Today, seaweeds are well recognized in the field of nutrition for their contribution to the fight against diseases linked to metabolic syndrome (obesity, type 2 diabetes, hypertension, cholesterol). And, recently, interest in these ingredients from the marine world has been growing around our intestinal flora. While the impact of seaweed on our transit is well known, its impact on our intestinal microbiota still holds many secrets.
The prebiotic effect of algal fibers has been demonstrated…
As explained by Charoensiddhi S. et al , algae possess polysaccharides and phenolic compounds which are resistant to digestion by enzymes present in the human gastrointestinal tract. These components also selectively stimulate the growth of beneficial intestinal bacteria and the production of fermentation products such as short-chain fatty acids. A dozen in vitro (anaerobic fermentation of human faeces) and in vivo animal studies have investigated the ability of polysaccharides extracted from algae (alginate, fucoidane, laminarin, porphyran, carrageenan, ulvane) to modulate the composition of the microbiota and increase the quantity of short-chain fatty acids produced. The latter are a source of energy for gastrointestinal epithelial cells. They play a protective role against pathogens, influence immunomodulation and induce apoptosis of colon cancer cells.
… but still many barriers to be overcome.
Although these 1st trials have demonstrated the prebiotic potential of algal fibers, several obstacles remain to be overcome:
- Bioaccessibility and bioavailability
As noted by Shannon E. et al , many factors can reduce the bioaccessibility and bioavailability of algal components. These include antagonistic or synergistic interactions with other physicochemical digestibility parameters such as solubility, polarity, molecular weight, the surrounding food matrix, the impact of first-pass metabolism, as well as variability in the composition of each individual’s gut microbiota. These can lead to the absence of certain bacterial families required for the metabolism of algal components.
- Studying the impact of other families of compounds of interest
As Cherry P. et al  point out, studies on the prebiotic effect of algae have mainly been carried out on purified extracts of algal fibers. However, other components such as polyphenols, carotenoids and polyunsaturated fatty acids can be metabolized by intestinal microbiota populations. They would therefore be of interest for future studies. Similarly, the prebiotic value of algae in their entirety has been little studied to date.
- Clinical studies focusing on the gut microbiota
Last but not least, all the authors raised the important issue of the lack of randomized, controlled clinical studies in large human cohorts, with measurable endpoints, to validate any putative health effects observed in animal, simulated digestion or in vitro models.
To go further:
 Charoensiddhi S, Abraham RE, Su P, Zhang W. Seaweed and seaweed-derived metabolites as prebiotics. Adv Food Nutr Res. 2020;91:97-156.
 Shannon E, Conlon M, Hayes M. Seaweed Components as Potential Modulators of the Gut Microbiota. Mar Drugs. 2021 Jun 23;19(7):358.
 Cherry P, Yadav S, Strain CR, Allsopp PJ, McSorley EM, Ross RP, Stanton C. Prebiotics from Seaweeds: An Ocean of Opportunity? Mar Drugs. 2019 Jun 1;17(6):327.