Fiber: And You Thought This Was Simple (ha!)

Many times over the years I’ve written about prebiotic fibers and gut health.  What seems so simple at first glance (i.e. “eat more fiber”) turns out to be anything but…just like everything else having to do with the gut biome!  I just finished reading a really interesting little article that was published in the proceedings of the Nutrition Society, from a conference held at the Royal Society of Medicine in the UK.[i]  It is a great follow up to my post of a few weeks ago, which talked about how too much of a good thing is just as bad as too little.  (As my old mentor, Dr. Sidney Baker, used to say (summarizing basically all of medicine):  does this patient have too much of something or too little?)

Dr. Rhodes summarizes some recent work looking at dietary fibers and to summarize in just a few words – they are not all the same. Historically, starting in the 1990s or so, we were all told to get at least 5 servings per day of fruit and vegetables, excluding starchy vegetables like potatoes, yams and plantains which didn’t “count.”  This, according to Dr. Rhodes, had very little evidence supporting it and in fact, there is “…some evidence that plantains for example might be particularly beneficial.”

The research on simply adding more fiber to the diet to stave off colorectal cancer has been contradictory at best.  Dr. Rhodes suggests that this might be an issue of over generalization:  “One possible conclusion from these contradictions is that it may not be helpful to generalize about health impacts of large food groups.” After all, fruit and vegetables, for example, contain many, many different nutritional components and too much of one or too little of another can have major health implications.

Looking then at colorectal cancer as an example, he points out that bacteria of the gut has come under great suspicion as playing a major role in its development:  “…cancer is so relatively rare in the small intestine (.4% life-time incidence) compared with the colon and rectum (6% life-time incidence in western countries) and bacteria re approximately 104 more numerous in the colon.” Research has shown some highly suspect bacteria include Fusobacterium and E. coli, among others.  E. coli has been found to adhere to the mucosal lining of the gut in both colon cancer and in Crohn’s disease, and promotes inflammation. In places where the diet includes high amounts of fiber (I’ve written about this before here), colon cancer and inflammatory bowel diseases are extremely rare.  Therefore, “…we investigated the possibility that soluble dietary fibres and other complex carbohydrates might be able to inhibit E. coli adherence to the epithelium.”  It turned out that fiber from plantains and broccoli, but not from apples or leeks, have a profoundly positive effect on protecting these bacteria from invading the mucosal lining.  And this applied to not just E.coli, but also other pathogenic bacteria including C. difficile.  Dr. Rhodes calls these inhibitory fibers contrabiotics.

You’ll remember that  bacteria eat these fibers and in fermenting them, produce short chain fatty acids (SCFA).  SCFAs are very, very beneficial to health, generally speaking, as you know, but…as I mentioned in the first paragraph of this post, too much of anything is NOT GOOD.  Studies on different soluble fibers have shown that higher doses of citrus pectin tend to be beneficial whereas inulin may be harmful.   Animal studies have shown that in mice in whom the anti-inflammatory cytokine, IL-10, is blocked, inducing inflammatory bowel disease, a diet rich in citrus peel pectin ameliorated the disease whereas a diet containing a similar amount of inulin exacerbated the colitis.

Why?  Inulin was correlated with high amounts of the short chain fatty acid, butyrate.  Now, butyrate, of course, is usually very good.  But that is the key phrase – in the right amount:  “Although butyrate is widely regarded as beneficial to the colonic epithelium, it is perhaps insufficiently recognized that this benefit is very much dose-related.”  Remember my article on propionic acid where I talked about research stating exactly the same thing?  High doses of butyrate, “…have been known for a long time to be toxic to colon epithelial cells in vitro…”

The ecosystem of the biome is unbelievably convoluted and science has barely begun to pick apart how each element affects others.  There are trillions of organisms interacting with each other, secreting an unbelievable array of metabolites (most of which are still unknown).  Throw food into the mix and – well, I reckon it will be decades and decades before we have even an inkling of how this all works. The question of fiber alone is not even vaguely simple:  “The impacts of fibre on microbiota are complex and also depend on the existing microbiota.”

Dr. Rhodes mentions other components of fruit and vegetables (like lectins – which I may write about soon) and also briefly addresses the question of emulsifiers in processed foods.  To summarize the latter: they are bad.  I’ve written about this before (just one example of several – here) so I won’t go into any detail now.  Just avoid them as much as you can.

So to conclude, two major points from this article:

  1. Some sound advice: “Dietary advice to the general public should therefore address factors that reduce the risk of a range of conditions, not just one, and should ideally impact beneficially on all-cause mortality….There is, for example, evidence that adherence to a Mediterranean diet, and its components; low meat, high fruit and vegetable, nuts, and olive oil impacts beneficially on all-cause mortality.”
  2. “If you are searching for a single dietary component that might prolong life there is arguably nothing that has a stronger case. Regular coffee drinking has been associated with reduced risk of type 2 diabetes, cardiovascular mortality, cancer, cirrhosis and most importantly, with a reduction in all-cause mortality.”

Waahooo! (Judy wrote, swallowing another mouthful of java…)


[i] Rhodes, J.M. Conference on ‘Diet and Digestive Disease’ Plenary Lecture 1: Nutrition and gut health: the impact of specific dietary components – it’s not just five-a-day.  Proceedings of the Nutrition Society. 2019.  doi:10.1017/S0029665120000026

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