ABSTRACT. The effects of purified fiber components and wheat bran on several indices of protein utilization were determined in growing rats. A control diet ...
Effect of Dietary Fiber Components on Fecal Nitrogen Excretion and Protein Utilization in Growing Rats1 NALINI SHAH, MOKHTAR T. ATALLAH, RAYMOND R. MAHONEY AND PETER L. PELLETT2
Department of Food Science and Nutrition, University of Massachusetts, Amtierst, MA 01003 ABSTRACT The effects of purified fiber components and wheat bran on several indices of protein utilization were determined in growing rats. A control diet containing 10% casein was diluted with either cellulose (C), pectin (P), lignin (L), guar gum (G), or wheat bran (W) at fiber levels ranging from 3% to 20%. All fibers except C caused a decrease in net protein ratio (NPR) as compared to the control casein diet. This depression in NPR increased as the dietary fiber level increased. Apparent and true nitrogen digestibilities also decreased with all fibers at all levels. At the highest level of fiber (20%) the depression was greatest for G and W and was least for C. NPR when divided by digestibility (analogous to biological value) decreased with P, L, W (all levels) and G (20% level) but not with C. When rats were fed fiber without protein, there was increased excretion of endogenous fecal nitrogen with all fibers at all levels. The results demonstrate that fiber(s) affected protein utilization as measured by NPR, digestibility and endogenous fecal nitrogen excretion and that the negative effect increased with the level of fiber consumed. J. Nutr. 112: 658-666, 1982. INDEXING KEY WORDS nitrogen •fecal nitrogen
dietary
fiber
Dietary fiber has been reported to decrease utilization of many nutrients, including pro tein (1,2). High fiber diets have been shown to increase fecal nitrogen excretion (3, 4). Decreased digestibility of dietary nitrogen has been reported with high fiber diets, in humans and laboratory animals (1, 5-7). In growing rats, guar gum produced a decrease in growth rate and nitrogen absorption (6); in addition, aliginate and pectin also reduced food intake (7). Because of the wide variations in level and source of dietary protein, level, type and source of fiber, experimental conditions used and method of evaluating protein utilization, the conclusions from these various investi gations remain equivocal. The present study was designed to measure the effect of puri fied fiber sources (cellulose, pectin, lignin and guar gum) and a nonpurified fiber source (wheat bran) at different levels in the diet on endogenous fecal nitrogen excretion and
• protein
utilization
• endogenous
on protein (casein) utilization in growing rats using digestibility, net protein ratio (NPR) and relative NPR (RNPR) as indices of pro tein utilization. It is recognized that conventional termi nology using the term fiber to include such wide ranging materials as purified pectin and wheat bran is unsatisfactory. However, for the want of a better term, fiber will be used throughout. MATERIALS AND METHODS
Animals, fibers and diets. Male, albino rats were obtained at 21 days of age [(CDCRL:CD[SD]BR), Charles River Breeding Laboratories, Wilmington, MA] and weighed 45 ±5 g. They were housed individually in ©1982 American Institute of Nutrition. Received for publication 13 July 1981. 1Research supported by the Massachusetts Agricultural Experiment Sta tion. 'To whom reprint requests should be sent.
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FIBER EFFECTS ON FECAL NITROGEN AND PROTEIN USE
suspended wire cages at an ambient temper ature of 21-22°and 50% relative humidity with lighting between 0700 and 1900 hours. The rats were fed stock diet (Rodent Labo ratory Chow, Ralston Purina Co., St. Louis, MO) ad libitum for the first five days. After this adjustment period, the rats were weighed and randomly assigned into groups of six so that the mean weights of the groups differed by less than ±1g and were then assigned to experimental diets for 21 days. Diets and water were fed ad libitum. The characteristics of the purified and nonpurified fiber sources used to prepare the diets are shown in table 1. Fiber can be added to a diet either by substituting an equivalent amount of one of the dietary components, e.g., cornstarch, or by adding fiber to the total diet. Since fiber does not supply significant energy (9), when substitution is used the per centages of nutrients, except food energy, remain constant but the protein:calorie ratio will change. In contrast, addition (dilution) with fiber keeps the protein:calorie ratio con stant but lowers the proportion of the indi vidual nutrients. A nonpurified fiber source such as wheat bran is an exception to this statement because of the nutrients contained in the wheat bran itself (8). These however
659
may not always be absorbed. In this study, dilution with fiber allowed for constancy of protein:calorie ratio (10). The diet composi tions used are shown in table 2. Chromic oxide (0.4%) was added to the diets for the determination of digestibility. Feces were collected from each rat starting from the 7th day on the diet. Fecal samples were kept frozen pending analysis. Food in take, food wastage and animal weights were recorded periodically and also at the termi nation of the study. Analysis. Fibers, diets and homogenized fecal samples were oven dried at 110°for 24 hours. Total nitrogen was determined by a semimicro-Kjeldahl method (11). Except for cellulose, in which the nitrogen content was negligible (table 1), the fiber samples were hydrolyzed and analyzed for their amino acid composition (12) using a Beckman model 120-C analyzer (Beckman Instruments, Inc., Spinco Division, Palo Alto, CA). Guar gum and pectin were analyzed for galacturonic acid by the carbazole method (13). Pectin was also analyzed for the degree of esterification by the Hinton method as outlined by Kertez (14). Chromic oxide in diets and fecal samples was determined by the procedure of Bolin et al. (15).
TABLE 1 Characteristics of fiber sources' Fiber
Chemical and physical properties
Distributor or manufacturer
Alphacel
Finely ground cellulose, moisture 7.0%, nitrogen 0.02% d.w.b.2
ICN Nutritional Biochemicals, Cleveland, OH
Guar Gum
Ratio of mannose:galactose:arabinose:glucose:xylose 59:36:2:3:trace3 Galacturonic acid: none, moisture 7.83%, nitrogen 0.58% d.w.b.
ICN Nutritional Biochemicals, Cleveland, OH
Pectin (citrus)
Anhydrogalacturonic acid 62.5%, esterification 71.0%, moisture 7.0%, nitrogen 0.17% d.w.b. Purity 99.5%,'' moisture 5%, nitrogen 0.25% d.w.b.
U. S. Biochemicals, Cleveland, OH
Lignin (indulin A. T.) Wheat bran (AACC-RO7 3691)
Dietary fiber 43.2%: cellulose 8.7%, hemicellulose 28.3%, lignin 3.2%, pectin 3.0%,4 moisture 10.3%, nitrogen 2.64% d.w.b.
1See analysis (Materials and Methods). 2d.w.b., dry weight basis. (8). ' As specified by the manufacturers.
Gift from Westvaco Chemical Division, Charleston Heights, SC American Association of Cereal Chemists, St. Paul, MN
! As analyzed by Anderson and Clydesdale
SHAH ET AL.
660 TABLE 2
increase caused presumably by the protein of wheat bran (table 1). Excretion of fecal nitrogen increased with all fibers at all levels Ingredients Parts, by wt Diet and the increase was the greatest for rats fed wheat bran. These animals also consumed the oil1Salt Basal, n = 1Corn mix-'Vitamin most food. mix3Chromic Cellulose (all levels) and guar gum (5% oxide4Cornstarch510430.482.6 level) in the diets had no effect on NPR but addition of pectin, lignin, wheat bran (all lev 10 parts casein replaces 10 parts els) and guar gum (at higher levels) to the Basal + casein,* n = 1 cornstarch in the basal diet diets, resulted in a significant decrease in NPR when compared to the basal + casein caseinpartsBasal Basal + diet. In general, except for cellulose, in creases in level of fiber increased the effect. fiber7"8(a) + casein + This trend is also illustrated in the values of 100Cuar Cellulose, pectin. RNPR where the NPR for rats fed a standard, orWheat gum 12(b) bran, n = basal + 8% lactalbumin fiber-free diet was Lignin, n = 2 100Addedfiberparts5.211.125.0316.4Fiber%5102036 defined as 100, and the other NPR are ex pressed relative to this value. Fiber sources added at same levels Addition of fiber significantly reduced ap as above to the basal diet (without parent nitrogen digestibility (AD), which fell Basal 4- fiber, n = 14 casein) further as the fiber levels rose in the diet. 1 Commercial Mazóla corn oil. * U. S. Biochemicals, salt This decrease was most pronounced with the mixture Rogers-Harper. ' U. S. Biochemicals, AIN vitamin addition of guar gum at the 20% level. Di mixture 76. * Fisher, Chromium oxide, sesqui. 5 U. S. Bio chemicals, cornstarch. * U. S. Biochemicals, vitamin-free ca viding NPR by nitrogen digestibility is an sein. 'Fiber see table 1. 8 Protein:calorie ratio kept con indication of the weight gain of rats per gram stant for all levels (see Materials and Methods). of nitrogen absorbed and is analogous to bi ological value (16). Pectin, lignin and wheat Apparent and true nitrogen digestibility, bran (all levels) when present in the diet pro NPR and RNPR were calculated as outlined duced significant decreases in NPR/AD. by Pellett and Young (16) except that data Guar gum and cellulose both showed effects were used from animals consuming nonproonly at the 20% level. The cellulose effect tein diets containing corresponding levels of seems anomalous in that it produced a small fiber for the calculation of NPR, rather than but statistically significant increase in NPR/ from a single nonprotein group. AD. Similar changes are seen for true nitro Statistical analysis. The data reported are gen digestibility (TD) and NPR/TD. True the means with their standard deviations and nitrogen digestibility was calculated using statistical significance was determined be the endogenous fecal nitrogen (corrected) tween means by use of Student's unpaired values shown in table 4. i-test (17). The effects of fibers on food intake, en dogenous fecal nitrogen and weight loss for RESULTS rats fed basal diets (without casein) are shown The effect of fiber(s) on food intake, ni in table 4. Food intake rose significantly with trogen digestibility and nitrogen utilization the addition of pectin (20% level), guar gum for rats fed the basal + casein diet is shown (10% and 20% level), lignin and wheat bran in table 3. Food intake fell significantly with (all levels). increasing levels of pectin and guar gum and Endogenous fecal nitrogen excretion or was increased by the addition of wheat bran. metabolic nitrogen (FJ is usually considered Weight gain was less when the diets con to be the fecal nitrogen excretion on a nontained cellulose (10 and 20%), pectin (all lev protein diet (16). A nonprotein diet, however, els), lignin (6%) and guar gum (all levels). It may contain some nitrogen, and fiber com remained unchanged for wheat bran except ponents also contain nitrogen (table 1). Thus at the 20% level, where it showed a marked endogenous fecal nitrogen values may be Composition of diets
661
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