Raffinose is sometimes called a fiber but more often is characterized as a trisaccharide, oligosaccharide, or complex carbohydrate. It’s all four.
A typical bowl of cabbage soup has three grams of fiber. If you eat two bowls, that’s six grams, still not all that much, but can predictably cause loose stools or diarrhea in many folks because of a particular type of fiber: raffinose.
The thing about raffinose is that it passes through the small intestine undigested because we lack the enzyme alpha-galactosidase. When raffinose hits the colon, bacteria start digesting it (aka fermentation), potentially leading to gas, bloating, and/or diarrhea. If your “dose” of raffinose is small enough, you won’t have any symptoms. To use cabbage soup as a constipation preventative or remedy, you have to experiment to see what dose works for you.
Raffinose is also found in beans and cruciferous vegetables like brussels sprouts and cauliflower
PS: Ever heard of Beano? The active ingredient is the enzyme alpha-galactosidase. It breaks down raffinose in the small intestine, to simple sugars we can absorb.
PPS: Raffinose is one of the oligosaccharides to avoid if you’re on a low FODMAPs diet.