Towards an updated methodology for measurement of dietary fiber, including associated polyphenols, in food and beverages

The analytical dietary fiber (DF) methods most widely used today were developed to determine non-starch polysaccharides and lignin. Updated dietary fiber definition includes all indigestible plant food constituents. Recent methods have proposed the measurement of resistant starch and oligosaccharides, but other major indigestible constituents such as polyphenolic compounds and resistant protein are still omitted in dietary fiber analysis. There is scientific evidence that an appreciable amount of dietary polyphenols are associated with the dietary fiber matrix, being a fermentable substrate for bacterial microflora. The objective of this work was to show polyphenols compounds are major constituents of dietary fiber and to propose a procedure for their measurement. Results showed that polyphenols are major constituents of DF, accounting from 1.4% to 50.7% (dry weight) of insoluble dietary fiber in plant foods and from 2.9% to 62.8% of soluble dietary fiber in common beverages.