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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.

Young adult rats were fed ad libitum for 4 wk on defined diets containing either no fiber, 10% levels of insoluble fiber sources [cellulose, wheat bran, alfalfa, mixed fibers (Fibyrax)], 5% levels of soluble fiber sources [pectin, guar gum, psyllium (Metamucil)] or 2% cholestyramine (Questran), a bile acid sequestrant. Fecal samples were obtained from paired rats over a 3-d period, were analyzed for neutral and acidic steroid levels and compositions and, together with the food, were assayed for divalent cations (Ca2+, Mg2+, Fe2+and Zn2+). Animals in all groups were in balance for divalent cations, and there appeared to be no relationship between the extent of cation balance and the type of fiber fed. All insoluble fiber diets and the guar gum and psyilium diets resulted in significantly higher daily fecal mass and, in general, resulted in significant dilution of total fecal steroids. With the insoluble fibers, there was a general dilution of fecal neutral steroids, which was not apparent with the soluble fibers or cholestyramine. In addition, except for the pectin- and mixed fiber-containing diets, there was reduced bacterial conversion of the primary bile acids to secondary bile acids or metabolites.

Adult male rats were surgically provided with a drainage catheter in the left thoracic lymphatic channel and an indwelling duodenal catheter for constant infusion of physiological saline–5% glucose. After an overnight fast, animals were given a single duodenal dose of an aqueous emulsion containing one of the following: oleic acid, corn oil, menhaden oil or a fish oil concentrate (FOC) and [1,2-3H]cholesterol. Digestion and absorption were estimated by recovering the total fatty acids in the thoracic duct lymph over a 24-h collection period (after subtraction of the “baseline” endogenous fatty acids in the lymph). Cholesterol absorption in the thoracic duct lymph was significantly reduced (P< 0.05) in the presence of menhaden oil or FOC compared to that in the presence of corn oil. With various fat feedings, the major increases in lymph fatty acids were directly related to the dietary fatty acid content. The relative amounts of eicosapentaenoic acid (EPA) and arachidonic acid (AA) in the thoracic lymph were influenced by the lipid content of the emulsion. The EPA/AA ratio in control, oleic acid and corn oil feedings ranged from 0.12 to 0.25. When marine oil was administered, the EPA/AA ratio was 0.78–0.98. The total amount of fatty acids found in the lymph after marine oil feeding was significantly less (P< 0.01) than that found after corn oil feeding. The results suggested that the digestion and absorption of menhaden oil and FOC were decreased as compared with corn oil. The EPA/AA ratio was increased in the thoracic lymph after dietary fish oil feeding.

Adult male rats were surgically provided with a drainage catheter in the left thoracic lymphatic channel and an indwelling gastric catheter for constant infusion of saline-glucose. After an overnight fast, animals were given a single gastric dose of an aqueous emulsion containing one of the following 1-14C-labeled fatty acids: oleic, arachidonic or eicosapentaenoic acid, and [1,2-3H]cholesterol. Absorption was estimated by the appearance of radioactivity in lymph over a 24-h collection period, and the lymph lipoprotein distributions and lipoprotein lipid distribution were determined in the 24-h samples. Although there were apparent differences in the rates of eicosapentaenoic and arachidonic acid absorption, the overall appearance (after 24 h) of these acids in lymph was quantitatively equivalent to that of oleate. Furthermore, cholesterol absorption from each fatty acid medium was quantitatively similar. The distributions of each fatty acid among major lymph lipoproteins were similar with 93–95% recovered in chylomicrons and very low density lipoprotein (VLDL) fractions. Furthermore in these lipoproteins, 85–91% of each of the three fatty acids was recovered as triglycerides. With both polyunsaturated fatty acids, there was greater incorporation into phospholipids and their precursor partial glycerides than with oleate. These studies suggest that unesterified eicosapentaenoic acid is absorbed efficiently into the lymphatic system, and has lymph lipoprotein and lipid distribution comparable to those of the more typical mono- and polyunsatured fatty acids.