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ANIMAL NUTRITION |
Department of Animal Sciences, University of Illinois at Urbana-Champaign, Urbana 61801
Abstract
The objectives of this research were to examine the palatability of stabilized rice bran (SRB) when included in a dry canine diet, and to determine the effects of SRB on food intake, digestion, fecal characteristics, blood lipid characteristics, and selected immune mediators. Experiment 1 tested the palatability of SRB. Diets contained poultry fat in Test 1 and soybean oil in Test 2, in conjunction with either 12% SRB or 12% defatted rice bran (DRB, as-fed basis), and were fed to 20 dogs. Diets contained approximately 32% protein and 22% fat (DM basis). Food intake data were collected and intake ratios calculated (grams of SRB diet consumed divided by total consumed of both diets). Intake ratios were 0.73 for Test 1 (P < 0.01) and 0.61 for Test 2 (P < 0.14) for SRB diets. Diets in Exp. 2 contained 12% SRB or DRB (as-fed basis), and poultry fat, beef tallow, or poultry fat:soybean oil (50:50) as the main fat sources, and were fed to 36 beagles. Diets contained approximately 32% protein and 22% fat (DM basis). The effects of SRB and DRB were determined on food intake, digestibility, fecal characteristics, and blood fatty acid, phospholipid, and eicosanoid concentrations. No differences were noted in food intake, digestibility, or fecal characteristics. Fat sources contributed much more to dietary fat than rice bran source; therefore, fat source profiles overwhelmed the rice bran source contribution. Dogs consuming a DRB diet had lower (P < 0.050) plasma phospholipid total monounsaturated fatty acids compared with those consuming a SRB diet (-1.17 vs. 0.95%, respectively), whereas plasma fatty acid concentrations tended (P < 0.119) to decrease more than with SRB diets. Total concentrations of red blood cell phospholipid SFA tended (P < 0.15) to be greater in dogs consuming a beef tallow-containing diet compared with those consuming a poultry fat or poultry fat:soybean oil diet. Total concentrations of red blood cell phospholipid PUFA and n-6 PUFA tended to be greater (P < 0.097 and P < 0.083, respectively) in dogs consuming a poultry fat-containing diet than in those consuming a beef tallow-containing diet. Statistical differences and tendencies were detected in individual plasma fatty acids and plasma and red blood cell phospholipids due to rice bran source, fat source, and their interaction. Eicosanoid concentrations did not change due to treatment. Stabilized rice bran is a highly palatable ingredient when included in a dry dog diet, and did not elicit an effect on inflammatory immune mediators in healthy dogs.
Key Words: Blood Fatty Acid Profile • Canine • Digestibility • Eicosanoid • Palatability • Stabilized Rice Bran
Introduction
Approximately 500 million metric tons of raw rice are produced each year (Sayre and Saunders, 1990
). During removal of the bran layer from brown rice, lipase from the testa and cross cells comes into contact with the oil in the aleurone layer and germ. Lipase rapidly hydrolyzes the lipid to glycerol and FFA, resulting in an unpalatable by-product (Sayre and Saunders, 1990). Inactivating the lipase results in the ingredient known as stabilized rice bran (SRB) (Saunders, 1990).
Stabilized rice bran has been shown to be a "functional ingredient," one that provides a health benefit beyond that of basic nutrition in humans by lowering blood cholesterol concentrations and exerting a laxative effect. Little is known about the functional properties of SRB in the canine. Stabilized rice bran contains approximately 20% lipid, including essential fatty acids with a decreased n-6:n-3 ratio. These fatty acids are used to synthesize eicosanoids, modulators of inflammation. By decreasing the ratio of n-6:n-3 fatty acids and thus altering the profile of cell membrane phospholipids, less inflammatory eicosanoids may be produced.
The high fat content and the fatty acid profile of SRB warrant examination of its effects on blood fatty acid and phospholipid concentrations. Determining the effects of SRB on palatability, digestibility, and immune response criteria when included in a canine diet will help define whether SRB is an appropriate functional ingredient for inclusion in commercial dog foods.
The objectives of this research were 1) to examine the acceptance of SRB when included in a high-quality, dry canine diet and 2) to determine the effects of SRB on food intake, digestion, fecal characteristics, blood lipid characteristics, and selected immune mediators in the dog.
Materials and Methods
Experiment 1
Animals.
Twenty pointer dogs (12 females and eight males), with weights ranging from 18.7 to 31.9 kg, were used in each of two palatability tests. Animal care procedures were approved by the University of Illinois Animal Care and Use Committee prior to initiation of the experiment. Dogs were individually housed in indoor–outdoor pens measuring approximately 1.2 x 1.5 m indoors and 1.2 x 3.0 m outdoors at Kennelwood Inc. (Champaign, IL). Dogs had access to the outside area of the kennel once daily.
Dietary Treatments.
Two tests were conducted to determine palatability of SRB in diets with different fat sources. For each test, diets were formulated to have similar GE values, and the exterior fat and exterior flavor enhancers were as similar as possible between treatments. Composition of the experimental diets is presented in Table 1. Dietary ingredients were identical except for the supplemental fat source and the type of rice bran added. The diets contained either 12% SRB or 12% defatted rice bran (DRB, as-fed basis). In Test 1, the supplemental fat source was poultry fat, and soybean oil was used as the supplemental fat source in Test 2. Testing poultry fat and soybean oil allowed the comparison of the palatability of SRB in a diet with both an animal- and a plant-based fat source. All diets were produced at the same time, with the same ingredients, and diets were used in the palatability tests within 1 wk of manufacturing. Before use, diets were stored at a cold temperature to prevent lipid oxidation.
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The experiment was designed as a two-pan, free-choice test, the most common palatability test used in the pet food industry (Griffin, 1996). This design results in the most reliable data (Hutton, 2002). Dogs were offered 1,000 g of each test diet for 1 h at the same time each day. Presentation of bowls was alternated each day to eliminate any "handedness" bias of the dogs. First choice (diet first consumed, an indicator of aroma preference more than taste preference) and first approach (diet first examined and smelled, another indicator of aroma preference more than taste preference) data were collected. At the end of the hour, any refused or unconsumed feed was weighed to determine the amount of each diet consumed.
Sampling Procedures.
A sample was taken of the four dietary treatments and SRB source and frozen at -4°C for subsequent analyses. Diets were ground with dry ice to pass a 2-mm screen in a Model 4 Wiley mill (Thomas-Wiley, Swedesboro, NJ) in preparation for chemical analyses.
Chemical Analyses.
Stabilized rice bran and diets were analyzed for DM, OM (AOAC, 1985), CP from Leco N values (AOAC, 1995), acid-hydrolyzed fat (AACC, 1983; Budde, 1952), and GE (Parr Instrument Manuals; Parr Instrument Co., Moline, IL).
Calculations.
The amount consumed of each diet was calculated by subtracting food refusals from the amount of food originally offered. Intake ratio (IR) was calculated by dividing the grams consumed of the DRB- or SRB-containing diet alone by the total grams consumed of both diets.
Statistical Analyses.
Data were analyzed by the paired t-test procedure of SAS (SAS Inst., Inc., Cary, NC). An additional t-test was conducted to determine if IR data were different from 0.50. This t-test indicates if the calculated IR implies a preference. Data collected in Test 1 were analyzed separately from data collected in Test 2. A probability of P < 0.05 was accepted as being statistically significant, but tendencies with P < 0.15 were noted and discussed.
Experiment 2
Animals.
Thirty-six beagles (22 females and 14 males), with weights ranging from 6.5 to 18.6 kg and ages ranging from 2 to 11 yr, were used in this study. One dog was removed during the course of the study due to a health problem unrelated to treatment. Thus, the study was completed with 35 dogs. Animal care procedures were approved by the University of Illinois Animal Care and Use Committee prior to initiation of the experiment. Dogs were individually housed in stainless steel metabolism cages at Kennelwood Inc. Cage size was approximately 0.7 x 0.9 x 0.9 m. Dogs were allowed daily exercise outside of their cages.
Dietary Treatments.
Six dry extruded diets were evaluated in this study. Ingredient composition of the dietary treatments is presented in Table 2. Diets were formulated to have similar GE values and exterior fat and exterior flavor enhancers were as similar as possible among treatments. Ingredients were identical except for the supplemental fat source and the type of rice bran added. Supplemental fat sources used were poultry fat, beef tallow, and poultry fat and soybean oil in a 50:50 ratio. Diets contained either 12% SRB or 12% DRB (as-fed basis).
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Sampling Procedures.
A sample was taken of the six dietary treatments and rice bran sources and frozen at -4°C for subsequent analyses. Diets were ground with dry ice through a 2-mm screen in a Model 4 Wiley mill in preparation for chemical analyses.
A total fecal collection occurred on d 39 through 42. Feces were scored and weighed at the time of collection. Scoring was determined as follows: 1 = hard, dry pellets; 2 = hard, formed, dry stool that remains firm and soft; 3 = soft, formed, moist stool that retains its shape; 4 = soft, unformed stool that assumes the shape of the container and is pudding-like; and 5 = watery liquid that can be poured. Feces were frozen at -4°C and then composited by dog at the end of each collection. All samples were dried at 57°C and ground to pass a 2-mm screen in a Model 4 Wiley mill.
Blood samples were collected on d 1 and 42. Approximately 30 mL of blood was collected via jugular or radial venipuncture and drawn into the appropriate Vacutainer tubes. Blood was drawn into Vacutainer EDTA tubes for fatty acid profiles and PG analysis, and into vacutainer serum separator tubes for leukotriene analyses. In addition, 9 µL of indomethacin (0.014 kg/L) was added immediately to blood for PG analyses to inhibit cyclooxygenase activity. Tubes were inverted after blood was collected. Vacutainer EDTA tubes were placed on ice, whereas serum tubes were kept at room temperature for transportation back to the laboratory. All tubes were centrifuged at 1,240 x g at room temperature for 10 min. Supernatant was removed from both PG and leukotriene tubes and stored in liquid nitrogen until further analyses could be conducted. Plasma and red blood cells were removed for fatty acid analyses and stored in liquid nitrogen until further analyses could be completed.
Chemical Analyses.
Rice bran, feed, and fecal samples were analyzed for DM and OM according to AOAC (1985). Crude protein was determined from Leco nitrogen values (AOAC, 1995). Fat content was determined by acid hydrolysis (AACC, 1983) followed by ether extraction according to Budde (1952). Gross enery was determined by use of a bomb calorimeter (Parr Instrument Co., Moline, IL; Parr Instrument Manuals). Rice bran and diet samples were further analyzed for long-chain fatty acids (Le Page and Roy, 1986) and total, soluble, and insoluble dietary fiber (Prosky et al., 1985).
Lipids were extracted from DRB, SRB, diet, plasma, and red blood cells as described by Campbell et al. (1997). Phospholipids were extracted from plasma and red blood cells according to Campbell et al. (1997). Fatty acids and phospholipids were analyzed by gas chromatography. An internal fatty acid (C25:0) was added to the samples prior to extraction to verify the procedure. Fatty acids were identified by comparing retention times to known standards.
Leukotrienes (LTB4 and LTB5) were isolated from serum and analyzed by HPLC according to Campbell et al. (1997). Prostaglandins were isolated from plasma as described by Campbell et al. (1997). The isolated PG were stored in liquid nitrogen until analyses could be completed. Prostaglandin E2, thromboxanes B2, and 6-keto-PGF1
concentrations were determined by ELISA using EIA kits 514010, 519031, and 515211, respectively (Cayman Chemical Co., Ann Arbor, MI).
Statistical Analysis.
Data were analyzed by ANOVA using the GLM procedure of SAS. The experimental design was a randomized complete block with a 2 x 3 factorial arrangement of treatments. The blood fatty acid profile, phospholipid, PG, and leukotriene data were analyzed as differences from baseline values (d 1 vs. d 42). A probability of P < 0.05 was accepted as statistically significant, although mean differences with P-values between 0.06 and 0.15 were accepted as a tendency and results were discussed accordingly. A power test (ß = 0.20) indicated that between 10 and 40 observations should have allowed detection of relevant differences of nutrient digestibility, blood fatty acid profiles, and PG concentrations among means at a level of P < 0.05. However, variation was unexpectedly high, so the P-value limit for a possible tendency was relaxed to 0.15. Reporting responses in the range of 0.06 > P < 0.15 allows us to discuss results deemed biologically important. When a significant effect or tendency was detected in fat source or the interaction of fat source and rice bran source, differences were determined by Tukey’s mean separation test. Overall model P-values are presented in tables, and Tukey’s mean separation P-values are presented in the text and table footnotes when a significant effect due to fat source or the interaction were noted.
Results and Discussion
Experiment 1
Chemical Composition.
Stabilized rice bran used in this experiment contained (DM basis) 95.4% DM, 90.1% OM, 15.3% CP, 21.0% acid-hydrolyzed fat, and 5.37 kcal/g GE. Chemical composition of the dietary treatments is reported in Table 1
. Diets had similar concentrations of DM, OM, CP, fat, and GE, which was as planned because the only variation among diets was the fat source and the form of rice bran added.
Palatability Data.
Palatability results are presented in Table 3. In both tests, dogs consumed more of the diet that contained SRB than DRB. In Test 1, dogs consumed more (P < 0.001) than twice as much of the diet containing SRB than DRB. Dogs consumed approximately 30% more of the diet containing SRB than DRB in Test 2.
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). In both of our tests, dogs consumed the diet that contained SRB first more often than was the case for the diet containing DRB. The percentage of dogs that consumed the SRB-containing diet first was approximately 79 and 68% in Tests 1 and 2, respectively. Regarding consumption data, first choice and first consumption data are not the best indicators of palatability because they are subjective measurements. Making an objective, valid measurement is difficult, and the repeatability of these measures is questionable (Griffin, 1996).
The IR for SRB-containing diet in palatability Test 1 was 0.73 (P < 0.01) and 0.61 (P < 0.14) for palatability Test 2. An IR greater than 0.5 implies a preference for a particular diet. The IR for the SRB-containing diet was statistically greater (P < 0.01) than 0.5 for Test 1, whereas the calculated IR for the SRB-containing diet tended to be greater (P < 0.14) than 0.5 for Test 2. This indicates that the calculated IR was reflective of a preference for the SRB-containing diet when the supplemented fat source was poultry fat. The calculated IR for the SRB-containing diet was not significantly different from 0.5 when soybean oil was the supplemented fat source, indicating a weak tendency. Intake ratios are the best indicators of overall palatability preference (Trivedi et al., 2000). Based on these IR, and in conjunction with the other measures discussed above, it can be concluded that SRB was preferred over DRB.
Results of both palatability tests indicate that SRB is highly palatable when included in a poultry by-product meal–brewer’s rice-based dry dog diet. Palatability results can be influenced by acquired taste and the nature of the whole food product (Trivedi et al., 2000). Animals accustomed to a particular flavor or ingredient will have a preference for a diet that contains similar flavors and ingredients. Mouth feel, shape, and size of the diet may also affect preference (Trivedi et al., 2000). Diets in these tests had identical kibble shape and size. Although the complete diets were similar in chemical composition, the DRB and SRB differed in CP, acid-hydrolyzed fat, and the insoluble portion of TDF. Addition of SRB vs. DRB perhaps resulted in a product with a different mouth feel due to the unique chemical composition of SRB compared with DRB. Diets were formulated to be isocaloric, so the diet that contained SRB had a lower concentration of the main fat source (poultry fat or soybean oil) than did the DRB-containing diet. Perhaps the chemical composition of SRB resulted in a mouth feel that was more palatable to the dog. Stabilized rice bran has a tasteless flavor with a nutty overtone (Saunders, 1990) as evaluated by humans, so the flavor of SRB is unlikely to have resulted in the strong preference shown in these tests. It is more likely that the mouth feel caused by SRB resulted in the strong preference in favor of SRB-containing diet.
Experiment 2
Chemical Composition.
Chemical composition of the SRB and DRB included in the test diets is presented in Table 4. Values for DM were similar. Organic matter was 87.3% for DRB and 91.5% for SRB. Acid-hydrolyzed fat concentrations were much higher in SRB than DRB (23.3 vs. 5.5%, respectively), but DRB had a higher CP concentration than SRB (20.2 vs. 15.6%, respectively). This was expected as SRB is processed so that the lipid will remain in the bran layer. Total dietary fiber was 21.4% for SRB and 28.8% for DRB. Both SRB and DRB were high in insoluble fiber, consistent with previous literature (Slavin and Lampe, 1992). Due to its higher fat content, SRB had a higher GE content than DRB (5.44 vs. 4.26 kcal/g).
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), present in almost every animal and vegetable fat. Linoleic acid is the major fatty acid of plant leaves, roots, and stems, and serves as the initial precursor for the synthesis of longer-chain n-6 PUFA, such as arachidonic acid (Ziboh and Miller, 1990). The longer-chain n-6 PUFA are precursors of the proinflammatory eicosanoids. Oleic acid is the most widely distributed of all fatty acids (Gunstone, 1986), serving as the precursor of other n-9 fatty acids. Both rice bran sources contained higher amounts of monounsaturated fatty acids (MUFA) than SFA or PUFA. Concentrations of n-6 and n-3 fatty acids were substantially higher in SRB than DRB. The ratio of n-6:n-3 in SRB was higher than that in DRB (30.8:1 vs. 23.0:1).
Chemical analyses of diets fed in Exp. 2 are presented in Table 2. Dietary treatments contained similar concentrations of all components measured. This was as planned since the only variation among diet was the fat source and the type of rice bran added.
The fatty acid composition of the experimental diets is presented in Table 5. Within each fat source, the DRB- and SRB-containing diet had similar concentrations of fatty acids. Concentrations of oleic acid (18:1n-9) were highest in all dietary treatments. Palmitic acid (16:0) was the most common SFA in all diets. -Linolenic acid (18:3 n-3) was the n-3 fatty acid present in the highest concentrations in all diets. This fatty acid serves as the precursor for long chain n-3 PUFA. Linoleic acid (18:2n-6) was the most common n-6 fatty acid in all diets. Although the poultry fat and poultry fat:soybean oil diet had similar fatty acid concentrations, the beef tallow diet contained greater amounts of SFA than did the poultry fat and poultry fat:soybean oil diet. This was expected as beef tallow itself contains high concentrations of SFA. The beef tallow-containing diet contained higher concentrations of MUFA. The beef tallow diet that contained DRB had higher concentrations of SFA, whereas inclusion of SRB in the beef tallow diet resulted in a higher concentration of MUFA. Polyunsaturated fatty acid concentrations were variable, ranging from 12.29% for the beef tallow:DRB diet to 37.01% for the poultry fat:soybean oil diet with SRB. Total n-3 and n-6 concentrations were highest in the poultry fat:soybean oil diet and lowest in the beef tallow diet. The ratio of dietary n-6:n-3 varied, but was similar among fat sources. Ratios ranged from 10.3:1 n-6:n-3 in the poultry fat:soybean oil diet with DRB to 18.1:1 for the poultry fat diet with SRB. Within each fat source, the ratio of n-6:n-3 was lower in diet containing DRB. Addition of DRB and SRB to the diet at the 12% level did not alter the fatty acid composition of the diet enough to affect the ratio of n-6:n-3.
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. Intakes were determined during the fecal collection period. No significant differences or tendencies were observed in intakes of DM, OM, CP, fat, or GE. The average ME requirement for the dogs used in this study was 512 kcal ME/d (range 347 to 702 kcal ME/d). Intakes were sufficient to maintain the dogs at maintenance or a positive state of growth.
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. No differences were observed in DM, OM, CP, fat, or GE digestibilities. Dogs fed the poultry fat:soybean oil diet tended to have lower CP digestibilities than those fed the beef tallow diet (P < 0.124). Crude protein total-tract digestibilities are not entirely reflective of an animal’s ability to digest a dietary protein source. Endogenous proteins, such as spent enzymes, sloughed intestinal mucosa, and bacteria, are present in feces along with indigestible dietary protein. When sloughed intestinal mucosa and spent enzymes are present, bacteria constitute the majority of endogenous protein loss (Sauer et al., 2000).
Fecal Characteristics.
Fecal characteristics of dogs are presented in Table 7. No differences were observed in fecal characteristics. Fecal outputs (DM basis) ranged from 31 g/d for dogs consuming the poultry fat-DRB diet to 49 g/d for dogs consuming the poultry fat:soybean oil–DRB diet. These values are similar to average fecal DM output values for dogs fed other dry extruded diets (e.g., 45 g/d; Murray et al., 1997). When fecal output (as-is basis) was expressed per gram of DMI, outputs were similar, ranging from 0.53 (poultry fat:soybean oil with SRB) to 0.60 (beef tallow with DRB). Fecal scores ranged from 2.5 for dogs consuming the beef tallow–DRB diet to 2.8 for dogs consuming the poultry fat–SRB diet. Dogs on this experiment had near ideal fecal scores, and values were not affected by dietary treatment.
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, 9, and 10). Fatty acids and phospholipids often were similarly affected by rice bran source and fat source. Changes in the plasma fatty acid profile are largely indicative of changes in phospholipids because circulating plasma fatty acids are incorporated into membrane phospholipids. Phospholipids have two fatty acids attached to a carbon chain. Although these fatty acids are typically an unsaturated fatty acid and a SFA, this arrangement can be altered by diet (Berdanier, 2000).
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, 9, and 10). Total concentrations of MUFA were affected by rice bran source. Dogs consuming the DRB-containing diet tended to have lower MUFA in plasma fatty acids (P < 0.119). Dogs consuming the DRB-containing diet also had lower MUFA in plasma phospholipids (P < 0.050).
Monounsaturated fatty acids are important for membrane structure and synthesis of complex lipids, but they do not serve as eicosanoid precursors. The n-5, n-7, and n-9 MUFA are desaturated and elongated using the same enzymes that act on n-3 and n-6 PUFA to yield dead-end, long-chain PUFA, such as eicosatrienoic acid. Whereas eicosatrienoic acid can be incorporated into phospholipids similarly to arachidonic acid and eicosapentanoic acid, it is not a substrate for eicosanoid production (Nelson, 2000).
Total concentrations of PUFA, n-3 PUFA, and n-6 PUFA were not affected by rice bran source. Individual n-3 PUFA in plasma, plasma phospholipids, and red blood cell phospholipids were not affected by rice bran source. Although individual n-6 PUFA in plasma were not affected by rice bran source, differences in n-6 PUFA were noted in plasma phospholipid and red blood cell phospholipid fractions. Dogs consuming a DRB-containing diet had higher concentrations of red blood cell phospholipid 20:2n-6 (P < 0.012) and they tended to have higher plasma phospholipid 20:4n-6 (P < 0.145). Concentrations of red blood cell phospholipid 18:2n-6 and 20:3n-6 tended to decrease (P < 0.124 and P < 0.149, respectively) in dogs consuming a DRB-containing diet.
Altering concentrations of n-6 PUFA will affect eicosanoid production. The n-6 PUFA are desaturated and elongated to arachidonic acid, which then is converted to eicosanoids during an inflammatory response. When concentrations of individual n-6 PUFA are lower, other fatty acids that compete with n-6 PUFA will be used for both the desaturation-elongation and eicosanoid pathways. When n-6 PUFA concentrations are increased, other PUFA are less likely to be incorporated into membrane phospholipids and utilized for eicosanoid production. Therefore, increased n-6 PUFA concentrations are undesirable when trying to decrease an inflammatory response. The individual n-6 PUFA affected by rice bran source are important for eicosanoid production.
Stabilized rice bran included in the diet contained higher concentrations than DRB of every fatty acid measured. Although the difference in fatty acid composition between rice bran sources may explain why dogs consuming a DRB-containing diet had decreases in some fatty acids, it does not explain why other fatty acids increased in dogs consuming a DRB-containing diet. Even though the fatty acid profiles of DRB and SRB are different, concentrations of dietary fatty acids are similar among fat sources. The differences in rice bran fatty acid profiles were largely negated by the supplemental dietary fat sources used.
Changes in plasma fatty acids, plasma phospholipids, and red blood cell phospholipids due to rice bran source cannot be explained by examining the rice bran fatty acid profile alone. The effect of rice bran source on plasma phospholipids is difficult to explain due to the similar fatty acid contents of dietary treatments within fat source. Based on the fatty acid analysis of the diets (Table 4), addition of the rice bran sources at the 12% level was perhaps not high enough to alter the fatty acid composition of the diets. It seems that alterations in the fatty acid profile due to rice bran source are more likely due to de novo synthesis from non-lipid precursors or alteration of other dietary fatty acids. Although de novo synthesis of fatty acids from carbohydrate results in high concentrations of SFA and MUFA, it is inhibited by dietary fatty acids. Dietary fatty acids are absorbed and transported to the liver, where they are converted to acyl CoA derivatives. They are further utilized for other metabolic processes, such as conversion to other fatty acids, and they inhibit acetyl CoA carboxylase and fatty acid synthestase, enzymes involved in fatty acid synthesis (Nelson, 2000). Because the unique fatty acid profiles of DRB and SRB were masked as a result of dietary formulation, differences noted due to rice bran source are probably due to desaturation or elongation of other fatty acids included in the diet.
Fat Source Effects.
Saturated fatty acid concentrations in plasma fatty acids and plasma phospholipids were not affected by fat source (Tables 8, 9, and 10). Total concentrations of red blood cell phospholipid SFA tended to be greater (P < 0.128) in dogs consuming a beef tallow-containing diet when compared with a poultry fat-containing diet. Although individual plasma SFA were not affected, plasma phospholipid SFA were affected by fat source. Dogs receiving a poultry fat:soybean oil-containing diet tended to have lower (P < 0.084) plasma phospholipid concentrations of 16:0 as compared to dogs receiving the beef tallow-containing diet.
Changes in phospholipid profiles due to fat source can be explained by examining the fatty acid profile of the dietary treatments. Total concentrations of SFA were highest in beef tallow-containing diet, which explains the increase in SFA red blood cell phospholipids in dogs consuming these diets. Concentrations of 16:0 were lower in dogs consuming the poultry fat:soybean oil-containing diet. When compared to other diets, concentrations of 16:0 were found in the lowest concentrations in this dietary treatment.
By decreasing the concentration of SFA, other fatty acids will be incorporated into the phospholipids and affect membrane fluidity. When more SFA are incorporated into phospholipids, membranes are less fluid due to attraction between the long linear hydrocarbon tails of SFA (deMan, 2000). Hydrogen bonding between the hydrocarbon tails causes the cell membrane to be more rigid. By decreasing SFA in plasma phospholipids, other fatty acids become incorporated. Palmitic acid (16:0) is the most widely occurring SFA in animals.
Plasma fatty acid, plasma phospholipid, and red blood cell phospholipid concentrations of MUFA were not affected by fat source.
Total concentrations of red blood cell phospholipid PUFA and n-6 PUFA tended to be greater (P < 0.097 and P < 0.083, respectively) in dogs consuming a poultry fat-containing diet when compared with those consuming a beef tallow-containing diet. Dietary concentrations of total PUFA and n-6 PUFA were markedly lower in the beef tallow-containing diet than in poultry fat-containing diet.
Differences were detected in individual n-6 PUFA. Plasma fatty acid concentrations of 20:3 n-6 were greater (P < 0.039) in dogs consuming a beef tallow-containing diet compared with dogs consuming a poultry fat:soybean oil-containing diet. When compared with dogs consuming a poultry fat diet and those consuming a beef tallow diet, dogs consuming a poultry fat:soybean oil diet had higher (P < 0.012, P < 0.001, respectively) plasma fatty acid concentrations of 18:2n-6. Concentrations of red blood cell phospholipid 18:2n-6 were greater (P < 0.035) in dogs consuming a poultry fat:soybean oil-containing diet compared to dogs consuming a beef tallow-containing diet. Compared with dogs consuming poultry fat, dogs consuming the beef tallow-containing diet had lower (P < 0.022) concentrations of plasma phospholipid 20:2n-6, whereas dogs consuming poultry fat:soybean oil diets had higher (P < 0.001) concentrations of this plasma phospholipid. Dogs consuming beef tallow diets had lower (P < 0.001) concentrations of plasma phospholipid 20:2n-6 compared with dogs consuming poultry fat:soybean oil diets. Concentrations of red blood cell phospholipid 20:2n-6 also were lower (P < 0.015 and P < 0.002, respectively) in dogs consuming a beef tallow-containing diet when compared with dogs consuming a poultry fat- and poultry fat:soybean oil-containing diet. Dogs consuming a beef tallow-containing diet had greater (P < 0.002 and P < 0.039, respectively) plasma phospholipid concentrations of 20:3n-6 compared with dogs consuming the poultry fat- and poultry fat:soybean oil- containing diet. When compared with dogs consuming the beef tallow- and poultry fat:soybean oil-containing diet, concentrations of plasma phospholipid 22:4n-6 were greater (P < 0.078 and P < 0.032, respectively) in dogs consuming the poultry fat-containing diet. Concentrations of red blood cell phospholipid 22:4n-6 tended to be greater (P < 0.091 and P < 0.107, respectively) in dogs consuming a poultry fat-containing diet when compared with dogs consuming the beef tallow- and poultry fat:soybean oil-containing diet.
The change in individual n-6 PUFA in plasma and phospholipids due to fat source can be explained largely by examining the fatty acid profile of dietary treatments. Beef tallow-containing diet had the lowest concentration of 18:2n-6. Concentrations of 20:2n-6 were highest in diets that contained poultry fat and lowest in the beef tallow-containing diet. Poultry fat diet also contained the highest concentration of 22:4n-6. Although dogs consuming the beef tallow-containing diet had greater 20:3n-6, beef tallow did not have the highest concentration of 20:3n-6.
A decrease in total PUFA concentrations indicates that other fatty acids have been incorporated into phospholipids. In the case of dogs consuming a beef tallow-containing diet, more SFA probably were incorporated because of the high SFA content of beef tallow. Polyunsaturated fatty acids are also important precursors for eicosanoids. If PUFA concentrations are lower, there is less substrate available for eicosanoid production.
Although total n-3 PUFA concentrations were not affected by fat source, tendencies were detected in individual plasma phospholipid n-3 PUFA. Dogs consuming a beef tallow-containing diet tended to have lower (P < 0.099) plasma phospholipid concentrations of 18:3n-3 compared with dogs consuming a poultry fat:soybean oil-containing diet. Red blood cell phospholipid concentrations of 18:3n-3 were lower (P < 0.058 and P < 0.001, respectively) in dogs consuming the poultry fat- and beef tallow-containing diet compared with those consuming the poultry fat:soybean oil-containing diet. Dogs consuming poultry fat-containing diets tended to have higher (P < 0.010) concentrations of red blood cell phospholipid 18:3n-3 compared with dogs consuming beef tallow-containing diets. Dogs consuming a beef tallow-containing diet had greater (P < 0.044) plasma phospholipid concentrations of 20:5n-3 compared with dogs receiving the poultry fat:soybean oil-containing diet. Dogs consuming the poultry fat-containing diet tended to have lower (P < 0.088) concentrations of plasma phospholipid 22:6n-3 compared to dogs consuming the beef tallow-containing diet. Dietary concentrations of 18:3n-3 were lowest in beef tallow-containing diet, whereas 20:5n-3 levels were lowest in the poultry fat-containing diet. Dietary concentrations of 22:6n-3 were highest in the beef tallow-containing diets. A decrease in n-3 fatty acids will affect the type of eicosanoid synthesized during an inflammatory response. Specifically, 18:3n-3 is desaturated and elongated to 20:5n-3, which competes with arachidonic acid for eicosanoid-producing enzymes. Decreasing the concentrations of these n-3 PUFA could lead to more inflammatory eicosanoid production.
Diet can alter the fatty acid content of phospholipids. The kind of dietary lipid and the amount of fatty acid included in the diet can affect the type of fatty acids available for incorporation into phospholipids (Berdanier, 2000).
Rice Bran Source x Fat Source Effects.
There was no interaction of rice bran source and fat source for total concentrations of SFA in plasma, plasma phospholipids, or red blood cell phospholipids (Tables 8, 9, and 10). There was no interaction of rice bran source and fat source for total concentrations of MUFA in plasma, plasma phospholipids, and red blood cell phospholipids. There was no interaction of rice bran source and fat source for total concentrations of PUFA, n-3 PUFA, and n-6 PUFA in plasma, plasma phospholipids, and red blood cell phospholipids. There was no interaction effect in individual n-6 PUFA for any of the criteria measured. There was no interaction of rice bran source and fat source for individual plasma fatty acid and plasma phospholipid n-3 PUFA. Compared with dogs consuming poultry fat:soybean oil-DRB diets, red blood cell phospholipid concentrations of 18:3n-3 (-linolenic acid) were lower (P < 0.005, P < 0.001, P < 0.011, respectively) in dogs consuming poultry fat-DRB, beef tallow-DRB, and beef tallow-SRB-containing diets. Dogs consuming the beef tallow-SRB diet had decreased (P < 0.024, P < 0.123, respectively) concentrations of red blood cell phospholipid 18:3n-3 compared with dogs consuming the poultry fat-SRB and poultry fat:soybean oil-DRB diets. The beef tallow-DRB diet had the lowest concentration (0.68%) of 18:3 n-3, whereas the poultry fat:soybean oil-DRB diet and poultry fat:soybean oil SRB diet had the highest concentrations (3.25 and 2.97%, respectively). A decreased intake of -linolenic acid would cause less -linolenic acid to be incorporated into membrane phospholipids, whereas an increased intake could result in a higher incorporation of this fatty acid into red blood cell phospholipids. Decreasing concentrations of this fatty acid will affect the type of eicosanoid produced during an inflammatory response.
Eicosanoid Production.
No differences or tendencies were detected in concentrations of leukotriene B4 (data not shown). Leukotriene B4 values ranged from 85.05 to 126.00 ng/mL. Leukotriene B5 concentrations were below the detectable range of the HPLC. Leukotriene concentrations were highly variable. Leukotriene concentrations in healthy dogs are minute and, to increase their concentration, animals must be injected with lipopolysaccharide, an acute immunostimulatory compound produced by gram-negative bacteria (Vaughn et al., 1994). The lack of response in leukotrienes to dietary SRB supplementation is due to its concentration of n-3 and n-6 phospholipids. Stabilized rice bran does not contain n-3 fatty acids in high enough concentrations to alter phospholipid concentrations and, therefore, eicosanoid production. Because the phospholipids were not altered by inclusion of SRB in the diet, the resultant eicosanoids would not be affected.
No differences or tendencies were detected in levels of PGE2, thromboxanes B2, or 6-keto-PGF1 (data not shown). This result is to be expected because phospholipid n-6 and n-3 levels were not altered by addition of SRB to the diet. Prostaglandin E2 concentrations ranged from 0.54 to 0.59 ng/mL, TXB2 levels ranged from 0.41 to 0.46 ng/mL, and 6-keto PGF1 levels ranged from 0.12 to 0.14 ng/mL. Stabilized rice bran does not contain n-3 fatty acids in high enough concentrations to alter phospholipid concentrations and, therefore, eicosanoid production. The ratio of n-6:n-3 fatty acids was not within the 10:1 range shown to affect prostaglandin production (Vaughn et al., 1994).
Implications
The addition of stabilized rice bran to a dry dog food at the 12% concentration (as-fed basis) was shown to be more palatable than defatted rice bran. Including stabilized rice bran in a diet with varying fat sources did not have a negative effect on nutrient digestibilities or fecal characteristics. Plasma fatty acid and phospholipid alterations were not large enough to elicit an effect on eicosanoid production in healthy dogs. Addition of stabilized rice bran to a high-quality dry food did not result in pronounced changes in the fatty acid profile of healthy dogs. More pronounced results might have been noted if dogs were acclimated to a diet with a known fatty acid composition for a period of time before the experiment began. Stabilized rice bran is a palatable ingredient when included in a high-quality, dry dog diet, even though it did not elicit an effect on inflammatory immune mediators in healthy dogs.
1 Correspondence: 132 Animal Sciences Laboratory, 1207 W. Gregory Dr. (phone: 217-333-2361; fax: 217 244-3169; e-mail: gcfahey{at}uiuc.edu).
Received for publication December 18, 2002. Accepted for publication December 16, 2003.
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