Seaweed polysaccharides

Fermentable fibers

Alginates, fucoidans, carrageenans, ulvans: fibers that no land plant carries.

Few microbiotas know how to cut them. Populations that have eaten seaweed for a long time acquired the enzymes to do so, often from marine bacteria.

Effects and benefits

Fibers that no land plant makes, and an astonishing story: in many Japanese people, gut bacteria borrowed from a marine bacterium the gene that lets them digest nori. Alginates and fucoidans, for their part, become short-chain fatty acids that talk to the rest of the body.

Seaweeds build their walls with sugars unknown to land plants: alginates in brown algae, carrageenans and porphyran in red algae, ulvans in green algae. Most of the bacteria in our gut cannot cut them.

In 2010, researchers found in Bacteroides plebeius, a gut bacterium of Japanese people, genes for digesting the porphyran of nori. These genes came from a marine bacterium: generations of seaweed meals are thought to have passed them on to the microbiota. In North Americans, they were almost absent (Hehemann et al., 2010).

The alginates and fucoidans that reach the colon are fermented slowly into short-chain fatty acids, the messengers the microbiota sends to the gut wall, the liver and even the brain.

One caution: brown seaweeds, kombu above all, are very rich in iodine. ANSES advises against eating a lot of them often, especially during pregnancy or with a thyroid problem (ANSES, 2018).

Sources: Hehemann et al., Nature, 2010 · ANSES, 2018

Where to find it (64)

Its category: Seaweed & microalgae