Sentence:  Evidence of the <WSD>prebiotic</WSD> properties of polydextrose (PDX), resistant maltodextrin (RMD), and other soluble fiber sources are slowly emerging, but the body of evidence is small compared to the gold standard of fructo oligosaccharides (FOS) and inulin. 
Meaning of the word marked with <WSD> tags:  Not digestible, but beneficial.