing TSH levels reflexively. Reports of the iodine content of commercially available macroalgae have recently raised concerns from national food safety authorities regarding potentially negative health effects of excessive iodine intakes.
In the Allsopp et al. (2016) human RCT, P. palmata-enriched bread consumption also significantly increased serum CRP (+16.1%, P=0.011) and triglycerides (+31.9%, P=0.001). While the mechanisms underlying these unexpected findings have not been fully elucidated, these data indicate that the health effects of dulse supplementation in humans are not uniformly favorable and that the assumption of benefit based on in vitro or traditional use data requires caution.
Total amounts of arsenic in Palmaria palmata specimens from several locations range from 1–10 µg/g (DW). According to one heavy metal analysis of a Palmaria palmata extract, antimony, arsenic, chromium, nickel, and vanadium were detected in amounts of 0.069, 1.480, 0.046, 0.433, and 2.29 ppm respectively, with approximately 3.8 ppm iodine detected in the same extract. No aflatoxins were detected. Heavy metal accumulation in seaweeds is harvest-location-dependent and is subject to regulatory monitoring in food-grade products in the European Union and elsewhere.
Due to naturally high iodine content, individuals with thyroid disorders — including Hashimoto's thyroiditis, Graves' disease, or those taking thyroid medication such as levothyroxine — should exercise caution before supplementing, as excess iodine can precipitate thyroid dysfunction. This is consistent with established pharmacology of thyroid-active iodine.
No controlled human safety studies, formal toxicology evaluations, or documented adverse event profiles exist for Palmaria palmata extract at supplemental doses, though centuries of dietary consumption as a whole food suggest reasonable tolerability at culinary quantities (up to approximately 15 g/day dried weight). Feeding trials in non-ruminants (animal studies) demonstrate that inclusion is feasible without adverse effects at moderate inclusion levels.
The scientific evidence base for Rhodymenia spp. (including the historically named Rhodymenia palmata / Palmaria palmata) in the context of human health is characterized by: (1) a rich body of in vitro chemistry demonstrating numerous bioactive properties; (2) a much smaller body of animal in vivo data; and (3) a very limited number of human clinical trials, with the best-powered RCT (Allsopp et al., 2016) producing results that were in several respects counter to hypothesized benefits and highlighted real safety signals (TSH elevation, CRP increase, triglyceride increase). The genus Rhodymenia sensu stricto (excluding Palmaria palmata) has been subject to even less clinical research, with species such as R. pseudopalmata and R. intricata studied primarily at the in vitro or laboratory extraction level.
Key research gaps include: randomized controlled trials with adequate statistical power examining clinically meaningful endpoints; bioavailability studies for key compounds (particularly phycobiliprotein peptides and sulfated polysaccharides); and long-term safety studies at supplemental doses. Molecular-assisted taxonomy continues to revise species boundaries within the genus, complicating extrapolation across studies conducted on differently identified specimens.
Health conditions that Rhodymenia may help support.
Body systems that Rhodymenia may help support.
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