Clinical nutrition

Struvite Diet in Cats: Effect of Ammonium Chloride andCarbonates on Acid Base Balance of Cats12

ELLEN KIENZLE3 AND SILKE WILMS-EILERS

Institute of Animal Nutrition, Tierärztliche Hochschule Hannover, 30173 Hannover, Germany

ABSTRACT Six healthy adult cats were fed a basalminced beef meat and rice diet (one meal per day) withvarying amounts or combinations of acidifying and alkalizing additives (ammonium chloride, calcium andsodium carbonate). The base excess in the food (mmol/kg dry matter) was calculated (data on food compoundsin g/kg dry matter) as follows: base excess = 49.9 *Ca+ 82.3*Mg + 43.5*Na + 25.6*K - 64.6*P -13.4*met - 16.6*cys - 28.2*C1. Base excess in theexperimental diets amounted to between +305 and-1079 mmol/kg dry matter. After an adaptation periodof 5 d, urine and blood pH as well as water and mineralbalance were determined in the cats over a 10-15-dperiod. The daily mean urine pH ranged between 6.1and 7.8. There was a highly significant correlation between the base excess in the food and the mean urinepH. The regression line was linear down to a base excess in the diet of ~ -400 to -500 mmol/kg dry matter and a pH in the urine of 6.2. The postprandial increase of urine pH was suppressed either by largeamounts of ammonium chloride (>780 mmol/kg drymatter) alone or in combination with calcium carbonatebut not in combination with sodium carbonate. The relationship between the decrease of the blood pH andthe amount of ammonium chloride added to the dietwas more marked than the relationship between bloodpH and base excess in the food. To avoid health risksfrom long term feeding of acidifying diets, it is recommended that struvite diets with low base excess beformulated and that they contain as few alkalizingcompounds as possible that must be neutralized byacidifiers. J. Nutr. 124: 2652S-2659S.

INDEXING KEY WORDS:

•cat •acid base balancechloride •struuite diet

ammonium

Acidifying diets are considered the method of choicein dietary treatment or prevention of struvite uroli-thiasis in cats, because struvite does not precipitatereadily in an acid milieu (pH < 7). This treatment hasbeen proven to be successful (Buffington et al. 1985,Cook 1985, Lewis et al. 1987, Taton et al. 1984), but

there have been reports of possible risks of a chronicacidifying nutrition such as disorders of calcium andpotassium metabolism (Buffington et al. 1988, Chingétal.1989).

In the first part of the present investigation (Kienzleet al. 1991, Kienzle and Schuknecht 1993), it wasdemonstrated that under practical conditions the effectof a diet on urine pH can be estimated from the baseexcess in the food. The base excess (BE)(mmol/kg drymatter) can be calculated as follows from the compounds in the food. (In contrast to the first part of theinvestigation, in this investigation minerals and aminoacids are given in g/kg dry matter instead of mmol,and the factors already include valence and molecularweight).

BE = 49.9*Ca + 82.3*Mg + 43.5*Na + 25.6*K

- 64.6*P - 13.4*met - 16.6*cys - 28.2*C1

This equation characterizes mainly the content ofalkalizing anions such as carbonates and organic an-ions that are not neutralized by acidifying compounds.The mechanism of action has been reviewed in detailby Kienzle (1991).

There is a highly significant relation between theBE in the food and the daily mean urine pH: Y = 6.72+ 0.0021X; r = 0.90**. From this regression equationit can be calculated that the BE in the food must be

1Presented as part of the Waltham Symposium on the Nutrition

of Companion Animals in association with the 15th InternationalCongress of Nutrition at Adelaide, SA, Australia, on September 23-25, 1993. Guest editors for this symposium were Kay Earle, JohnMercer and D'Ann Finley.

2 This paper is based on a translation of the paper by E. Kienzleand S. Wilms-Eilers (1993): "Untersuchungen zur Struvitstein-

diätetik:2. Einflußvon Ammoniumchlorid und Carbonaten auf denSuren-Basen- und Mineralstoffhaushalt der Katze," published inDeutsche Tierärztliche Wochenschrift 100: 399-405.

3To whom correspondence should be addressed: Institute of

Physiology, Biochemistry and Animal Nutrition, Veterinary Faculty,Ludwig-Maximilians-University, Munich, Veterinärstr. 13, 80539München,Germany.

0022-3166/94 S3.00 ©1994 American Institute of Nutrition.

2652S

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decreased to >130 mmol/kg dry matter if the pH inurine is to be reduced below 7 (Kienzle and Schuknecht1993). If the variance of the regression line is considered, an even lower base excess of 0 mmol/kg dry matter may be necessary to ensure a decrease of urine pHbelow 7.

Because of possible risks of chronic acidifying nutrition in the present investigation, it was tested

•whether the relations between BEin the food andeffects of acidifying diets on acid base balance ormineral metabolism are similar to the relation between BE in the food and urine pH

•and/or whether there are other important factorsinfluencing these parameters.

In addition we investigated whether the totalamount of acidifiers is still important if they are partlyneutralized by alkalizing compounds. To answer thesequestions, the effects of large or small doses of anacidifier (ammonium chloride) with or without alkalizing additives (carbonates) were investigated.

MATERIALS AND METHODS

Various doses of an acidifier (ammonium chloride[AC]) with or without alkalizers (calcium carbonate[CC] or sodium carbonate [SC])were added to a basaldiet (B) of minced beef meat and cooked rice (Table1).To obtain considerable differences of acidifiers andalkalizers, calcium, sodium and chloride content, aswell as calcium phosphorus ratio, were not always keptwithin recommendations. The BE was calculated asdescribed above. It is always given in the respectiveabbreviation of the diet, for example, B/-224 for thebasal diet with a base excess of -224 mmol/kg drymatter. These research studies were approved bythe appropriate Animal Welfare Authority (Bezirksregierung Hanover, Germany).

A diet with a positive BEof 305 mmol/kg dry matterwas formulated by addition of calcium carbonate (CC/+305), and another was formulated with a BEof -479mmol/kg dry matter (AC/-479) by addition of ammonium chloride. Two other diets with a similar BEwere formulated with combinations of ammoniumchloride and calcium carbonate (ACCC/-478) or sodium carbonate (ACSC/—470).Thus three diets witha marked negative BE in the same range but withwidely differing contents of ammonium chloride andcalciunrphosphorus ratio were compared. All fivediets were fed for several weeks (5 d adaptation, 10-19 d balance trial) to six adult cats (2.75-4.1 kg bodyweight, age 3-5 years). Food was offered in the morning (16g dry matter/kg body weight/d^ for 30 min.The cats had free access to water.

Urine and blood pH, mineral and water balance,plasma mineral concentration, ammonia in urine and

TABLE 1

Expfrinicn tal design

Diet1B/-224AC/-479ACCC/-478ACSC/-470CC/+305AC/-

1079Food

additives(g/kg

drymatter)Basal

diet withoutadditives213.6

NH4C142.0NH4 Cl + 26.7CaCO341.9NH4Cl

+28.5Na2CO326.5CaCO345.7

NH4C1Ca

NaCl(g/kg

drymatter]0.8

3.70.83.711.53.70.8

16.111.43.70.8

3.74.013.132.031.94.034.4

1Abbreviation gives BE in mmol/kg dry matter.2 Mixture of raw, minced beef and cooked rice (ratio of uncooked

rice and beef 1:10) per kg of dry matter; 0.5 g of vitamin mixture(Rovimix) and 10 g of trace element mixture were added. Composition of basal diet: dry matter 22%. Crude nutrients (% dry matter):protein, 58.4; fat, 11.3; ash, 3.2; N-free extract, 27.1. In g/kg drymatter: met, 13.8; cys, 5.7; Mg, 0.63; P, 5.11; K, 9.67. In mg/kg drymatter: Fe, 270; Cu, 15; Zn, 210; Mn, 11.

digestibility of dry matter, as well as fecal pH, wereinvestigated.

In a second trial ammonium chloride was added tothe basal diet to test palatability. Starting from a BEof —528,a stepwise reduction for 50 or 100 mmol/kgdry matter every 2 d was carried out. When a BE of—1079 mmol/kg dry matter was reached, the cats refused to eat on the third day of the experiment. Theacceptance of diets with such a low BE was not expected. Because it might have been because of thestepwise reduction, a third trial with a sudden reduction of base excess to -876 was carried out. The abbreviations are AC/-528 to AC/-1079. Urine pH wasalso determined throughout the palatability trials, andin the trial with diet AC/-1079 blood pH (second dayof trial) and urine volume (first and second day) wereinvestigated.

The animals were kept in cages that allow a separatecollection of urine and feces. They were offered foodin an amount of 16 g dry matter/kg body weight oncedaily. Urine samples were collected hourly for 8 h afterfeeding and in the morning before feeding.

Crude nutrients were determined by Weender analysis (Nehring 1963), methionine and cystine by ionexchange chromatography, chloride with an ion sensitive electrode, other minerals after wet digestionwith atomic absorption spectography (Ca, Mg), flameemission spectography (Na, K) or colorimetrically (P).Urine pH was measured with an electric pH-meter,ammonia enzymatically (testkit from Boehringer,Mannheim, Germany).

After 7 d blood samples for mineral determinationwere taken 6 h after feeding. Blood samples for bloodpH were drawn before feeding on the last day of thetrial. Blood was drawn from the V. cephalica ante-bracchii without compression. From one animal who

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showed severe resistance no blood samples weredrawn. The blood was analyzed immediately afterwardin the clinic for cattle of Hanover Veterinary Schoolwith a Corning Blood Gas System 278. Apparent digestibility was determined by the collection method(collection period > 10 d).

Statistical evaluation was carried out by calculationof mean and SD. For "daily mean" urine pH all valuesobtained during the day and night from all cats duringthe trial were included in the mean. Multiple comparisons of means were carried out by one-way analysisof variance and Tukey test. Tables show mean and SDas well as the least significant differences. This termindicates the smallest difference between two meansthat differed significantly (p < 0.05). Correlation coefficients are marked with * for p < 0.05,** forp < 0.01.

Before the beginning of the experiment a methodfor preservation of urine (especially night urine collected in the morning) was tested. Thymol (pinch) andparaffin (sufficient to cover the surface of the sample)were put into the receptacle (de Wilde, R. O., personalcommunication). pH was determined as usual in themorning. The mean pH of 2 nights with or withoutpreservation were compared. The efficiency of preservation was tested with fresh samples. pH was measured, and the samples were divided into two. Onewas preserved; the other one was not. Then they wereleft at room temperature for 20 h, and pH again wasdetermined. The preservation method proved to beefficient. In preserved samples even after 20 h pH increased <0.1 pH-units. In untreated samples the observations of Kienzle and Schuknecht (1993) were repeated. An increase of pH with time occurs mainly insamples with a pH > 6.5 (increase of <0.5 pH-unitsin samples with pH 8.2), whereas more acid samplesdo not change much (increase of 0.15 pH-units insamples with pH 6.2). Because most samples collectedduring the night were acid, preservation had only aweak effect on the total result. The means of two consecutive nights with and without preservation did notdiffer significantly. The maximum difference was 0.15pH-units. To improve comparison with previous results (Kienzle and Schuknecht 1993), results for samples without preservation are displayed.

RESULTS

During the adaptation period in all groups receivingammonium chloride, occasional vomiting occurred.No other side effects were observed in the first part ofthe experiment, but in the second part after feedingdiet AC/-1079 (lowest BE), a mild apathy was observed before the food was completely refused. Oneanimal showed strong vomiting.

In the first part of the experiment, 97% of the foodoffered was eaten. Food intake averaged 15.5 ±1.0 gdry matter/kg body weight in all trials of balance. Allthe food was eaten within 30 min after feeding, butsubjective criteria of behavior (begging during feeding)indicate differences in palatability. It was lowest indiet ACCC/-478 and also rather low in diet ACSC/-470, whereas the other diets were eaten rapidly.

If BE was reduced stepwise by addition of ammonium chloride from -528 to -1079 mmol/kg drymatter (steps: -578, -628, -778, -876, -980, -1079),in the second part of the experiment all food offered(16 g dry matter/kg body weight/d) was eaten until aBE of -1079 mmol/kg dry matter was reached. DietAC/-1079 was eaten on the first day by all cats. Onthe following day food intake was reduced by half thecats, and on the third day all cats refused to eat. Aftera sudden change from B/-224 to AC/-876 with a BEof —876mmol/kg dry matter, the food was acceptedreadily for several days.

Fecal pH was not significantly affected by diet, butthere was a tendency to higher values in diets withcarbonates (B/-224, 6.58 ±0.51; ACCC/-478, 6.92±0.43; ACSC/-470, 6.81 ±0.24; CC/+305, 7.15±0.25; n = 6). The combination of carbonates andammonium chloride also increased fecal dry matter(B/-224, 36.2 ±4.0; ACCC/-478, 50.4 ±6.7; ACSC,49.9 ±7.0% dry matter; n = 6). Apparent digestibilityof dry matter was not affected (mean of all diets: 94.5±2.5%; n = 30).

In diets with high ammonium chloride content,water intake was markedly increased by increaseddrinking. Water intake as a part of the food did notvary considerably (52-57 ml/kg body weight/d). Totalwater intake increased from between 61 and 66 ml/kg body weight/d in the groups B/-224, AC/-479 andACSC/-470 to 74 ml in group ACCC/-478 and highlysignificant to 102 ml in group AC/-1079. Urine volume (Y, ml/kg body weight/d) is given in Table 2. Itcorrelated strictly with total water intake (X, ml/kgbody weight/d; Y = -11.5 + 0.84X; r = 0.77**; n = 36(Table 2). Fecal water excretion was almost negligible(0.8-1.3 ml/kg body weight/d).

The mean of urine pH varied in the balance trialsbetween 6.35 (diet AC/-479) and 7.80 (diet CC/+305).There were no differences between the three diets withthe same BE (Table 2). In the trials with stepwise reduction of BE,urine pH decreased proportionally untila base excess of -400 to -500 mmol/kg dry matter,after which no further reduction of urine pH occurred(Figure 1). The lowest mean pH amounted to 6.13.The linear equation for the relation between BEin thefood and urine pH as calculated by Kienzle and Schuknecht (1993) could be used only for the range above-500 to -400 mmol/kg dry matter, for lower valuesthere was a parabolic equation (Figure 1).

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ACID BASE BALANCE IN CATS 2655S

TABLE 2

Urine and blood pH, urine volume, renal phosphorus excretion and plasma phosphorus level

DietB/-224AC/-479ACCC/-478ACSC/-470CC/+305AC/-528AC/-578AC/-628AC/-685AC/-778AC/-876AC/-980AC/-10794Least

significantdifference5NH,C1

dosemg/kg

bw/d021265365002522943353824595406267094Total

dailymean6.94

±0.556.35±0.496.43±0.426.43±0.377.80±0.636.36±0.366.23+0.316.24±0.296.27±0.276.

18±0.296.18±0.256.13±0.286.18±0.350.26n19980133108953550345159434076Day

mean7.11

±0.536.55±0.506.43

±0.456.48±0.408.09±0.496.42±0.386.30±0.326.28±0.306.31

±0.276.18±0.306.16±0.246.07

±0.246.20±0.370.31n169529774602333223442312959Night

mean6.57

±0.42*5.97±0.15'6.44

±0.346.31±0.28'7.31±0.52'6.23

±0.316.10±0.25'6.18

±0.286.20±0.276.18

±0.266.22±0.276.26±0.336.12

±0.270.37n130283634351217121717121117Blood

pH*7.300

±0.0337.296±0.0487.

180±0.0387.222±0.0317.277

±0.0387.079

±0.0400.071Urine

Volume3(ml/bw/d)39.1

±3.140.8±2.453.0±8.449.6±6.541.9+2.073.2

±14.213.1Renal

Pexcretion3(mg/bw/d)55.1

±5.358.3±2.521.

3±4.555.6±2.013.5

±3.26.3Plasma

P(mg/1)55

±759±539±753±339±75.5

±0.61.1

1n = number of samples from six cats.1 n = 5 samples from five cats.3n = 6 samples from six cats.4 No complete food intake on second and third day, thus the NH4C1 dose is only valid for the first day of experiment.5 Indicates the minimum difference between two means that differ significantly |P < 0.05).* Significant difference (P < 0.05) between night and day means.

After the intake of the basal diet and of the dietsAC/-479, ACSC/-470, CC/+305 and AC-578, themean pH of urine samples collected during the daywas higher than that of samples collected in the night(Table 2).

The postprandial urine pH showed surprising differences among diets with nearly identical BE. In dietACCC/—478there were hardly any alterations of pHduring the day or the night, whereas after the intakeof the two other comparable diets ACSC/-470 andAC/-479 there was a considerable alkaline tide 2-4h after feeding (Figure 2), which persisted only for a

0.0019X+9.7»10~7X2

6.4

-800 -400 0 400

base excess mmol/kg dry matter

FIGURE 1 Relation between BE in the food and meanurine pH.

short time. In contrast to that, the postprandial alkaline tide persisted longer in the basal diet. This effectwas even more marked in group CC/+305 (Figure 3).In the diet with the highest ammonium chloride concentration (AC/-1079) urine pH decreased postpran-dially (Figure 3).

Blood pH depended mainly on the ammoniumchloride intake and did not show any immediate relation to the BE in the food. If blood pH was low,urine pH was acid as well. On the other hand, in somediets with low BE but also low ammonium chlorideconcentration, urine pH was decreased, whereas bloodpH remained unchanged (Table 2). There was a sig-

8 -

7

6-

ACSC/-470

ACCC/-478AC/-479

0-2 2-4 4-6 6-8hours postprandially

night

FIGURE 2 Postprandial urine pH for diets AC/-479,ACCC/-478 and ACSC/-470 (with comparable BE; 6 catsper group, 2-36 samples per mean).

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ni'

9

8

7-

6

CC/+305

i l

0-2 2-4 4-6 6-8hours postprandially

night

FIGURE 3 Postprandial urine pH for diets CC/+305 andAC/-1079 compared to basal diet (B/-224) (6 cats per group,2-35 samples per mean).

nificant negative correlation between ammoniumconcentration in urine (Y, mg/1) and urine pH (X, Y= 3240 - 335X; r = 0.74**; n = 60).

In this paper only data on plasma phosphorus leveland renal phosphorus excretion as well as chloride digestibility are reported, other details of mineral balance and plasma mineral levels are published elsewhere(Kienzle and Wilms-Eilers 1993).

Renal phosphorus excretion and plasma phosphoruslevels were affected by the calcium:phosphorus ratio(Table 2). Both parameters decreased in diets withadded calcium. Chloride digestibility was high in alldiets tested (mean of all diets 99.3 ±0.8%; n = 30).There was no effect of either calcium or sodium carbonate on chloride digestibility.

DISCUSSION

In this investigation the minimum of the daily meanof urine pH was >6. This is partly because of the calculation of the daily mean from all samples collectedduring 24 h, because the daily mean includes severalsamples collected during postprandial alkaline tide andonly one from the night. Single values were considerably lower (minimum 5.7), and also means of varioustimes after feeding were somewhat lower than thedaily mean (Figures 2 and 3). Another factor might beadaptation to acidifying diets, especially in those dietsfrom the second part of the experiment with the step-wise reduction of base excess, which lasted for 3 wk.Consequently, the lower the BEin the diet, the longerwas the adaptation period. A methodical problem insampling is unlikely but cannot be completely excluded, because the preliminary experiment on thestability of urine samples after treatment with thymoland paraffin covered a range from 6.2 to 8.2. On theother hand it seems unlikely that samples <6.2 willchange more rapidly than the samples tested, and it is

also most unlikely that such changes will stop at a pHof ~6.

In the present investigation it was demonstratedagain that the mean urine pH depends on BE in thefood, regardless of whether large amounts of alkalizingcompounds were neutralized by corresponding dosesof ammonium chloride or whether the total amountof alkalizing and acidifying substances was low. Theresults of the present investigation agree very well withresults from the first part of the investigation (Kienzleand Schuknecht 1993), as well as with data obtainedin dogs (Behnsen 1992) (Figure 4). Finkensiep (1993)and Krohn (1993) report similar regression equationsbetween BE in the food and urine pH in pigs. In thepresent investigation as well as in the experiments ofKrohn (1993) in pigs, the relation between both parameters is only linear with a base excess above -500to —400mmol/kg dry matter (Figure 1).

The good correspondence between observations indifferent species is not surprising, because manipulation of acid base balance are mainly governed byphysiochemical rules. The greater part of dietary acidequivalents must be excreted by the kidney if acid basebalance of the organism is to be maintained.

The postprandial alkaline tide in urine showed considerable differences between diets with comparableBE and comparable mean urine pH (Figures 2 and 3).There was no obvious influence of the total amountof acidifiers. This observation cannot be because ofeating behavior. Although both diets with ammoniumchloride and carbonates were not eaten as readily asthe other diets, the food was completely eaten within30 min. Also postprandial alkaline tide differed markedly between these two diets with the lower palata-bility. A time shift of the effect of various additives

18.4-

o£ 8.0-3

7.6 -

7.2 -

6.8-

6.4-

6.0-

-800 -400 0 400 800base excess mmol/kg dry matter

• present investigation; cats*• Kienzle and Schuknecht (1993); catso Behnsen (1992): dogs

FIGURE 4 Relation between BE in the food and urinepH in the present investigation compared to results of Kienzle and Schuknecht (1993) in cats and Behnsen (1992) indogs.

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on urine pH could be explained by a different velocityof absorption from the gastrointestinal tract. If an earlier onset of effects is suggested for ammonium chloride and sodium carbonate than for calcium carbonate,all the contradictory results can be satisfactorily explained. In the diets ACSC/-478 and AC/-1079 thehigh ammonium chloride dose suppresses postprandialalkaline tide. This is not prevented by the alkalizercalcium carbonate that is not easily soluble and absorbed rather slowly. On the other hand, sodium carbonate is rapidly absorbed and contributes to postprandial alkaline tide, thus impairing the effects ofammonium chloride. This theory is supported by thepostprandial alkaline tide of diet CC/+305, which wasat first parallel to the basal diet but persisted muchlonger. There were also considerable differences in thenight urine (Figure 3). These findings indicate that notonly calcium but also carbonate from calcium carbonate is absorbed preferably in the large bowel of the cat.This agrees with observations in dogs (Lass 1988),where a considerable percentage of calcium is absorbedfrom the large bowel. In summary it is postulated thatthe postprandial alkaline tide is suppressed only indiets with considerable amounts of rapidly absorbableacidifiers (ammonium chloride) and without rapidlyabsorbable alkalizers (sodium carbonate). These observations also demonstrate that general recommendations for feeding technique (number of meals perday) might possibly not apply to all diets.

Although the determination of blood pH from venous blood leads to some inaccuracies, this methodwas chosen because the cats resisted arterial bloodsampling (A. femoralis) more vehemently than bloodsampling from V. cephalica antebracchii. In horseseven moderate activity for a few minutes leads to considerable shifts in arterial blood pH (Coenen 1992).Strong resistance to blood sampling in cats appears tobe quite comparable to moderate activity in horses.Therefore pH analysis from arterial blood may alsolead to considerable errors. Middleton et al. (1981)compared arterial and venous blood pH in cats underidentical conditions. They concluded that larger differences in the acid base balance can be analyzed withvenous blood pH. The data from the basal diet groupwithout additives (7.300 ±0.033; n = 5) and the groupCC/+305 with calcium carbonate (7.277 ±0.038;n = 5) agree with normal values of Middleton et al.(1981), which amounted to 7.300 ±0.044 (n = 30),whereas high ammonium chloride intake reduced venous blood pH.

As expected, blood pH was not primarily related tothe BEin the food. Low blood pH values only occurredin diet groups with negative BE. The reverse is nottrue; however, because not all acidifying diets led toan alteration of blood pH. Relations between bloodand urine pH were corresponding. If urine is acid, it

is possible but not unavoidable that blood pH is alsodecreased.

The discrepancy between the reduction of blood andurine pH in various diets with a negative BE is partlybecause of a lower limit of daily mean urinary pH,which did not decrease below 6.2 (Figures 1 and 4).Possibly the maximum long-term capacity for acid excretion is reached at that point. A stronger acidifyingeffect of diet was compensated by a decrease of foodintake and not by an increase of urinary acid excretion.

In contrast to that, it is less obvious that diets withthe same BE but different acidifying or alkalizing additives result in different blood pH. On one hand, thismay be because of a specific effect of high ammoniumchloride doses, regardless of the BEof the diet. On theother hand, differences between the physiologicalneutral point at pH = 7.4 and the chemical neutralpoint, which is the basis for the BE of the food, canplay a role. Provided that the urine pH equals the bloodpH (pH = 7.4), it can be expected that, for the regulation of the acid base balance, there is only minorrenal excretion of acid or alkaline equivalents. According to the regression equations from the data presented in this paper and the experiments of Kienzleand Schuknecht (1993), a urine pH of 7.4 can be expected at a BE of 300 mmol/kg diet dry matter. Thisvalue can be considered as the physiological neutralpoint of the BE, whereas a BE of zero has a strongacidifying effect on the organism. In the range between0 and 300 mmol/kg dry matter, the body has to bufferand excrete neutralized acid equivalents of the food.Therefore, if there are more neutralized acid components, as is the case with the diets containing ammonium chloride and carbonate, the buffer capacity ofthe body is required more than with diets containingless neutralized acid equivalents. For example, the effect of diets with more or less acid and alkaline components or the same BE could be compared with theeffect of more or less buffered solutions of the samepH on another buffer system.

Plasma phosphorus content and the renal phosphorus excretion phosphorus mainly depended on the cal-cium:phosphorus ratio of the diet, but there was a tendency toward higher renal phosphorus excretion atacid urine pH, when the calcium:phosphorus ratio ofthe diets were comparable. Including the results reported by Kienzle and Schuknecht (1993), a multipleregression between the calciunrphosphorus ratio ofthe diet (XJ, the average urine pH (X2)and the renalphosphorus excretion (Y,in percent of the phosphorusintake) could be calculated: Y = 183- 19.4X! - 17.2X2;r = 0.83*; n = 11. Thereby, 67.5% of the variationdetermined by the independent variables resulted fromthe calciunrphosphorus ratio, 32.5%, from the urinepH. It is possible that the weak effect of the ACSC/-470 diet on the blood pH compared with the ACCC/-478 diet is because of these interactions. The acid

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TABLE 3

Recommendations for the composition ofstruvite diets1

Dissolution of calculi Prevention of calculi

[per kg dry matter]

BE1(mmol)Calg)Mg

(g)Na(g)K(g)Ca:P-2606.0-8.00.6-0.74.0-6.05.0-6.01.1:104.0-6.00.5-0.74.0-6.04.0-6.01.1:1

1Contents of minerals based on minimum requirements for adult

cats after Meyer and Heckotter (1986). Based on results of this investigation, no long-term clinical studies have been carried out sofar. Higher safety margin for dissolution.

1 Calculated on the basis of food content in g/kg dry matter: BE= 49.9»Ca + 82.3»Mg + 43.5»Na + 25.6.K - 64.6.P - 13.4»met- 16.6*cys - 28.2*C1. These values are calculated under consid

eration of the statistical variance of the regression between BE andurine pH. Therefore it is possible that a higher base excess in a dietstill leads to a sufficient acidification of urine pH.

excretion is facilitated by a higher phosphorus excretion via urine. There are consequences for the feedingpractice. The calcium:phosphorus ratio should not bewider than absolutely necessary (1.1:1), when the dietis chronically acidifying.

Water intake and urine volume were similar tovalues reported for diets with a water content >70%in the first part of the study (Kienzle and Schuknecht1993). Fecal water losses were practically negligible,as already reported for highly digestible diets byvarious authors (Figge 1989, Kienzle 1989, Schneider1988). There was no diuretic effect from the additionof ammonium chloride until the dose of 800 mmol/kg dry matter was reached. The diuretic effect isprobably because of chloride intake. Kienzle andSchuknecht (1993) reported a similar threshold fordiuretic effects of sodium at ~780 mmol/kg drymatter.

Based on the first part of the experiment (Kienzleand Schuknecht 1993) and the present results, the following recommendations for struvite diets can begiven. It is necessary to make a clear difference between diets for dissolving calculi (short-term feedingfor some weeks, not over 3 mo) and diets for prevention of stone formation. Diets for dissolution of struvite calculi should have 3) a BE below -260 mmol/kg dry matter to ensure an acid mean urine pH below6.5 (Kienzle and Schuknecht 1993) and 2) contain asufficient quantity of easily absorbable acidifiers tosuppress the postprandial alkaline tide (according toresults of the present investigation, ~780 mmol ammonium chloride/kg dry matter are sufficient if thebase excess is negative, 250 mmol/kg, however, arenot). A decrease of blood pH is probably inevitable in

this case. Alkalizing components should be only addedin quantities that cover the minimum requirementsincluding a small safety margin for possible increaseof requirements because of the acidifying effects ofthe diet (Table 3). Easily absorbable alkalizing substances must be avoided. A calcium:phosphorus rationot above 1.1:1 is recommended.

The BE in the food in diets for struvite preventionshould amount to 0 mmol/kg dry matter (mean urinepH < 7). The reduction of the base excess should bedone mainly by eliminating alkalizing components(considering the minimum requirements; table 3) and,only when the possibility of this method has been exhausted, by adding acidifiers. Thus, the amount of acidifiers is kept as small as possible. For the calcium:phosphorus ratio see above. Under such conditions itis possible to acidify urine without inducing changesof blood pH.

ACKNOWLEDGEMENT

We are grateful to Prof. Dr. H. Scholz from the Cattle Clinic of TierärztlicheHochschule Hannover forblood gas analysis.

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