Dietary modulation of bacterial fermentative capacity by edible seaweeds in rats

The effect of edible seaweeds [nori (Porphyra tenera) and wakame (Undaria pinnatifida)] on the modulation of colonic microbiota was studied in adult male Wistar rats. Each alga was fed to rats as the only source of dietary fiber and compared with cellulose. After 12 days, animals were sacrificed and cecal contents used as inoculum to ferment lactulose, citrus pectin, cellulose, nori, and wakame in vitro. Dietary treatment did not affect food intake or food efficiency, yet alga caused a significant increase in cecal weight. Nori and wakame were poorly fermented by the cellulose inoculum, with intermediate substrate degradation (76 and 57% for nori and wakame, respectively) and low metabolism to short-chain fatty acids (SCFA) (30% fermentability compared with lactulose). Cecal contents from rats fed nori and wakame showed a reduced ability to ferment all of the studied substrates compared with the cellulose inoculum, causing a reduction in SCFA production and dry matter disappearance. Only nori induced a bacterial adaptation that brought about a higher fermentation of this substrate. The different behaviors of the two algae could be due to their distinct chemical compositions. In conclusion, nondigestible components of edible seaweeds modified the metabolic activity of intestinal microflora, leading to a reduction of its fermentative capacity.