Fucoidans, laminarin, and alginate are rich sources of compounds that may have beneficial effects on the mammalian gut microbiota by increasing bacterial diversity and abundance. Gut bacterial imbalances have been linked to the development of diseases such as inflammatory bowel disease, immune deficiency, hypertension, type 2 diabetes, obesity, and cancer.

Polysaccharides act as prebiotics to enhance the bacterial community and often promote the production of short-chain fatty acids, which serve as an energy source for gastrointestinal epithelial cells, provide defense against pathogens, and participate in immunomodulation. In this blog, I review the study “Seaweed Components as Potential Modulators of the Gut Microbiota” by Ton That Huu Dat

et al discuss the advantages and limitations of in vitro gastrointestinal models compared to ex vivo and in vivo approaches.

Seaweed polysaccharides are largely composed of soluble and insoluble fiber. They produce endogenous carbohydrate-degrading enzymes, such as β-glucanase and β-glucosidase, that can hydrolyze the glycosidic bonds in polysaccharides. The polysaccharides contained in seaweed that are not digested in the upper gastrointestinal tract are thought to have bioactive effects such as glycemic control, acting as prebiotics in vitro and in vivo studies, and promoting gut microbiota and immune regulation.

Fucoidan, a brown alga, is a water-soluble sulfated polysaccharide composed of repeating fucose and sulfate groups and may also contain galactose, mannose, xylose, rhamnose, arabinose, glucose, acetyl groups, and glucuronic acid. Fucoidan also plays a role in adapting to osmotic stress caused by changes in salinity, as the sulfate groups can bind cations such as sodium, potassium, magnesium, and calcium. Fucoidan has previously been shown to have potential as an anticancer, antiviral, antioxidant, and anti-inflammatory agent. However, the oral bioavailability of fucoidan may be limited due to its highly polar nature and its inability to cross intestinal epithelial cells, despite its recognized prebiotic properties in vitro and in humans and animals.

The energy storage polysaccharide of brown algae is laminarin, which is water-soluble, but the more branched the molecule, the lower the temperature required for dissolution. It accounts for 3-35% of the dry mass of brown algae and is most abundant in Laminaria species. Laminarin has previously been shown to be effective in in vitro studies. It may be used as an anticancer, antimetastatic, antioxidant, and immunostimulant. In vivo, it has been reported to act as an immunomodulator and prebiotic (in animal models) to regulate dysbiosis of the gut microbiota.

Alginic acid accounts for up to 45% of the brown algae’s dry mass and is a water-soluble linear polysaccharide consisting of β-D-mannuronic acid and α-L-guluronic acid (1-4) linked together. It is the most abundant polysaccharide in brown algae, and the prebiotic effect of alginic acid on the gut microbiota has been demonstrated in other studies.

Alginic acid may also be useful in preventing metabolic syndrome. It increases the viscosity of gastric contents and reduces postprandial glucose absorption and insulin response, potentially affecting hyperlipidemia and hypertension.

In vitro, animal, and human studies summarized in the study indicate that the consumption of seaweed ingredients may beneficially modulate the mammalian gut microbiota. Seaweed polysaccharides, such as fucoidan, laminarin, and alginate, have shown particular efficacy as regulators of the gut environment by acting as prebiotics to increase gut bacterial populations and short-chain fatty acid production. However, many factors reduce the bioaccessibility and bioavailability of seaweed ingredients. These factors include antagonistic or synergistic interactions with other food ingredients, physicochemical digestibility parameters such as solubility, polarity, molecular weight, and the surrounding food matrix, and the effects of first-pass metabolism. However, the low bioavailability of some seaweed ingredients can be improved by gastric, enterohepatic, and bacterial biotransformation to compounds with enhanced bioactivity. Another factor influencing bioaccessibility and bioavailability is the composition of each individual’s gut microbiota, which varies widely. For this reason, they may lack certain bacterial families required for the metabolism of seaweed ingredients. This can be augmented by introducing bacterial strains capable of digesting them. In vitro studies and in vivo animal studies suggest the potential of seaweed ingredients as prebiotics, but do not fully reflect the metabolism of ingredients in humans. Further randomized controlled clinical trials in large human cohorts with measurable endpoints are needed to validate the putative health benefits observed in animal models, simulated digestion models, or in vitro. However, seaweed could be a sustainable source of bioactive compounds that could potentially be used as regulators of the gut microbiota.

Source: Mar Drugs. 2021 Jun 23;19(7):358. doi: 10.3390/md1907035