[jfv]

Maybe someone with a knowledge of nutrition and medicine can clear this up for me.

As I understand it, there are two types of fiber that are lumped into the term "dietary fiber": soluble fiber and insoluble fiber. I believe that both are fermented in the large intestine but soluble fiber is the one that primarily supports flora through fermentation, while insoluble fiber is used for more of a physical effect, to draw water into the stuff that's going through. Apparently soluble fiber also helps to increase the viscosity of the stuff going through the stomach, which slows digestion (considered to be a good thing).

One of the studies mentioned in the article uses inulin (soluble fiber -- the kind you'd find in a supplement like Benefiber). With regards to regulating sugar, metabolism, and gut flora I imagine that soluble fiber is the one we should be focusing on. Yet it seems rare to distinguish the two.

I know that both fibers are often found in the same food (typically fruits and vegetables) but some processes (like juicing) remove insoluble fiber while leaving the soluble fiber intact. So it's commonly believed that juice "doesn't have fiber", which is untrue.

Isn't it time we start distinguishing between the two when we talk about the health effects? Am I just completely wrong in how I understand this?

[twic]

AIUI, insoluble fiber didn't get fermented. Insoluble fiber basically means cellulose; to digest cellulose, you need some pretty serious engineering, like four stomachs [1], or a huge caecum [2], or, er, a two-pass process [3]. Humans don't traditionally have any of those, so we don't digest cellulose to a significant degree. There are other components of food classified as insoluble fiber, but the only significant one is lignin, at on the order of 10-20% of the mass of cellulose [4], and that's so hard to digest you basically have to be a fungus to do it [5]. Apparently even termites don't break it down much.

Soluble fibres are oligosaccharides with weird bonds that we can't break down, but that bacteria can. They're nothing like insoluble fibre, except that they come from plants. They're polymers, but this is biology, every other thing is a polymer. They're not really even fibrous. We absolutely should distinguish them.

This review is pretty accessible, and covers a lot of ground:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614039/

I would write more, but i'm going to cook some lentils instead!

[1] https://en.wikipedia.org/wiki/Ruminant

[2] https://en.wikipedia.org/wiki/Equine_nutrition#The_cecum_and...

[3] https://en.wikipedia.org/wiki/Cecotrope

[4] http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.1981....

[5] https://en.wikipedia.org/wiki/Lignin#Biodegradation

[epmaybe]

I think the main reason we can't break down cellulose is because we don't have the enzymes to do so. It's not as if cellulose is actually all that different to polysaccharides we are able to digest, it simply has beta 1->4 glycosidic bonds [1] between the glucose molecules instead of alpha 1->6 glycosidic bonds that are digestable by humans.

edit: upon some more skimming of the review article, even with the enzymes we may not have the conditions in which cellulose could be degraded, hence the "four stomachs" and other methods.

[1]: https://en.wikipedia.org/wiki/Cellulose

[bL79]

Thanks for this comment. I totally forgot about this when I posted mine. You're understanding of soluble/insoluble matches mine although I think soluble does contribute to drawing water into the stool and giving it shape. Edit - Soluble fiber dissolves in water and forms a gel - insoluble absorbs water and provides a fibrous matrix. Both are involved in stool firming

Seems most supplements are soluble in nature. So maybe if you need to speed digestion or solidify loose stools you should focus on getting some insoluble fiber naturally rather than relying on supplements? Definitely seems like an important distinction