What is the difference between grains and starchy vegetables to gut bacteria?
There are two basic divisions of prebiotics from grains/vegetables - energy storage material such as starch (large polymers of glucose) and fructans (aka inulins, relatively small polymers of fructose) and structural material (cell wall), lumped together as non-starch polysaccharides (NSP).
NSPs include cellulose, hemi-cellulose, pectins, xylans (very rich in grains) and are complex polymers of several different sugars.
There is third source of prebiotics used by a select few organisms, host derived mucous, and for the sake of completeness, infants take advantage of a fourth source, breast milk.
Out of all of these, the only one that is at least partially digested by us is starch. However there is a significant proportion of it which is indigestible by us, called resistant starch. This is a major source of food for gut microbes.
There is a hierarchy of digestion. Very few organisms can deal with particles of insoluble plant material, starch globules and lumps of mucous in their crude state. Those that can are primary degraders.
Bacteroides is the absolute standout here. It can and does tackle everything. Some Firmicutes such as
Ruminococcus and
Roseburia, along with
Bifidobacterium, tend to avoid the NSPs and mucous and concentrate on starch and fructans (some bifido can cope with mucous).
Akkermansia focuses solely on mucous.
After they break up the huge complexes into slightly smaller elements, a few more semi-primary degraders move in. Some like
Prevotella make a beeline for the xylans, others prefer cellulose (eg
Pseudobutyrivibrio) or pectin (eg
Faecalibacterium) etc.
There are still only a relatively few organisms working at this stage.
These break down the polymers into much smaller units, and this is where the vast bulk of gut anaerobes (and the gut is almost entirely anaerobes, despite what culture-based gut tests try to tell us) get their food. They can select from a huge array of different oligosaccharides and simple sugars made available by the primary degraders.
These secondary degraders are termed sacchrolytic and constitute most of the gut flora. While there are varying preferences among them there is also a great deal of overlap.
A much smaller group are asacchrolytic (use amino acids or other sources) and the aerobes and facultative anaerobes (often potential pathogens) have the option of a respiratory metabolism much like ours, as well as fermentation of sugars.
Paraprevotella ferments glucose amongst other sugars
Well almost all anaerobes do. However I think you are probably referring to a study like
this which characterises two species of
Paraprevotella from faeces.
When I first began trying to understand the properties of the different gut bacteria, I used to get misled by such studies.
The tests of fermentation of various substrates (usually simple sugars) and determination of enzyme activities are all about classification, establishing phenotype, and finding ways to distinguish between organisms.
Until the relatively recent advent of DNA sequencing, this was the only way to classify. Isolated cultures with an abundance of a single simple sugar substrate, however, bear little resemblance to what goes on in the gut.
The study does tell us though that it is a sacchrolytic bacterium and it gives another important clue. Growth is stimulated by xylan. This tells me that,
in situ, it is likely to behave like
Prevotella and prefer xylans, which are abundant in grains.
Chris Kresser recommends excluding FODMAPS in treating dysbiosis
The developers of the FODMAPS diet and of the SCD had a very limited understanding of the gut flora (it was an era when our understanding was very distorted by culture-based tests). The GAPS diet stuff has updated the SCD a bit in this respect but there is still something fundamentally lacking I think.
Chris Kesser does have a much better understanding but I think we need to consider which form of dysbiosis might benefit from a FODMAPS diet for example.
These substances are all prebiotics, things our gut bacteria like to eat and which are essentially good for us, not things we should be avoiding (this applies also to the carbohydrate sources excluded in the SCD, with the proviso that this is also about treating caeliac disease, so exclusion of gluten and hence grain is essential).
Some people do have trouble with unpleasant symptoms from fermentation of these various carbohydrates. The idea was that it is bad bacteria causing this problem and that restricting these substances will starve them out. Well no, these things are food for most of the bacteria in the gut.
The unpleasant symptoms can arise for a variety of reasons, in the case of FODMAPS, often because the fermenting bacteria are too highly concentrated in the small intestine rather than remaining in the colon. As a temporary measure, restriction of the offending substances and concentration on other gut healing measure may be necessary. But it should be a temporary restriction.
When you asked about the SCD I did think it was worth a try. It does seem to be very helpful, particularly for people with serious gut problems, though I think it works for reasons other than it's original proponents ideas (apart from gluten restriction). In your case restriction of grains would be a good thing and supplying plenty of fruit and veg would help your Firmicutes.
You would be lacking resistant starch so it might be a good idea to experiment with sources of this after an initial period on SCD.
Your situation, though, is unique. It's all guesswork.
