Carb, fat and protein changes in vegetable fermentation, specifically a ballpark reduction arc in carbohydrates I have been fermenting vegetables, kimchi for the most part, for a few years and am familiar with the general arc of the process, as well as observing how active my ferment is.
Yesterday, I started a half gallon of sauerkraut, a half gallon of white kimchi, and a couple pints of fermented beets. Today, I am working up nutritional values for each in support of a Keto eating plan. I've had no trouble working up the raw calories, fat, net carbs and protein for each recipe. Yet I know that fermenting will change the carbohydrates and perhaps other values.
I am aware there is no one-size-fits-all for this calculation, rather the opposite, it seems the number of variables is too broad to pin down any kind of equation. Case in point, I have three ferments going, all burning at a different pace. That said, I'm looking for any kind of rough approximation based on experience.
Two facts seem certain. One, that the fermented product will have a lower carbohydrate value than the cumulative raw ingredients. And two, the resulting carbohydrates will never reach zero. That leaves the rate of burn. Based on your experience, what would you expect the carbohydrate burn to be in a highly active ferment for something like Napa Cabbage, Red Cabbage, or Beets?
Less important, but of keen interest, should I expect the other values (cals, fat, or protein) to change as the result of fermentation?
on the &quot;Less important, but of keen interest&quot; question:
let's talk carbs instead of cals here.  cals are potential chemical energy stored in carbohydrates, fats, proteins, etc.  in vegetable fermentation, the fat and protein content is not a significant contributor to potential calories, so i'll confine this to carbohydrates
microbial fermenters' (effective, not comprehensive or quantitative) ...
most who do this don't track a carb burn rate, but they definitely notice when the jars quit bubbling.  CO2 output rate is directly proportional to carb burn rate, but those are very difficult to measure without specialized equipment
there are two ways that fermenting usually stops, when fermenters

run out of food (mostly carbs)
byproducts (here, lactic acid) are concentrated enough to halt reproduction

so if the jar's quit bubbling, it's important to understand that it might not be down to near zero carbs
inputs are

carbs
minuscule amounts of protein and trace minerals

outputs are

more fermenters
carbon dioxide, which is not retained by most canners
lactic acid
compounds that lend funk to flavor and aroma

note that fermenters consist mostly of:  water, lipids (structural and energy storage), and protein (structural and information storage)
to answer the keen interest ...

net change in carbs?  yes.  down
net change in protein?  yes.  up, but not by much
net change in fat/lipids?  yes.  up, but not by much