Nutrient Requirements
and Interactions
Taurine Balance is Different in Cats Fed Purified and Commercial Diets MARY A. HICKMAN, QŒNTON R. ROGERS ANDJAMES G. MORRIS Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616 generation and dilated cardiomyopathy (2, 3). The National Research Council's taurine requirement was based on studies of kittens fed purified diets and did not account for nutrient interactions or processing effects. Attempts to determine taurine requirements by traditional balance studies, which have measured fecal and urinary taurine excretion in cats fed purified and commercial diets, have failed to account for ap proximately 30-50% of ingested taurine (4). These balance discrepancies could be the result of taurine degradation by anaerobic bacteria in the lower in testine (5, 6). A recent study (7) with cats fed com mercial and purified diets showed that substantial amounts of ingested 14C-labeled taurine may be con verted to 14CC>2and that the quantity recovered in 14CC>2depends on the type of diet. Taurine balance studies that depend on the collection and analysis of feces to assess intestinal taurine loss are therefore inappropriate. The purpose of this study was to determine taurine balance and intestinal taurine loss in cats fed com mercial and purified diets by measuring taurine in ileal digesta instead of fèces.A technique was deve loped for permanent ileal cannulation of cats, to allow daily sampling of ileal digesta. By collecting samples from the terminal ileum, where bacterial numbers are significantly lower than in the colon (107/g digesta vs. 1010-10n/g digesta) (8), it was anticipated that intes tinal degradation of taurine by microorganisms would be substantially reduced. This technique has been used previously in other species to determine the digestibility of many dietary constituents, including protein, amino acids, starch and lignin (9, 10). The response to four diets was evaluated. Two of the diets were derived from a single commercial formulation processed by heat or preserved by freezing. This diet when heat-processed was previously shown to not maintain adequate plasma taurine concentration when fed to cats, although the same diet preserved without heat maintained adequate plasma taurine concentration (2, 7). Additionally, significantly greater amounts of ingested 14C-labeled taurine were converted to 14CU2 in cats fed the heat-processed
ABSTRACT Deal fluxes, urinary losses and taurine balance were determined in six taurine-replèteand four taurine-depleted cats. Digesta samples collected at the terminal ileum were used to assess ileal flux of taurine. Four diets were tested: a commercial diet in two forms (heat-processed and frozen) and two purified diets con taining either 1225 or 0 mg taurine/kg diet. Five-day balance trials were performed on d 3-7 with measurement of food intake and taurine in urine and ileal digesta. Substantially greater quantities of total taurine (free + bound) were found in ileal digesta from cats fed the heat-processed rather than the frozen preserved diet (205 vs. 101% of the average daily taurine intake, re spectively), with calculated taurine balances of -609 vs. -212 |amol/d, respectively. The quantity of taurine in ileal digesta from taurine-replètecats fed the 1225 or 0 mg taurine/kg purified diets was not significantly dif ferent, indicating that taurine found at the terminal ileum is mostly of endogenous origin. Taurine-depleted cats had significantly lower amounts of taurine in ¡leal digesta, with a taurine balance of -77 jjmol/d. These results demonstrate that a heat-processed diet causes substantially greater losses of taurine from the intestine than does a frozen diet. This phenomenon may explain the inability of some heat-processed diets to maintain normal plasma taurine concentrations in cats. J. Nutr. 122: 553-559, 1992. INDEXING KEY WORDS: •taurine •taurine balance •Ileal digesta taurine •heat processing •cats
Taurine is an established essential nutrient for the cat. However, the minimal dietary intake that will maintain consistent plasma and tissue taurine con centrations in cats fed all types of diets has not been clearly defined. Adult cats fed some commercial diets containing 1500 mg taurine/kg, a concentration threeto fourfold that of the 1986 National Research Council (1) minimal requirement for growing kittens (400 mg/kg), develop abnormally low plasma and tissue taurine concentrations and acquire associated clinical diseases, such as feline central retinal de
0022-3166/92 $3.00 ©1992 American Institute of Nutrition. Received 13 May 1991. Accepted 16 September 1991. 553 Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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than in those fed the frozen preserved commercial diet, indicating increased bacterial taurine degra dation as a result of heat processing. Similar effects indicating a decreased taurine status have been demonstrated in cats fed three other heat-processed proprietary diets (unpublished data). Two purified diets were also used: a diet containing 1225 mg taurine/kg and one containing no taurine. The diet containing 1225 mg taurine/kg was used to measure taurine losses in cats fed a taurine adequate diet. The taurine-free diet was used to deplete cats of tissue taurine and to measure endogenous synthesis of tau rine.
MATERIALS AND METHODS Animal*. Ten male specific-pathogen-free cats (Nutrition and Pet Care Center, University of Cali fornia, Davis, CA), 18- to 21-mo old, were used. Body weights at the start of the experiment ranged from 4029 to 5387 g (mean 4594 ±149 g). Cats were housed in individual metabolism cages in a light-controlled (14 h light: 10 h dark) room at 21 ±3°C.Permanent cannulae were surgically placed in the terminal ileum of all cats 2-4 wk before the start of the ileal collec tions, using a technique similar to that described for dogs (11). The cats were anesthetized with sodium thiamylal (Bio-tal, Boehringer Ingelhelm, St. Joseph, MO) and halothane (Halocarbon Laboratories, North Augusta, SC), the peritoneal cavity was opened with a ventral midline incision and the terminal ileum iso lated. A longitudinal incision was made in the ileum -2 cm proximal to the ileocolic juncture and a plastic T-shaped cannula was inserted. The cannula was similar in design to that previously described for adult cockerels (12). The ileum was closed with two purse string sutures (4-0 Proline, Ethilon Inc., Somerville, NJ) and the cannula secured to the abdominal wall by a Teflon retaining plate. The stem of the cannula was exteriorized through a small stab incision -8 cm to the right of midline and the abdomen was closed. Cats tolerated the ileal cannulae extremely well with minimal post-surgical complications and no deaths. Cannula failures were limited to leakage of digesta around the cannula stem and in these animals the cannulae were surgically removed. Animal care throughout the study was in compliance with the Guide for the Use and Caie of Laboratory Animals developed by the Institute of Laboratory Animal Resources of the National Research Council. The ex perimental protocol was approved by the University of California at Davis, Animal Use and Care Adminis trative Advisory Committee, utilizing the guidelines provided by the American Association for Laboratory Animal Care. Diets. A commercial diet (Theracon, Topeka, KS), identical to that described previously (7), was used in Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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TABLE 1 Composition
of purified
diets
Dietary componentSoybean protein1 Starch2Sucrose
202200
Animal tallow3 10050 Mineral mix Vitamin mix4Cholinc 103 chloride TaurineAmountS/ks435 0 'Ardex F-disp., SPI Group, San Leandro, CA. 2Melojel, food grade cornstarch, National Starch and Chemical, Bridgewater, NJ. 3Rendered animal tallow, Florin Tallow Co., Dixon, CA. 4For composition see Williams et al. (13). laurine replaced an equal weight of starch in the 1225 and 10,000 mg/kg taurine diets.
two forms, heat-processed and frozen. The diet con tained 12 g/100 g crude protein, 7 g/100 g fat, 1.5 g/ 100 g crude fiber, 72 g/100 g moisture and had a measured taurine concentration of 1050 mg/kg dry wt (8400 |imol/kg). Purified diets (Table 1) with 0, 1225, or 10,000 mg taurine/kg dry matter (0, 9800 or 80,000 (¿moltaurine/kg) were used, with an equal weight of starch replacing the taurine in the 0 mg taurine/kg diet. All diets were mixed with 0.2 g/100 g chromic oxide (Cr2U3, certified, Fisher Scientific, Pittsburgh, PA) as a marker throughout the test cycles in which balance studies were performed. Food and water were freely available and food intake was measured daily. Design. Ten cats were randomly assigned to two groups. Group 1 (six cats) was fed the 1225 mg taurine/kg purified diet (1225 Tau) and Group 2 (four cats) the 0 mg taurine/kg diet (0 Tau) for 6-8 mo prior to the start of ileal collections. Group 2 cats were fed the 0 Tau diet to deplete tissue taurine. The experi mental protocol for Group 1 consisted of three con secutive test cycles (Fig. 1), with each cycle designed to test the effects of a different diet on ileal digesta taurine concentration and to determine taurine bal ance. During each of the three cycles, cats were first fed the 1225 Tau diet as a control diet for 7 d. Heal digesta and urine were collected on d 3-7 to establish control values and in cycles 2 and 3 to determine any residual effect of the previous cycle. Following the control diet, cats were fed in a randomized order one of three test diets for 7 d: the heat-processed commer cial, frozen commercial or 0 Tau diet. Heal digesta and urine were collected on d 3-7 and compared with the values obtained using the 1225 Tau control diet. For the heat-processed commercial diet only, the test period was extended to 28 d and an additional balance trial was performed on d 24—28.This balance study
ILEAL FLUXES OF TAURINE IN CATS
Period1225
DietDaysControl Tau7dExperimental
PeriodTest
Diet7d'Taurine
Repletion10
000 Tau5dTaurine
Stabilization1225
TauId
FIGURE 1 Feeding regimen followed for cats in Group 1. Each cat went through three consecutive test cycles. An expanded diagram of one of the three test cycles is shown. Test diets included the heat-processed commercial, frozen commercial and 0 Tau diets. *For heat-processed com mercial diet only, the test period was extended to 28 d and an additional balance trial was performed on d 24-28.
was used to determine the effects of short-term taurine depletion on ileal digesta taurine and taurine balance. Following each test diet, cats were fed the 10,000 mg taurine/kg purified diet for 5 d to replete tissue taurine, followed by the 1225 Tau diet for 9 d to stabilize tissue taurine pools, before the next test cycle. Feeding the 10,000 mg taurine/kg purified diet for 5 d was previously shown to replete taurinedepleted cats (unpublished data); this was verified in this study by the return of both plasma and whole blood taurine concentrations to initial baseline values. Group 2 cats were fed the 0 Tau diet throughout the experiment to determine the effects of taurine depletion on taurine balance and ileal digesta taurine concentration. A 5-d balance experiment involving collection of ileal digesta and urine was performed following 6-8 mo of depletion. Procedures. Cats were fed at 0800 h, and ileal contents collected from 1000 until 1330 h. Collec tions were made by allowing digesta to flow by gravity drainage into small latex bags containing 2 ml, of antibiotic solution (12.5 g potassium penicillin G and 2 g streptomycin sulfate/L) to inhibit bacterial metabolism (14). Chromic oxide in ileal digesta was determined by a modification of the procedure of Arthur (15). Heal digesta (~1 g) were placed in 75-mL digestion tubes with 5 mL of concentrated nitric acid and 2 mL of 11.7 mol/L perchloric acid and heated at 150°Cfor 90 min followed by 230°Cfor 180 min in a block digester. Samples were diluted and chromium concentration was then determined by atomic absorption spectrophotometry (Perkin-Elmer Model 3030 B, Norwalk, CT). Taurine in all samples was determined on an amino acid analyzer (Beckman Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
555
121MB amino acid analyzer, Fullerton, CA). Free taurine concentration was measured in a water ex tract of the ileal digesta made by mixing 3 g of digesta with 15 mL of distilled water. Total taurine in the water extract was determined after acid hydrolysis, as previously described (7).Bound taurine was calculated by subtracting free taurine from total taurine. Plasma and whole blood taurine concentrations, and free and bound taurine in urine samples preserved with 3 mol/ L sulfuric acid, were determined as previously described (7). Because several compounds present in urine elute close to taurine during amino acid analysis, urine taurine concentrations were also deter mined by HPLC using the method of Hirschberger et al. (16). Prior to HPLC, decontamination of urine samples was performed with ion-exchange columns using the method of Larsen et al. (17). Similar taurine concentrations were obtained using amino acid analysis or HPLC determination (for the four diet groups analyzed, the mean difference was 0.047 ± 0.009 (irnol/L). All taurine concentrations reported were determined by amino acid analysis. Statistical methods. Results are expressed as means ±SEM. Food intake and urinary and ileal taurine from cats fed the four diets were analyzed by SAS ANOVA and least significant difference test (18). Values were logarithmically transformed to achieve homogeneity of variances and then analyzed using a two-way ANOVA. Analysis of covariance using food intake as a covariant with diet showed a linear rela tionship with the dependent variables free, bound and total ileal taurine when expressed as micromoles per day (regression coefficient of £0.9).Accordingly, these variables were divided by food intake to normalize all values prior to ANOVA. Because no significant effects of diet order were found, results from the three control periods when cats were fed the 1225 Tau diet were averaged and treated as one diet period. A Stu dent's t test was used to determine whether calcu lated taurine balance was significantly different from zero.
RESULTS Substantial quantities of both free and bound taurine were found in the terminal ileal digesta of cats fed all diets (Table 2). Highest amounts of taurine were found in digesta of cats fed the heat-processed commercial diet, being equivalent to about twice the average daily intake of taurine on d 3-7. On d 24-28 ileal taurine decreased to about 1.5 times dietary taurine, which was the result of a lower proportion of bound taurine in ileal digesta. Total taurine was sig nificantly lower in digesta of cats when fed the frozen preserved rather than the heat-processed diet on d 3-7 (10.6 vs. 17.3 jimol-d-^g diet"1) and was equivalent to the daily taurine intake. Free and bound taurine made
556
HICKMAN
ET AL.
TABLE 2 diets1DietnFreeHeal
Taurin« in Heal digesta in cats fed fonr different
taurineBoundTotal\unold-1-g
taurineBoundTotal% diet'1Commercial processed^Days (heat 3-7Days 24-28Commercial (frozen)2Purified Tau)1Purified (1225
dietary taurine intakeFreeHeal
11"99 ± 11"43 ± 6b32± ± 5b109
19a51 ± 10b58 ± 21b40 ± ±llb205
Tau)Group (0 I2Group 2346466496
29a149 ± 14"101 ± 18b72 ± ±16C8.1
0.9a10.5 ± ±1.1*4.5 0.7b3.5 ± O.S1*02.5 ±
1.6a5.3 ± 1.1a6.1 ± 2.2a154.3 ± 1.2b4.4 ±
0.5°0.5 ± 0.7*0.3 ± ±O.ld9.2 ±O.lc17.3
2.4"15.8 ± 1.5a10.6 ± 1.9b7.8 ± 1.6C6.9 ± 1.0°0.8 ± ±0.2d
'Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different [P < 0.05) as determined by ANOVA and least significant difference test, taurine-replète cats. 3Taurine-depleted cats.
approximately equal contributions to total ileal taurine of the cats fed the frozen preserved diet. Taurine excretion in the urine of cats fed the four diets is shown in Table 3. Total (free + bound) ex cretion for d 3-7 was not significantly different when Group 1 cats were fed the commercial heat-processed, frozen or purified diets. Excretion was markedly lower on d 24—28than on d 3-7 in cats fed the heatprocessed diet and in taurine-depleted cats, indicating active conservation of taurine by the kidney. Bound taurine accounted for a substantial portion of total urine taurine in all dietary groups. Intestinal taurine fluxes calculated from ileal taurine are shown in Table 4, together with food intake, dietary taurine intake, urine taurine loss and taurine balance. Heal taurine flux was calculated by multiplying micromoles of taurine per milligram of Cr2U3 in ileal digesta by milligrams of CriOa ingested per day. Taurine balance was calculated in micromoles per day as: dietary taurine intake - (ileal taurine flux + urine taurine loss). This balance as sumes that none of the taurine passing the terminal ileum is absorbed in the colon and does not include endogenous synthesis by the cat. The calculated taurine balance for cats when fed the heat-processed diet compared with frozen preserved commercial diet on d 3-7 was -609 vs. -212 nmol/d. Taurine balance was significantly different from zero in cats fed the heat-processed commercial or 0 Tau purified diets, but not in cats fed the commercial frozen or 1225 Tau purified diets. Flux of taurine at the ileum was significantly less when cats were fed the purified rather than com mercial diets. Because there was no dietary taurine intake when cats were fed the 0 Tau diet, results from this period are only expressed as micromoles per day per gram of diet. Values shown for cats fed the 1225 Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
Tau diets are averages of the three control periods, because no significant effects of diet order were found. Similar amounts of taurine were present in ileal chyme from Group 1 cats when they were fed either the 1225 Tau or the 0 Tau diets. This suggests that the majority of the taurine appearing at the terminal ileum is of endogenous origin. Using ileal taurine flux from Group 1 cats when fed the 0 Tau diet, endogenous intestinal taurine loss in cats previ ously fed a 1225 mg taurine/kg diet approximates 440 umol/d. Calculated taurine balance in Group 1 cats ranged from about zero when fed the 1225 Tau diet (-15 (imol/d) to a very negative balance when fed the 0 Tau diet (-574.7 jimol/d). A negative balance would be expected for cats fed a diet containing no taurine. Results from the taurine-depleted cats (Group 2) fed the 0 Tau diet are shown in Tables 2-4. These cats had significantly lower amounts of taurine in ileal digesta than cats fed all other diets. Because these cats were fed no taurine for 6-8 mo before ileal samples were taken, it was assumed that their tissues were depleted to the extent that an equilibrium had been established between synthesis and loss. If it is assumed that none of the taurine passing the terminal ileum is absorbed, the sum of urinary and ileal taurine fluxes can be used to estimate endogenous taurine synthesis, which was 77 (imol/d.
DISCUSSION This study demonstrated that heat processing a diet significantly increased taurine at the terminal ileum and caused a marked negative taurine balance. These observations are consistent with our previous report that greater amounts of 14CC>2are recovered from cats given a pulse-labeled dose of 14C-labeled
557
ELEAL FLUXES OF TAURINE IN CATS
TABLE 3 Excretion of taurine in urine of cats fed four different diets1
DietCommercial processed)2Days (heat 3-7Days 24-28Commercial (frozen)2Purified Tau)2Purified (1225 Tau)Group ¡0 I2Group 23n464664Free
taurine29
taurine\unol/d105
taurine133
9a12 ± &*35 ± 9"24± 5*18 ±
23»37 ± 19b154 ± 44a101 ± 21"116 ±
24"49 ± 19b189 ± 43a125 ± 24"134 ±
3ab9 ± ±3CBound
13a1 ± ± IeTotal
15"11 ± ± 3C
Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different (P < 0.05) as determined by ANOVA and least significant difference test. •^Taurine-replète cats. 3Taurine-depleted cats.
taurine when fed a heat-processed rather than a frozen diet (7). The changes induced by heat processing could account for the inability of some heat-processed com mercial diets to maintain plasma and tissue taurine concentrations in cats. Taurine from the ileum that is not reabsorbed by the colon would be available for degradation by anaerobic bacteria. Taurine absorption from the colon has not been studied, but short-chain fatty acids such as propionate and butyrate (similar in size to taurine but carrying only a single negative charge rather than being a zwitterion), are passively absorbed from the colon (19). If some of the taurine entering the colon is absorbed, taurine balance calcu lated using ileal digesta flux would represent an overestimation of intestinal taurine loss.
The source(s) of the large amounts of free and bound taurine appearing at the terminal ileum of cats fed all diets was not determined in this study, al though the taurine seems to be mostly of endogenous origin. This conclusion was reached by comparing results from Group 1 cats fed the 1225 Tau vs. 0 Tau diets. Equal amounts of taurine were found in ileal digesta of cats fed these two diets, despite dietary taurine intakes of 577 vs. 0 (imol/d, respectively. Taurine is absorbed from the small intestine by a carrier-mediated active transport mechanism (20), and dietary taurine uptake would therefore be assumed to be quite efficient. Previous studies have shown that heat processing neither destroys nor results in covalent binding of taurine to dietary components in
TABLE 4 Food and taurine intakes and taurine losses on d 3-7 in cats fed four different diets1'2
DietCommercial (heat processed)4 Commercial (frozen!4 Purified (1225 Tau)4 Purified (0 Tau) Group I4 Group 2sn4
4 66
intake3g/d58
taurine intake485
taurine loss\unol/d969 flux
± 6a 71 ±12ab 5ab66 59 ±
± 54b 749 ±129a 57752ab0 ±
±108a 772 ±205a 462 ±110b 18a440
Urinary taurine balance-609 133 ±24a 189 ±43a 131 ±
± 91a* -212 ±141b 105b-574 -15 ±
± 98a* ±13ab ± 87b 134 ±15a 66 ± 15° 11 ± 3bTaurine -77 ± 18b* 4Food 85 ± 6bDietary 0Heal 'Balance was calculated as dietary intake - (fecal loss calculated from ileal values + urine loss). 2Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different (P < 0.05) as determined by ANOVA and least significant difference test. 'Significantly different from zero as determined by a Student's t test. 3Dry-matter basis, taurine-replète cats. Taurine-depleted cats. Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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HICKMAN
commercial diets, because it is water extractable (un published data). Therefore, it is likely that the heatprocessed diet increased endogenous taurine at the terminal ileum rather than decreased absorption of dietary taurine in the small intestine. However, the possibility of a heat-induced inhibitor of intestinal taurine transport or changes in the gut microflora cannot be excluded. The chemical composition of the bound taurine in terminal ileal digesta was not determined in this study, but may largely represent taurine-conjugated bile acids. Several other compounds are detoxified by hepatic taurine conjugation prior to biliary secretion, but would not be expected in the concentration found at the terminal ileal in this study (21). Greater con jugated bile acids at the terminal ileum from the heatprocessed diet than from the frozen diet could be due to decreased intestinal bile acid reabsorption or in creased biliary secretion. Additions of water-soluble fibers such as pectin to the diet of rats resulted in elevated secretion of bile acids by the liver, decreased hepatic taurine concentration and increased losses of bile acids in the feces (22, 23). It seems that physical properties of the water-soluble fibers cause en trapment of bile acids rather than actual binding (24), and a similar mechanism may explain the greater taurine found at the terminal ileum of cats fed heatprocessed rather than frozen commercial or purified diets. Heating the diet may cause a change in the physical properties of some componenti s) °ftne diet/ allowing either entrapment or actual bile acid binding. Bile acids bind to dietary protein such as soybean protein isolate, with greater losses of bile acids in the feces of animals fed soybean rather than casein diets (25, 26). Heat processing changes protein structure by coagulation and denaturation, which may result in a greater affinity for bile acids in the cooked protein than in unprocessed protein. In addition, heat processing may change the number, species and/or location of bacterial flora in the small intestine, causing increased deconjugation of bile acids and an increased turnover of taurine. Anaerobic bacteria play a role in the normal intestinal loss and compensatory biliary secretion of bile acids by actively deconjugating taurine and glycine from bile acids and by converting primary bile acids into secondary bile acids (27). A recent study (28) demon strated that bacterial bile acid hydrolase activity was greater in the ileum of chickens fed rye rather than com or sucrose diets. Although the mechanism may not be related, bacterial overgrowth of the small in testine has been associated with a decrease in plasma and retinal taurine concentrations in rats and humans (29). Bile acid deconjugation and modification may be involved in this phenomenon. Ileal digesta levels of taurine and taurine balance in cats were shown to be significantly affected by taurine depletion. Taurine-depleted (Group 2) cats had Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
ET AL.
substantially less free, bound and total taurine in ileal digesta, the total taurine being -12% of that found in taurine-replète (Group 1) cats. Several factors could explain this decrease in ileal taurine. Taurinedepleted cats decrease the conjugation of bile acids with taurine by approximately half (30). Therefore, less taurine would appear at the terminal ileum in the form of taurine-conjugated bile acids. Depleted cats also have substantially decreased intestinal tissue taurine concentration (30), which would lead to decreases in intestinal taurine released from sloughed mucosal cells. Calculated taurine balance in taurine-depleted cats (-77 njtnol/d) was used as an estimate of taurine syn thesis. With no dietary taurine intake and minimal taurine available from peripheral tissues due to chronic depletion, losses of taurine in the urine and feces should approximate the amount of taurine syn thesized. This is a reasonable assumption because taurine-depleted cats are unable to completely con jugate bile acids with taurine and instead secrete into bile significant amounts of free cholic acid (30). Free bile acids are not found in the bile of healthy animals (31). Taurine synthesis determined in this study would be consistent with the limited ability of cats to synthesize taurine (32) and their inability to maintain normal taurine status with no dietary taurine intake. In summary, cats fed a heat-processed commercial diet had a greater intestinal taurine loss, resulting in a more negative taurine balance, than did cats fed the same diet preserved by freezing. Until the mechanism of this effect is defined, it will be difficult to predict how individual commercial cat diets will affect taurine status without actual in vivo diet testing. Cats fed commercial diets also had greater intestinal taurine losses than did cats fed purified diets; thus, recommendations of an adequate concentration of di etary taurine based on studies that use purified diets are not valid for commercial foods.
ACKNOWLEDGMENT The authors thank Hoffman-La Roche Inc., Fresno, CA, for kindly providing the vitamin mixture used in the purified diets.
LITERATURE CITED 1. National Research Council (1986) Nutrient Requirements of Cats. National Academy Press, Washington, DC. 2. Morris, J. G., Rogers, Q. R. & Pacioretty, L. M. (1990) Taurine: an essential nutrient for the cat. J. Small Anim. Pract. 31: 502-509. 3. Pion, P. D., Kittleson, M. D., Rogers, Q. R. &. Morris, J. G. (1987) Myocardial failure in cats associated with low plasma taurine: a reversible cardiomyopathy. Science (Washington, DC) 237: 764-768.
ELEAL FLUXES OF TAURINE 4. Cooke, J. A., Rogers, Q. R. &.Morris, J. G. (1989) Urinary and fecal excretion of taurine by cats fed commercial canned diets. FASEB J. 3: A1617 (abs.). 5. Ikeda, K., Yamada, H. &. Tanaka, S. (1963) Bacterial degra dation of taurine. Biochem. J. 54: 312-316. 6. Fellman, J. H., Roth, E. S., Avedovech, N. A. &.McCarthy, K. D. (1980) The metabolism of taurine to isethionate. Arch. Biochem. Biophys. 204: 560-567. 7. Hickman, M. A., Rogers, Q. R. &.Morris, J. G. (1990)Effect of processing on fate of dietary [14C]taurine in cats. J. Nutr. 120: 995-1000. 8. Gorbach, S. L. (1971) Intestinal microflora. Gastroenterology 60: 1110-1129. 9. Haydon, K. D., Knabe, D. A & Tanksley, T. D., Jr. (1984) Effects of level of feed intake on nitrogen, amino acid and energy digestibilities measured at the end of the small in testine and over the total digestive tract of growing pigs. J. Anim. Sci. 59: 717-724. 10. Fadel, J. G., Newman, R. K., Newman, C. W. &. Graham, H. (1989) Effects of baking hulless barley on the digestibility of dietary components as measured at the ileum and in the feces in pigs. J. Nutr. 119: 722-726. 11. Rubinstein, A, Li, V.H.K., Gruber, P., Bass, P. & Robinson, J. R. (1988)Improved intestinal cannula for drug delivery studies in the dog. J. Pharmac. Methods 19: 213-217. 12. Gumsey, M. P. & Johns, D. C. (1986) An improved ileal cannula for adult cockerels. Res. Vet. Sci. 41: 283-284. 13. Williams, J. M., Morris, J. G. &.Rogers, Q. R. (1987) Phenylalanine requirement of kittens and the sparing effect of tyrosine. J. Nutr. 117: 1102-1107. 14. Windham, W. R. &.Akin, D. F. (1984)Rumen fungi and forage fiber degradation. Appi. Environ. Microbiol. 48: 473-476. 15. Arthur, D. (1970) The determination of chromium in animal feed and excreta by atomic absorption spectrophotometry. Can. J. Spectrosc. 15: 134-141. 16. Hirschberger, L. L., De LaRosa, J. & Stipanuk, M. H. (1985) Determination of cysteinesulfinate, hypotaurine and taurine in physiological samples by reversed-phase high-performance liquid Chromatography. J. Chromatogr. 343: 303-313. 17. Larsen, B. R., Grosso, D. S. & Chang, S. Y. (1980) A rapid method for taurine quantitation using high performance liquid chromatography. J. Chromatogr. Sci. 18: 233-236. 18. SAS Institute Inc. (1985) SAS User's Guide: Statistics. SAS Institute, Cary, NC. 19. McNeil, N. L, Cummings, J. H. & James, W.P.T. (1978) Short chain fatty acid absorption by the human large intestine. Gut
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19: 819-822. 20. Barnard, J. A., Thaxter, S., Kikuchi, K. & Ghisnan, F. K. (1988) Taurine transport by rat intestine. Am. J. Physiol. 254: G334-G338. 21. Killenberg, P. G. & Webster, L. T., Jr. (1980) Conjugation by peptide bond formation. In: Enzymatic Basis of Detoxication, vol. 2 (Jakoby, W. B., éd.), pp. 141-176. Academic Press, New York, NY. 22. Ide, T., Horii, M., Kawashima, K. &.Yamamoto, T. (1989)Bile acid conjugation and hepatic taurine concentration in rats fed on pectin. Br. J. Nutr. 62: 539-550. 23. Ide, T., Horii, M., Yamamoto, T. &. Kawashima, K. (1990) Contrasting effects of water-soluble and water-insoluble di etary fibers on bile acid conjugation and taurine metabolism in the rat. Lipids 25: 335-340. 24. Ebihara, K. &.Schneeman, B. O. (1989)Interaction of bile acids, phospholipids, cholesterol and triglycéride with dietary fibers in the small intestine of rats. J. Nutr. 119: 1100-1106. 25. Iwami, K., Sakakibara, K. &. Ibuki, F. (1986) Involvement of post-digestion 'hydrophobic' peptides in plasma cholesterollowering effect of dietary plant proteins. Agrie. Biol. Chem. 50: 1217-1222. 26. Choi, Y., Ikeda, I. & Sugano, M. (1989)The combined effects of dietary proteins and fish oil on cholesterol metabolism in rats of different ages. Lipids 24: 506-510. 27. Hayakawa, S. (1973) Microbiological transformation of bile acids. Adv. Lipid Res. 11: 143-192. 28. Feighner, S. D. & Dashkevicz, M. P. (1988) Effect of dietary carbohydrates on bacterial cholyltaurine hydrolase in poultry intestinal homogenates. Appi. Environ. Microbiol. 54: 337-342. 29. Sheikh, K. (1981) Taurine deficiency and retina defects asso ciated with small intestinal bacterial overgrowth. Gastroen terology 80: 1363 (abs.). 30. Rentschler, L. A., Hirschberger, L. L. &.Stipanuk, M. H. (1986) Response of the kitten to dietary taurine depletion: effects on renal reabsorption, bile acid conjugation and activities of en zymes involved in taurine synthesis. Comp. Biochem. Physiol. 84B: 319-325. 31. Elliott, W. H. (1985) Metabolism of bile acids in liver and extrahepatic tissues. In: Sterols and Bile Acids (New Compre hensive Biochemistry), vol. 12 (Danielsson, H. & Sjövall,J., eds.), pp. 303-329. Elsevier, Amsterdam, The Netherlands. 32. Hardison, W.G.M., Wood, C. A. &.Proffitt, J. H. (1977) Quanti fication of taurine synthesis in the intact rat and cat liver. Proc. Soc. Exp. Biol. Med. 155: 55-58.
and Interactions
Taurine Balance is Different in Cats Fed Purified and Commercial Diets MARY A. HICKMAN, QŒNTON R. ROGERS ANDJAMES G. MORRIS Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616 generation and dilated cardiomyopathy (2, 3). The National Research Council's taurine requirement was based on studies of kittens fed purified diets and did not account for nutrient interactions or processing effects. Attempts to determine taurine requirements by traditional balance studies, which have measured fecal and urinary taurine excretion in cats fed purified and commercial diets, have failed to account for ap proximately 30-50% of ingested taurine (4). These balance discrepancies could be the result of taurine degradation by anaerobic bacteria in the lower in testine (5, 6). A recent study (7) with cats fed com mercial and purified diets showed that substantial amounts of ingested 14C-labeled taurine may be con verted to 14CC>2and that the quantity recovered in 14CC>2depends on the type of diet. Taurine balance studies that depend on the collection and analysis of feces to assess intestinal taurine loss are therefore inappropriate. The purpose of this study was to determine taurine balance and intestinal taurine loss in cats fed com mercial and purified diets by measuring taurine in ileal digesta instead of fèces.A technique was deve loped for permanent ileal cannulation of cats, to allow daily sampling of ileal digesta. By collecting samples from the terminal ileum, where bacterial numbers are significantly lower than in the colon (107/g digesta vs. 1010-10n/g digesta) (8), it was anticipated that intes tinal degradation of taurine by microorganisms would be substantially reduced. This technique has been used previously in other species to determine the digestibility of many dietary constituents, including protein, amino acids, starch and lignin (9, 10). The response to four diets was evaluated. Two of the diets were derived from a single commercial formulation processed by heat or preserved by freezing. This diet when heat-processed was previously shown to not maintain adequate plasma taurine concentration when fed to cats, although the same diet preserved without heat maintained adequate plasma taurine concentration (2, 7). Additionally, significantly greater amounts of ingested 14C-labeled taurine were converted to 14CU2 in cats fed the heat-processed
ABSTRACT Deal fluxes, urinary losses and taurine balance were determined in six taurine-replèteand four taurine-depleted cats. Digesta samples collected at the terminal ileum were used to assess ileal flux of taurine. Four diets were tested: a commercial diet in two forms (heat-processed and frozen) and two purified diets con taining either 1225 or 0 mg taurine/kg diet. Five-day balance trials were performed on d 3-7 with measurement of food intake and taurine in urine and ileal digesta. Substantially greater quantities of total taurine (free + bound) were found in ileal digesta from cats fed the heat-processed rather than the frozen preserved diet (205 vs. 101% of the average daily taurine intake, re spectively), with calculated taurine balances of -609 vs. -212 |amol/d, respectively. The quantity of taurine in ileal digesta from taurine-replètecats fed the 1225 or 0 mg taurine/kg purified diets was not significantly dif ferent, indicating that taurine found at the terminal ileum is mostly of endogenous origin. Taurine-depleted cats had significantly lower amounts of taurine in ¡leal digesta, with a taurine balance of -77 jjmol/d. These results demonstrate that a heat-processed diet causes substantially greater losses of taurine from the intestine than does a frozen diet. This phenomenon may explain the inability of some heat-processed diets to maintain normal plasma taurine concentrations in cats. J. Nutr. 122: 553-559, 1992. INDEXING KEY WORDS: •taurine •taurine balance •Ileal digesta taurine •heat processing •cats
Taurine is an established essential nutrient for the cat. However, the minimal dietary intake that will maintain consistent plasma and tissue taurine con centrations in cats fed all types of diets has not been clearly defined. Adult cats fed some commercial diets containing 1500 mg taurine/kg, a concentration threeto fourfold that of the 1986 National Research Council (1) minimal requirement for growing kittens (400 mg/kg), develop abnormally low plasma and tissue taurine concentrations and acquire associated clinical diseases, such as feline central retinal de
0022-3166/92 $3.00 ©1992 American Institute of Nutrition. Received 13 May 1991. Accepted 16 September 1991. 553 Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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than in those fed the frozen preserved commercial diet, indicating increased bacterial taurine degra dation as a result of heat processing. Similar effects indicating a decreased taurine status have been demonstrated in cats fed three other heat-processed proprietary diets (unpublished data). Two purified diets were also used: a diet containing 1225 mg taurine/kg and one containing no taurine. The diet containing 1225 mg taurine/kg was used to measure taurine losses in cats fed a taurine adequate diet. The taurine-free diet was used to deplete cats of tissue taurine and to measure endogenous synthesis of tau rine.
MATERIALS AND METHODS Animal*. Ten male specific-pathogen-free cats (Nutrition and Pet Care Center, University of Cali fornia, Davis, CA), 18- to 21-mo old, were used. Body weights at the start of the experiment ranged from 4029 to 5387 g (mean 4594 ±149 g). Cats were housed in individual metabolism cages in a light-controlled (14 h light: 10 h dark) room at 21 ±3°C.Permanent cannulae were surgically placed in the terminal ileum of all cats 2-4 wk before the start of the ileal collec tions, using a technique similar to that described for dogs (11). The cats were anesthetized with sodium thiamylal (Bio-tal, Boehringer Ingelhelm, St. Joseph, MO) and halothane (Halocarbon Laboratories, North Augusta, SC), the peritoneal cavity was opened with a ventral midline incision and the terminal ileum iso lated. A longitudinal incision was made in the ileum -2 cm proximal to the ileocolic juncture and a plastic T-shaped cannula was inserted. The cannula was similar in design to that previously described for adult cockerels (12). The ileum was closed with two purse string sutures (4-0 Proline, Ethilon Inc., Somerville, NJ) and the cannula secured to the abdominal wall by a Teflon retaining plate. The stem of the cannula was exteriorized through a small stab incision -8 cm to the right of midline and the abdomen was closed. Cats tolerated the ileal cannulae extremely well with minimal post-surgical complications and no deaths. Cannula failures were limited to leakage of digesta around the cannula stem and in these animals the cannulae were surgically removed. Animal care throughout the study was in compliance with the Guide for the Use and Caie of Laboratory Animals developed by the Institute of Laboratory Animal Resources of the National Research Council. The ex perimental protocol was approved by the University of California at Davis, Animal Use and Care Adminis trative Advisory Committee, utilizing the guidelines provided by the American Association for Laboratory Animal Care. Diets. A commercial diet (Theracon, Topeka, KS), identical to that described previously (7), was used in Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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TABLE 1 Composition
of purified
diets
Dietary componentSoybean protein1 Starch2Sucrose
202200
Animal tallow3 10050 Mineral mix Vitamin mix4Cholinc 103 chloride TaurineAmountS/ks435 0 'Ardex F-disp., SPI Group, San Leandro, CA. 2Melojel, food grade cornstarch, National Starch and Chemical, Bridgewater, NJ. 3Rendered animal tallow, Florin Tallow Co., Dixon, CA. 4For composition see Williams et al. (13). laurine replaced an equal weight of starch in the 1225 and 10,000 mg/kg taurine diets.
two forms, heat-processed and frozen. The diet con tained 12 g/100 g crude protein, 7 g/100 g fat, 1.5 g/ 100 g crude fiber, 72 g/100 g moisture and had a measured taurine concentration of 1050 mg/kg dry wt (8400 |imol/kg). Purified diets (Table 1) with 0, 1225, or 10,000 mg taurine/kg dry matter (0, 9800 or 80,000 (¿moltaurine/kg) were used, with an equal weight of starch replacing the taurine in the 0 mg taurine/kg diet. All diets were mixed with 0.2 g/100 g chromic oxide (Cr2U3, certified, Fisher Scientific, Pittsburgh, PA) as a marker throughout the test cycles in which balance studies were performed. Food and water were freely available and food intake was measured daily. Design. Ten cats were randomly assigned to two groups. Group 1 (six cats) was fed the 1225 mg taurine/kg purified diet (1225 Tau) and Group 2 (four cats) the 0 mg taurine/kg diet (0 Tau) for 6-8 mo prior to the start of ileal collections. Group 2 cats were fed the 0 Tau diet to deplete tissue taurine. The experi mental protocol for Group 1 consisted of three con secutive test cycles (Fig. 1), with each cycle designed to test the effects of a different diet on ileal digesta taurine concentration and to determine taurine bal ance. During each of the three cycles, cats were first fed the 1225 Tau diet as a control diet for 7 d. Heal digesta and urine were collected on d 3-7 to establish control values and in cycles 2 and 3 to determine any residual effect of the previous cycle. Following the control diet, cats were fed in a randomized order one of three test diets for 7 d: the heat-processed commer cial, frozen commercial or 0 Tau diet. Heal digesta and urine were collected on d 3-7 and compared with the values obtained using the 1225 Tau control diet. For the heat-processed commercial diet only, the test period was extended to 28 d and an additional balance trial was performed on d 24—28.This balance study
ILEAL FLUXES OF TAURINE IN CATS
Period1225
DietDaysControl Tau7dExperimental
PeriodTest
Diet7d'Taurine
Repletion10
000 Tau5dTaurine
Stabilization1225
TauId
FIGURE 1 Feeding regimen followed for cats in Group 1. Each cat went through three consecutive test cycles. An expanded diagram of one of the three test cycles is shown. Test diets included the heat-processed commercial, frozen commercial and 0 Tau diets. *For heat-processed com mercial diet only, the test period was extended to 28 d and an additional balance trial was performed on d 24-28.
was used to determine the effects of short-term taurine depletion on ileal digesta taurine and taurine balance. Following each test diet, cats were fed the 10,000 mg taurine/kg purified diet for 5 d to replete tissue taurine, followed by the 1225 Tau diet for 9 d to stabilize tissue taurine pools, before the next test cycle. Feeding the 10,000 mg taurine/kg purified diet for 5 d was previously shown to replete taurinedepleted cats (unpublished data); this was verified in this study by the return of both plasma and whole blood taurine concentrations to initial baseline values. Group 2 cats were fed the 0 Tau diet throughout the experiment to determine the effects of taurine depletion on taurine balance and ileal digesta taurine concentration. A 5-d balance experiment involving collection of ileal digesta and urine was performed following 6-8 mo of depletion. Procedures. Cats were fed at 0800 h, and ileal contents collected from 1000 until 1330 h. Collec tions were made by allowing digesta to flow by gravity drainage into small latex bags containing 2 ml, of antibiotic solution (12.5 g potassium penicillin G and 2 g streptomycin sulfate/L) to inhibit bacterial metabolism (14). Chromic oxide in ileal digesta was determined by a modification of the procedure of Arthur (15). Heal digesta (~1 g) were placed in 75-mL digestion tubes with 5 mL of concentrated nitric acid and 2 mL of 11.7 mol/L perchloric acid and heated at 150°Cfor 90 min followed by 230°Cfor 180 min in a block digester. Samples were diluted and chromium concentration was then determined by atomic absorption spectrophotometry (Perkin-Elmer Model 3030 B, Norwalk, CT). Taurine in all samples was determined on an amino acid analyzer (Beckman Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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121MB amino acid analyzer, Fullerton, CA). Free taurine concentration was measured in a water ex tract of the ileal digesta made by mixing 3 g of digesta with 15 mL of distilled water. Total taurine in the water extract was determined after acid hydrolysis, as previously described (7).Bound taurine was calculated by subtracting free taurine from total taurine. Plasma and whole blood taurine concentrations, and free and bound taurine in urine samples preserved with 3 mol/ L sulfuric acid, were determined as previously described (7). Because several compounds present in urine elute close to taurine during amino acid analysis, urine taurine concentrations were also deter mined by HPLC using the method of Hirschberger et al. (16). Prior to HPLC, decontamination of urine samples was performed with ion-exchange columns using the method of Larsen et al. (17). Similar taurine concentrations were obtained using amino acid analysis or HPLC determination (for the four diet groups analyzed, the mean difference was 0.047 ± 0.009 (irnol/L). All taurine concentrations reported were determined by amino acid analysis. Statistical methods. Results are expressed as means ±SEM. Food intake and urinary and ileal taurine from cats fed the four diets were analyzed by SAS ANOVA and least significant difference test (18). Values were logarithmically transformed to achieve homogeneity of variances and then analyzed using a two-way ANOVA. Analysis of covariance using food intake as a covariant with diet showed a linear rela tionship with the dependent variables free, bound and total ileal taurine when expressed as micromoles per day (regression coefficient of £0.9).Accordingly, these variables were divided by food intake to normalize all values prior to ANOVA. Because no significant effects of diet order were found, results from the three control periods when cats were fed the 1225 Tau diet were averaged and treated as one diet period. A Stu dent's t test was used to determine whether calcu lated taurine balance was significantly different from zero.
RESULTS Substantial quantities of both free and bound taurine were found in the terminal ileal digesta of cats fed all diets (Table 2). Highest amounts of taurine were found in digesta of cats fed the heat-processed commercial diet, being equivalent to about twice the average daily intake of taurine on d 3-7. On d 24-28 ileal taurine decreased to about 1.5 times dietary taurine, which was the result of a lower proportion of bound taurine in ileal digesta. Total taurine was sig nificantly lower in digesta of cats when fed the frozen preserved rather than the heat-processed diet on d 3-7 (10.6 vs. 17.3 jimol-d-^g diet"1) and was equivalent to the daily taurine intake. Free and bound taurine made
556
HICKMAN
ET AL.
TABLE 2 diets1DietnFreeHeal
Taurin« in Heal digesta in cats fed fonr different
taurineBoundTotal\unold-1-g
taurineBoundTotal% diet'1Commercial processed^Days (heat 3-7Days 24-28Commercial (frozen)2Purified Tau)1Purified (1225
dietary taurine intakeFreeHeal
11"99 ± 11"43 ± 6b32± ± 5b109
19a51 ± 10b58 ± 21b40 ± ±llb205
Tau)Group (0 I2Group 2346466496
29a149 ± 14"101 ± 18b72 ± ±16C8.1
0.9a10.5 ± ±1.1*4.5 0.7b3.5 ± O.S1*02.5 ±
1.6a5.3 ± 1.1a6.1 ± 2.2a154.3 ± 1.2b4.4 ±
0.5°0.5 ± 0.7*0.3 ± ±O.ld9.2 ±O.lc17.3
2.4"15.8 ± 1.5a10.6 ± 1.9b7.8 ± 1.6C6.9 ± 1.0°0.8 ± ±0.2d
'Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different [P < 0.05) as determined by ANOVA and least significant difference test, taurine-replète cats. 3Taurine-depleted cats.
approximately equal contributions to total ileal taurine of the cats fed the frozen preserved diet. Taurine excretion in the urine of cats fed the four diets is shown in Table 3. Total (free + bound) ex cretion for d 3-7 was not significantly different when Group 1 cats were fed the commercial heat-processed, frozen or purified diets. Excretion was markedly lower on d 24—28than on d 3-7 in cats fed the heatprocessed diet and in taurine-depleted cats, indicating active conservation of taurine by the kidney. Bound taurine accounted for a substantial portion of total urine taurine in all dietary groups. Intestinal taurine fluxes calculated from ileal taurine are shown in Table 4, together with food intake, dietary taurine intake, urine taurine loss and taurine balance. Heal taurine flux was calculated by multiplying micromoles of taurine per milligram of Cr2U3 in ileal digesta by milligrams of CriOa ingested per day. Taurine balance was calculated in micromoles per day as: dietary taurine intake - (ileal taurine flux + urine taurine loss). This balance as sumes that none of the taurine passing the terminal ileum is absorbed in the colon and does not include endogenous synthesis by the cat. The calculated taurine balance for cats when fed the heat-processed diet compared with frozen preserved commercial diet on d 3-7 was -609 vs. -212 nmol/d. Taurine balance was significantly different from zero in cats fed the heat-processed commercial or 0 Tau purified diets, but not in cats fed the commercial frozen or 1225 Tau purified diets. Flux of taurine at the ileum was significantly less when cats were fed the purified rather than com mercial diets. Because there was no dietary taurine intake when cats were fed the 0 Tau diet, results from this period are only expressed as micromoles per day per gram of diet. Values shown for cats fed the 1225 Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
Tau diets are averages of the three control periods, because no significant effects of diet order were found. Similar amounts of taurine were present in ileal chyme from Group 1 cats when they were fed either the 1225 Tau or the 0 Tau diets. This suggests that the majority of the taurine appearing at the terminal ileum is of endogenous origin. Using ileal taurine flux from Group 1 cats when fed the 0 Tau diet, endogenous intestinal taurine loss in cats previ ously fed a 1225 mg taurine/kg diet approximates 440 umol/d. Calculated taurine balance in Group 1 cats ranged from about zero when fed the 1225 Tau diet (-15 (imol/d) to a very negative balance when fed the 0 Tau diet (-574.7 jimol/d). A negative balance would be expected for cats fed a diet containing no taurine. Results from the taurine-depleted cats (Group 2) fed the 0 Tau diet are shown in Tables 2-4. These cats had significantly lower amounts of taurine in ileal digesta than cats fed all other diets. Because these cats were fed no taurine for 6-8 mo before ileal samples were taken, it was assumed that their tissues were depleted to the extent that an equilibrium had been established between synthesis and loss. If it is assumed that none of the taurine passing the terminal ileum is absorbed, the sum of urinary and ileal taurine fluxes can be used to estimate endogenous taurine synthesis, which was 77 (imol/d.
DISCUSSION This study demonstrated that heat processing a diet significantly increased taurine at the terminal ileum and caused a marked negative taurine balance. These observations are consistent with our previous report that greater amounts of 14CC>2are recovered from cats given a pulse-labeled dose of 14C-labeled
557
ELEAL FLUXES OF TAURINE IN CATS
TABLE 3 Excretion of taurine in urine of cats fed four different diets1
DietCommercial processed)2Days (heat 3-7Days 24-28Commercial (frozen)2Purified Tau)2Purified (1225 Tau)Group ¡0 I2Group 23n464664Free
taurine29
taurine\unol/d105
taurine133
9a12 ± &*35 ± 9"24± 5*18 ±
23»37 ± 19b154 ± 44a101 ± 21"116 ±
24"49 ± 19b189 ± 43a125 ± 24"134 ±
3ab9 ± ±3CBound
13a1 ± ± IeTotal
15"11 ± ± 3C
Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different (P < 0.05) as determined by ANOVA and least significant difference test. •^Taurine-replète cats. 3Taurine-depleted cats.
taurine when fed a heat-processed rather than a frozen diet (7). The changes induced by heat processing could account for the inability of some heat-processed com mercial diets to maintain plasma and tissue taurine concentrations in cats. Taurine from the ileum that is not reabsorbed by the colon would be available for degradation by anaerobic bacteria. Taurine absorption from the colon has not been studied, but short-chain fatty acids such as propionate and butyrate (similar in size to taurine but carrying only a single negative charge rather than being a zwitterion), are passively absorbed from the colon (19). If some of the taurine entering the colon is absorbed, taurine balance calcu lated using ileal digesta flux would represent an overestimation of intestinal taurine loss.
The source(s) of the large amounts of free and bound taurine appearing at the terminal ileum of cats fed all diets was not determined in this study, al though the taurine seems to be mostly of endogenous origin. This conclusion was reached by comparing results from Group 1 cats fed the 1225 Tau vs. 0 Tau diets. Equal amounts of taurine were found in ileal digesta of cats fed these two diets, despite dietary taurine intakes of 577 vs. 0 (imol/d, respectively. Taurine is absorbed from the small intestine by a carrier-mediated active transport mechanism (20), and dietary taurine uptake would therefore be assumed to be quite efficient. Previous studies have shown that heat processing neither destroys nor results in covalent binding of taurine to dietary components in
TABLE 4 Food and taurine intakes and taurine losses on d 3-7 in cats fed four different diets1'2
DietCommercial (heat processed)4 Commercial (frozen!4 Purified (1225 Tau)4 Purified (0 Tau) Group I4 Group 2sn4
4 66
intake3g/d58
taurine intake485
taurine loss\unol/d969 flux
± 6a 71 ±12ab 5ab66 59 ±
± 54b 749 ±129a 57752ab0 ±
±108a 772 ±205a 462 ±110b 18a440
Urinary taurine balance-609 133 ±24a 189 ±43a 131 ±
± 91a* -212 ±141b 105b-574 -15 ±
± 98a* ±13ab ± 87b 134 ±15a 66 ± 15° 11 ± 3bTaurine -77 ± 18b* 4Food 85 ± 6bDietary 0Heal 'Balance was calculated as dietary intake - (fecal loss calculated from ileal values + urine loss). 2Values are means ±SEM.Means in a column not sharing a common superscript letter are significantly different (P < 0.05) as determined by ANOVA and least significant difference test. 'Significantly different from zero as determined by a Student's t test. 3Dry-matter basis, taurine-replète cats. Taurine-depleted cats. Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
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HICKMAN
commercial diets, because it is water extractable (un published data). Therefore, it is likely that the heatprocessed diet increased endogenous taurine at the terminal ileum rather than decreased absorption of dietary taurine in the small intestine. However, the possibility of a heat-induced inhibitor of intestinal taurine transport or changes in the gut microflora cannot be excluded. The chemical composition of the bound taurine in terminal ileal digesta was not determined in this study, but may largely represent taurine-conjugated bile acids. Several other compounds are detoxified by hepatic taurine conjugation prior to biliary secretion, but would not be expected in the concentration found at the terminal ileal in this study (21). Greater con jugated bile acids at the terminal ileum from the heatprocessed diet than from the frozen diet could be due to decreased intestinal bile acid reabsorption or in creased biliary secretion. Additions of water-soluble fibers such as pectin to the diet of rats resulted in elevated secretion of bile acids by the liver, decreased hepatic taurine concentration and increased losses of bile acids in the feces (22, 23). It seems that physical properties of the water-soluble fibers cause en trapment of bile acids rather than actual binding (24), and a similar mechanism may explain the greater taurine found at the terminal ileum of cats fed heatprocessed rather than frozen commercial or purified diets. Heating the diet may cause a change in the physical properties of some componenti s) °ftne diet/ allowing either entrapment or actual bile acid binding. Bile acids bind to dietary protein such as soybean protein isolate, with greater losses of bile acids in the feces of animals fed soybean rather than casein diets (25, 26). Heat processing changes protein structure by coagulation and denaturation, which may result in a greater affinity for bile acids in the cooked protein than in unprocessed protein. In addition, heat processing may change the number, species and/or location of bacterial flora in the small intestine, causing increased deconjugation of bile acids and an increased turnover of taurine. Anaerobic bacteria play a role in the normal intestinal loss and compensatory biliary secretion of bile acids by actively deconjugating taurine and glycine from bile acids and by converting primary bile acids into secondary bile acids (27). A recent study (28) demon strated that bacterial bile acid hydrolase activity was greater in the ileum of chickens fed rye rather than com or sucrose diets. Although the mechanism may not be related, bacterial overgrowth of the small in testine has been associated with a decrease in plasma and retinal taurine concentrations in rats and humans (29). Bile acid deconjugation and modification may be involved in this phenomenon. Ileal digesta levels of taurine and taurine balance in cats were shown to be significantly affected by taurine depletion. Taurine-depleted (Group 2) cats had Downloaded from https://academic.oup.com/jn/article-abstract/122/3/553/4754935 by University of Minnesota Law Library user on 27 May 2018
ET AL.
substantially less free, bound and total taurine in ileal digesta, the total taurine being -12% of that found in taurine-replète (Group 1) cats. Several factors could explain this decrease in ileal taurine. Taurinedepleted cats decrease the conjugation of bile acids with taurine by approximately half (30). Therefore, less taurine would appear at the terminal ileum in the form of taurine-conjugated bile acids. Depleted cats also have substantially decreased intestinal tissue taurine concentration (30), which would lead to decreases in intestinal taurine released from sloughed mucosal cells. Calculated taurine balance in taurine-depleted cats (-77 njtnol/d) was used as an estimate of taurine syn thesis. With no dietary taurine intake and minimal taurine available from peripheral tissues due to chronic depletion, losses of taurine in the urine and feces should approximate the amount of taurine syn thesized. This is a reasonable assumption because taurine-depleted cats are unable to completely con jugate bile acids with taurine and instead secrete into bile significant amounts of free cholic acid (30). Free bile acids are not found in the bile of healthy animals (31). Taurine synthesis determined in this study would be consistent with the limited ability of cats to synthesize taurine (32) and their inability to maintain normal taurine status with no dietary taurine intake. In summary, cats fed a heat-processed commercial diet had a greater intestinal taurine loss, resulting in a more negative taurine balance, than did cats fed the same diet preserved by freezing. Until the mechanism of this effect is defined, it will be difficult to predict how individual commercial cat diets will affect taurine status without actual in vivo diet testing. Cats fed commercial diets also had greater intestinal taurine losses than did cats fed purified diets; thus, recommendations of an adequate concentration of di etary taurine based on studies that use purified diets are not valid for commercial foods.
ACKNOWLEDGMENT The authors thank Hoffman-La Roche Inc., Fresno, CA, for kindly providing the vitamin mixture used in the purified diets.
LITERATURE CITED 1. National Research Council (1986) Nutrient Requirements of Cats. National Academy Press, Washington, DC. 2. Morris, J. G., Rogers, Q. R. & Pacioretty, L. M. (1990) Taurine: an essential nutrient for the cat. J. Small Anim. Pract. 31: 502-509. 3. Pion, P. D., Kittleson, M. D., Rogers, Q. R. &. Morris, J. G. (1987) Myocardial failure in cats associated with low plasma taurine: a reversible cardiomyopathy. Science (Washington, DC) 237: 764-768.
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