Alginate Oligosaccharides in Lipid Metabolism: A Dual-Action Marine Prebiotic

Alginate oligosaccharides (AOS) are degradation products of brown seaweed alginate, known as dietary bioactive components and used as health-beneficial nutrients. Unlike conventional prebiotics such as fructo-oligosaccharides, galacto-oligosaccharides, and xylo-oligosaccharides, AOS carry carboxyl groups and are negatively charged at physiological pH. This narrative review uses charge as an organizing framework for understanding their lipid-regulating effects. Neutral oligosaccharides act mainly through gut microbial fermentation, whereas charged oligosaccharides can also act on host cells directly, although charge and backbone structure cannot be fully separated in the available studies. The gut-dependent actions of AOS (microbiota modulation, short-chain fatty acid production, bile acid metabolism, and barrier protection) and the direct actions supported mainly by in vitro evidence or parenteral administration in animals, including AMP-activated protein kinase (AMPK) activation, brown adipose tissue thermogenesis, and candidate receptor-mediated effects, are discussed, although the systemic exposure achievable after oral intake remains to be established. The beneficial functions of AOS depend on structural parameters such as degree of polymerization, M/G ratio, and terminal unsaturation; the AOS product approved in China in 2026 is produced by acid hydrolysis, and its relationship to the more active unsaturated forms remains unresolved. Structurally defined AOS are to be developed for nutritional supplementation and as potential candidates for clinical evaluation.