Fucoidan is a sulfated polysaccharide (MW: average 20,000) found mainly in various species of brown algae and brown seaweed such as mozuku, kombu, bladderwrack, wakame, and hijiki (variant forms of fucoidan have also been found in animal species, including the sea cucumber). Fucoidan is used as an ingredient in some dietary supplement products.
Fucoidan designates a group of certain fucose-containing sulfated polysaccharides (FCSPs) that have a backbone built of (1→3)-linked α-l-fucopyranosyl or of alternating (1→3)- and (1→4)-linked α-l-fucopyranosyl residues, but also include sulfated galactofucans with backbones built of (1→6)-β-d-galacto- and/or (1→2)-β-d-mannopyranosyl units with fucose or fuco-oligosaccharide branching, and/or glucuronic acid, xylose or glucose substitutions. These FCSPs offer several potentially beneficial bioactive functions for humans. The bioactive properties may vary depending on the source of seaweed, the compositional and structural traits, the content (charge density), distribution, and bonding of the sulfate substitutions, and the purity of the FCSP product. The preservation of the structural integrity of the FCSP molecules essentially depends on the extraction methodology which has a crucial, but partly overlooked, significance for obtaining the relevant structural features required for specific biological activities and for elucidating structure-function relations.
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The physiological and biochemical effects of fucoidan have been examined in several small-scale in vitro and animal studies. F-fucoidan was reported to inhibit hyperplasia in rabbits  and induce apoptosis in isolated human lymphoma cell lines in vitro. It has been hypothesized that these two effects may involve a common mechanism, but the evidence is inconsistent and no mechanism for the putative induction of apoptosis by fucoidan has been identified. A study in rats indicated that pre-treatment with fucoidan increases mortality subsequent to meningitis infection. In a clinical study, orally-ingested Undaria-derived-fucoidan was reported to produce a small increase in the total number of CD34+ cells, and a more pronounced increase in the proportion of CD34+ cells that expressed CXCR4 (connected to over 23 types of cancers). The authors of the study hypothesized that the ability of fucoidan to mobilize hematopoetic cells with high levels of CXCR4 expression could be clinically valuable.
- Ale, Marcel Tutor; Mikkelsen, Jørn D; Meyer, Anne S (2014). "Important Determinants for Fucoidan Bioactivity: A Critical Review of Structure-Function Relations and Extraction Methods for Fucose-Containing Sulfated Polysaccharides from Brown Seaweeds". Marine Drugs 2106 (10): 2130. doi:10.3390/md9102106.
- Jean-François Deux; Anne Meddahi-Pellé; Alain F. Le Blanche; Laurent J. Feldman; Sylvia Colliec-Jouault; Françoise Brée; Frank Boudghène; Jean-Baptiste Michel; Didier Letourneur (2002). "Low molecular weight fucoidan prevents neointimal hyperplasia in rabbit iliac artery in-stent restenosis model". Arteriosclerosis, Thrombosis, and Vascular Biology 22 (10): 1604–9. doi:10.1161/01.ATV.0000032034.91020.0A. PMID 12377737.
- Aisa Y; Miyakawa Y; Nakazato T; Shibata H; Saito K; Ikeda Y; Kizaki M (Jan 2005). "Fucoidan induces apoptosis of human HS-sultan cells accompanied by activation of caspase-3 and down-regulation of ERK pathways". American Journal of Hematology 78 (1): 7–14. doi:10.1002/ajh.20182. PMID 15609279.
- Wu XZ, Chen D (September 2006). "Effects of sulfated polysaccharides on tumour biology". West Indian Med J 55 (4): 270–3. PMID 17249315.
- Brandt CT, Lundgren JD, Lund SP, Frimodt-Moller N, Christensen T, Benfield T, Espersen F, Hougaard D, Ostergaard C (2003). Pretreatment with fucoidan promotes lethal infection in a rat model of experimental pneumococcal meningitis. Interscience Conference on Antimicrobial Agents and Chemotherapy (43rd). Chicago, Illinois. ISBN 978-1-55581-284-3.
- Irhimeh MR, Fitton JH, Lowenthal RM (Jun 2007). "Fucoidan ingestion increases the expression of CXCR4 on human CD34+ cells". Exp Hematol 35 (6): 989–94. doi:10.1016/j.exphem.2007.02.009. PMID 17533053.