RS is a special starch, not sugar. I guess you could think of fructooligosaccharides as being like resistant sugar. Fermentation is key, because if it's fermentable, that means the bacteria can eat it. Resistance is also key, because if it's resistant to stomach and small intestine digestion, then it can get down to the colon bacteria. Resistant starch is a fermentable fiber, as is inulin, pectin, fructooligosaccharides, etc., and resistant starch seems to be the most important one.
Not many people are eating much of it on any of the popular current diets in America--the SAD, typical versions of the USDA food pyramid, most raw "Paleo" and cooked "Paleo", low carb, Peatarian, and even some vegetarian and vegan, etc. Part of the problem is that the foods highest in RS tend to be rare or unpopular in the USA. Lots of folks in less modern old world countries still eat a fair amount of RS. Unfortunately, they too are eating less and less as more of their foods become processed in factories.
