Page 79 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 79

Apparent and true digestibility of fats, starch and energy are Table 5-8. Mineral availability as affected by different fiber
sources fed at 5% total dietary fiber.*
VetBooks.ir al, 1996; Muir et al, 1996). Fiber (%)** Calcium (%)*** Phosphorus
unaffected by the type and amount of fiber in foods (Silvio et
Zinc
Iron
†
(%)**
Apparent digestibility of protein is lower in foods contain-
source
(%)
ing fiber because of increased nitrogen in feces from the Apple pectin ND ND 55 a 75
Beet pulp 24 a 44 a 29 a 67 a
increased fecal biomass. Research using ileal-cannulated dogs Cellulose 88 100 100 100
has shown that true digestibility of protein is unaffected by Citrus pulp cells ND ND 49 a 75
dietary fiber type or content (Silvio et al, 1996; Muir et al, Corn bran 94 92 100 ND
Guar gum ND ND 100 35 a
1996). However, a study that evaluated varying levels of cellu- Gum arabic ND ND 81 100
lose and pectin at 10% total dietary fiber (100% cellulose, 66% Pea fiber 41 a 58 a ND ND a
cellulose/34% pectin, 66% pectin/34% cellulose and 100% Peanut hulls 30 100 67 58
Soy cotyledon fiber ND ND 86 66 a
pectin) showed that total crude protein digestibility decreased Soy hulls 78 100 ND 64 a
with increasing pectin levels (Silvio et al, 2000).The discrep- Sunflower hulls 54 100 100 a ND
ancy between the results obtained with total tract apparent Wheat middlings 70 100 51 ND
Key: ND = not determined.
digestibility and true digestibility can be explained by taking *Availability (%) of the mineral in a food with 5% iso-total dietary
into account fermentation in the colon. One of the assump- fiber relative to the same food with no fiber.
tions when calculating digestibility across the entire GI tract is **Adapted from Wedekind K, Walker L, Hancock J, et al.
Bioavailability of zinc and calcium is affected by certain fiber
that all fecal material is of dietary origin. Fermentation of sources. Federation of American Societies for Experimental
nutrients that pass into the large intestine, however, results in Biology Journal 1995; 9: A450.
significant amounts of bacterial protein in the feces. Bacterial ***Adapted from Wedekind K, Walker L, Beyer S, et al.
Bioavailability of iron is affected by certain fiber sources in
protein is then confounded with undigested dietary protein chicks and puppies. Ninth International Symposium on Trace
resulting in lower apparent digestibility. Therefore, it is best to Elements in Man and Animals (TEMA-9) 1996; A20.
† Adapted from Wedekind K, Beyer S, Titgemeyer E. Bioavailability
use true protein digestibility to obtain an unbiased evaluation
of phosphorus is affected by certain fiber sources. Federation of
of pet foods containing fiber. When evaluating protein American Societies for Experimental Biology Journal 1996; 10:
digestibility data, it is essential that the fiber content of the A524.
a Within the same mineral, a superscript “a” indicates significant
food be known, and care must be taken when interpreting and
reductions (p <0.10) relative to the no-fiber standard.
comparing results among foods.
The main excretory product of fiber digestion is additional
bacterial protein in the feces. It is typically analyzed as addi-
tional fecal nitrogen content and can confound protein when most of the fiber is slowly fermentable; however, the
digestibility measurements made using total fecal collections. analysis excludes the more rapidly fermentable pectins and
Intestinal fermentation of fiber accounts for hydrogen,methane gums. Because the crude fiber analysis underestimates fer-
and other gases in expired breath. Some short-chain fatty acids mentable fiber, it does not accurately represent the total fiber in
are excreted in the stool. a pet food.
Fiber affects mineral availability (BNF, 1990). Some fiber Fiber can also be measured by the total dietary fiber method
types reduce and others enhance mineral absorption and use (Prosky et al, 1984) (Figure 5-12). This analysis is used to
(Table 5-8). It is not clear what factors in fiber are responsible determine total fiber and is commonly used for measuring
for the effects on mineral availability. Water-holding capacity, fiber content of human foods. In the total dietary fiber
viscosity, cation exchange capacity, particle size, tannin content, method, lipids are first extracted with ethanol and then the
oxalate content and presence of phytates, uronic acid and phe- sample is digested with α-amylases to convert readily
nolic groups are among the properties of fibers that have been digestible starches to soluble sugars. All the water-soluble
evaluated to predict effects on mineral availability (BNF, 1990; components (sugars, degraded starch, pectin, gums and most
Robinson, 1987; Southgate, 1987). Unfortunately, a direct rela- of the hemicellulose) are separated from the water-insoluble
tionship appears not to exist between these physiochemical components or insoluble fibers (cellulose, lignin and a small
properties measured on fibers in vitro and mineral availability fraction of hemicelluloses). The water-soluble components are
measured in vivo. This disparity reflects the complex nature of further extracted with ethanol to remove the sugars and
the absorption processes within the intestine and interactions degraded starch. The residue that remains is termed soluble
that occur with other food components. fiber, which includes pectins and gums.
The Van Soest fiber analysis system was developed as an
Analyses improvement to the crude fiber method (1963). The Van Soest
The fiber content of pet foods or ingredients can be measured analysis uses detergents to more accurately estimate the differ-
by several different laboratory methods; the most common is ent types of fiber and fractionate them into relatively digestible
the crude fiber method (Figure 5-12). In the United States, and indigestible fractions (Figure 5-12). Neutral detergent fiber
regulations require that the maximum amount of crude fiber be (NDF) is the residue remaining after samples are boiled in a
listed on the label of all pet foods (Chapter 9). Determination solution containing neutral pH detergent and EDTA. The
of crude fiber adequately represents total fiber in a pet food residue that remains is mostly plant cell walls including hemicel-