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Seaweed’s Exceptional Fibres

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Measurement of Dietary Fibre

In its simplest terms, dietary fibre content in a sample is measured in the laboratory by what is called an enzymatic-gravimetric method.

Digestible carbohydrates are broken down into simple sugars using enzymes mimicking human digestion and removed from the sample by precipitation and filtration. This mimics absorption of these sugars in the body. The non-digestible precipitate contains the dietary fibre but also contains protein and inorganic material. These should not be included in dietary fibre so protein and inorganic material are measured separately and subtracted from the weight.

Further designation can be made to give soluble and insoluble dietary fibre.

Seaweed dietary fibre

Here are some dietary fibre contents of seaweeds, expressed as percentage of dry weight. On a dry weight basis the dietary fibre content of seaweeds varies from 30% to over 50%, which is quite high.

Soluble Insoluble Total Reference
Red (carrageenan)
Nori – Porphyra 17.9 16.8 34.7 Jimenez-Escrig
26 9 34 MacArtain
Dulse– Palmaria 19 15 34 MacArtain
Gigartinapistillata 21.9 7.4 29.3 Gomez-Ordonez
 
Brown
Hijiki – Sargassum 32.9 16.3 49.2 Jimenez-Escrig
Wakame - Undaria 30.0 5.3 35.3 Jimenez-Escrig
30 5 35 MacArtain
Kelp/kombu - Laminaria 33 5 38 MacArtain
17.1 13.1 30.2 Gomez-Ordonez
Sea spaghetti - Himanthalia 25.7 7.0 32.7 Jimenez-Escrig
  26 7 33 MacArtain
  23.6 13.5 37.1 Gomez-Ordonez
 
Green
Sea lettuce - Ulva 21.3 16.8 38.1 Jimenez-Escrig
21 16 37 MacArtain
20.5 34.4 54.9 Yaich

Effects of dietary fibre

The effects of dietary fibre can vary depending on whether it is soluble or insoluble, upon its composition and structure and whether it is fermentable by our microbiota (bacteria in our bowels).   The benefits of dietary fibre include laxation, attenuation of blood cholesterol and blood glucose, and prevention of bowel diseases such as bowel cancer, as a result of fermentation of fibre polysaccharides by bowel bacteria.

Fermentability of seaweed polysaccharides

For a polysaccharide to be considered a prebiotic it must be preferentially fermented by bacteria that are considered to confer health benefits to the host (i.e. us).  Originally, prebiotic was synonymous with bifidogenic (preferentially used by bifidobacteria), but now, as our understanding has increased, prebiotic could refer to fermentation by a whole range of beneficial bacteria. The products of fermentation are short-chain fatty acids (SCFA), such as acetate, lactate, propionate and butyrate.  While all are thought of as conferring a health benefit the latter two are considered particularly beneficial.

The soluble polysaccharides from brown seaweeds are mostly alginate, laminarin and fucan/fucoidan.  Studies on the fermentability of these polysaccharides suggests that laminarin is readily fermented by gut bacteria, alginate is fermented to some extent (but not by all individuals) and fucan/fucoidan is not fermented.  Thus, laminarin and alginate may be considered as prebiotic, although more work is needed.  Fucan/fucoidan is not prebiotic, but may have other beneficial effects through direct interaction with the gut immune system.

Studies by Bobin-Dubigeon et al (1997) showed that 50 % of the insoluble dietary fibre of Ulva (a green seaweed) was consumed, but only 30 % of the soluble fibre (mostly ulvan, a soluble polysaccharide from Ulva species).

There is not much known about the fermentation of carrageenan and agars.  However, it does seem that algal xylans, such as found in Palmaria species (dulse), are fermented by human colonic microbiota.

There is not much known about colonic fermentation of the insoluble polysaccharides from seaweeds. Like plants, these are mostly insoluble glucans, mannans and xylans.

To summarise, here are some of the fibres in seaweed characterised with their known degree of colonic fermentability  which effects vary according to groups & types:

Brown Seaweeds:
Alginates**
Laminarans***
Fucans*

Red Seaweeds:
Sulphated galactans*
Xylans***

Green Seaweeds:
Ulvans*
Xylo-galactans

*represent the degree of fermentation & degradation by gut microflora and levels production of short-chain fatty acids

Sources:
Red seaweeds & gut function
Prebiotics from marine macroalgae for Health
Prebiotics: metabolic & health benefits
Probiotics, prebiotics, and synbiotics
jimenez-2000-nutrition-research-20
gomez_2010_dietary-fibre-spanish-edible-seaweeds
dietary-fibre-definition-2

Disclaimer: This material is provided for educational purposes only and IS NOT intended as a substitute for professional medical advice, diagnosis, or treatment. This information is generic and may not include the latest research. We encourage you to do your own research and discuss your findings with a qualified health practitioner who can help you validate the outcomes in the context of your specific & individual health situation.

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