Some clarification:
There are some different things going on in fermentation.
-When you use YEASTs to ferment - like when making beer - the yeasts are digesting the sugars and producing ACETIC acid.
-When you use BACTERIA to ferment - like when making sauerkraut, yogurt, etc. = the bacteria are digesting the sugars and producing LACTIC acid.
I prefer to propagate the LABs (Lactic Acid Bacteria) rather than using yeasts for many reasons.
When using bacteria to ferment you will also get some "wild fermentation yeasts" joining the crowd as they're in the air and on the items you are fermenting. However, there are ways to help the bacteria along and continue to create a balance that favors the LABs.
As far as them digesting the sugars - yes, that is the process in fermenting. The sugars are being digested by either the yeasts, bacteria or both. That is what the process of fermentation is all about.
As far as reducing energy-producing sugars - your body is able to produce plenty of energy without them...no problem. But the reality of it is, most of us don't ferment so long that every sugar (carb) has been fully digested so there are still plenty in there. If you ever fermented to that point, the bacteria would begin to die for lack of food since that's what they digest.
This has been a very condensed version to explain what's happening. Could put a lot of technical stuff in there, but there it is in laymans terms.