Dietary Fibers and Absorption of Nutrients 1

Abstract

The increased intake of foods containing dietary fiber can generally be shown to result in increases in fecal volume and dry weight and in fecal energy content (1-3). These changes are in part directly attributable to increased consumption of undigestible plant cell wall material, but there is also evidence that the certain dietary fibers may also affect the digestion and absorption of other nutrients.

The effect of dietary fiber on increased fecal energy content can be expressed by fruit, vegetable, or cereal fiber (4, 5) and has been reported to vary between 58 and 321 kcal/day, depending on the type and amount of dietary fiber ingested (4-7). This energy loss is largely in the form of fat and protein (7) and has been suggested to be of both bacterial and metabolic origin (8, 9).

Generally, the effects of increasing intakes of fiber in the form of wheat bran have, at best, only a mild effect on fecal energy loss (10). A portion of this loss is due to fat which is closely associated with the bran and largely inaccessible to the lipases of the intestinal contents. However, increased intakes of fiber in the form of pectin, which, when pure, lacks endogenous fat, cause more apparent increases in fecal fat content (11, 12), and this loss is therefore of endogenous or metabolic origin.

Southgate (2) has pointed out the difficulties of assessing the effects of dietary fibers on the digestibility of dietary macronutrients. In many nutritional studies, assessment of nutrient digestibility is based on comparisons of nutrient intakes and fecal recoveries. This, however, only represents apparent digestibility (2), since even when the digestion and/or absorption of a nutrient is negligible, there is a finite fecal recovery.