FOS, XOS and GOS are short, linear chains of the sugars fructose, xylose and galactose respectively. The relatively small size means they are classified as oligomers, rather than larger polymers and the presence of sugar molecules makes them oligosaccharides.
The chemical linkage between the sugar molecules is one that humans can't digest so the molecules pass into the colon unaltered. Many gut bacteria can digest the linkage and use the sugar molecules for energy.
All three substances are laboratory creations though they are related to naturally occurring substances.
As already noted, FOS is similar to inulin. It is produced by enzymatic treatment of inulin to produce shorter chain lengths. Generally, oligomers with greater than about 10 fructose residues are considered to be inulins, less than 10, FOS. The longest inulins contain about 60 residues.
XOS is an extremely simplified version of xylans in plant cell walls, likewise GOS is a poor approximation of human milk oligosaccharides (HMOs).
These laboratory creations are easier to handle and sell as supplements - readily soluble, easy to produce etc. In general they appear to be readily fermented by a wide variety of bacteria.
You would need to do specific research if you were interested in promoting growth of particular bacteria in your gut.
The arabinogalactan you mention consists of a basic chain of galactose molecules with side chains of arabinose. So it is most like GOS though more complex.
Thanks! I haven't tried resistant starch... It seems like something that I shouldn't be eating b/c it's too carby (starchy?)
Starch, like inulin, is used by plants for energy storage. It is a very large polymer of glucose, often with a complex branched structure. Unlike inulin, we can digest some of the starch, but some we can't. This is resistant starch, which our gut bacteria can ferment and which is an important prebiotic food.
Resistant starch is usually classified into four types. RS1 is trapped in plant cell walls which we can't digest and so becomes available to gut microbes. There is a lot of this in food like beans.
RS2 is found in green bananas, plaintains and raw potatoes. It can be found in pure form in
raw potato starch which we don't digest at all - it is not "starchy". If cooked though it does become digestible.
RS3 is retrograde starch, formed when cooked starch cools. So hot, cooked potatoes are largely digested by us but cooled into potato salad and a significant proportion of this starch becomes undigestible and can feed our gut bacteria.
RS4 is a laboratory creation akin to FOS, GOS and XOS. It is often called dextrin, usually derived from wheat.
Personally I think that all these laboratory creations should be used with caution. Small amounts are probably fine but if you want to boost prebiotic intake with concentrated preparations (as opposed to relying only on vegetable intake) then a variety of less processed options such as raw potato starch, larch extract (your arabinogalactans), pectins, acacia and guar gums, baobab extract, aloe vera - to name just a few off the top of my head - are more like what our gut bacteria have evolved to eat.
