Effect of seeds on bile-enzymatic-gravimetric analysis of total dietary fiber

Dietary fiber sometimes is defined chemically as nonstarch polysaccharides plus lignin or as other specific chemical entities. Analysis of dietary fiber according to a chemical definition typically involves gas chromatography, which allows separation and quantitation of chemical constituents that are added to arrive at a dietary fiber value. Other definitions of fiber are broader, defining it to be whatever is not digested in the alimentary tract. Analytically, this definition translates into the gravimetric sum of the material remaining after a series of enzymatic and chemical treatments that simulate in vivo digestion. Various methods reflect the gravimetric definition, which might include as dietary fiber some protein, resistant starch, and even lipids that are not digested by particular assay conditions. We used a recently proposed bile-enzymatic-gravimetric assay for total dietary fiber on commonly consumed seeds (hulled and unhulled sesame, caraway, and poppy) and visually found these seeds to be undigested. We then determined the impact of the undigested seeds on measured dietary fiber content by spiking homogenized daily menus with 5% by weight of these seeds and calculating recoveries with 2 assumptions: seeds are 100% fiber because they are not digested, and the fiber content of seeds is as determined by assay. Calculated recoveries were very different depending on which assumption was made (71-90% or 99-109%, respectively), and the difference was closely related to the seed's protein content.