Only raw, unpasteurized sauerkraut carries live cultures. Heat kills them, so the shelf-stable jars and cans in the center aisle are pickled cabbage rather than a living ferment. Look in the refrigerated section for the words raw, unpasteurized, or live cultures.
4.1g of fiber per cup for 27 calories, the kind of fiber your gut bacteria can actually use.
The ferment is run by lactic acid bacteria, largely Lactobacillus species, which are among the microbes research associates with a more varied gut community.
Vitamin C at 23% DV per cup. Cabbage holds on to its vitamin C through fermentation, which is why barrels of it once sailed with crews who had no other source of it.
One honest caveat: 939mg of sodium per cup, roughly 41% of the daily limit. Salt is what makes the ferment work, so sauerkraut earns its place as a condiment rather than a side dish.