Influence of acidifying or alkalinizing diets on bone mineral density and urine relative supersaturation with calcium oxalate and struvite in healthy cats

Joseph W. Bartges Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Claudia A. Kirk Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Sherry K. Cox Department of Biomedical and Diagnostic Services, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Tamberlyn D. Moyers Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Abstract

Objective—To evaluate the influence of acidifying or alkalinizing diets on bone mineral density and urine relative supersaturation (URSS) with calcium oxalate and struvite in healthy cats.

Animals—6 castrated male and 6 spayed female cats.

Procedures—3 groups of 4 cats each were fed diets for 12 months that differed only in acidifying or alkalinizing properties (alkalinizing, neutral, and acidifying). Body composition was estimated by use of dual energy x-ray absorptiometry, and 48-hour urine samples were collected for URSS determination.

Results—Urine pH differed significantly among diet groups, with the lowest urine pH values in the acidifying diet group and the highest values in the alkalinizing diet group. Differences were not observed in other variables except urinary ammonia excretion, which was significantly higher in the neutral diet group. Calcium oxalate URSS was highest in the acidifying diet group and lowest in the alkalinizing diet group; struvite URSS was not different among groups. Diet was not significantly associated with bone mineral content or density.

Conclusions and Clinical Relevance—Urinary undersaturation with calcium oxalate was achieved by inducing alkaluria. Feeding an alkalinizing diet was not associated with URSS with struvite. Bone mineral density and calcium content were not adversely affected by diet; therefore, release of calcium from bone caused by feeding an acidifying diet may not occur in healthy cats.

Abstract

Objective—To evaluate the influence of acidifying or alkalinizing diets on bone mineral density and urine relative supersaturation (URSS) with calcium oxalate and struvite in healthy cats.

Animals—6 castrated male and 6 spayed female cats.

Procedures—3 groups of 4 cats each were fed diets for 12 months that differed only in acidifying or alkalinizing properties (alkalinizing, neutral, and acidifying). Body composition was estimated by use of dual energy x-ray absorptiometry, and 48-hour urine samples were collected for URSS determination.

Results—Urine pH differed significantly among diet groups, with the lowest urine pH values in the acidifying diet group and the highest values in the alkalinizing diet group. Differences were not observed in other variables except urinary ammonia excretion, which was significantly higher in the neutral diet group. Calcium oxalate URSS was highest in the acidifying diet group and lowest in the alkalinizing diet group; struvite URSS was not different among groups. Diet was not significantly associated with bone mineral content or density.

Conclusions and Clinical Relevance—Urinary undersaturation with calcium oxalate was achieved by inducing alkaluria. Feeding an alkalinizing diet was not associated with URSS with struvite. Bone mineral density and calcium content were not adversely affected by diet; therefore, release of calcium from bone caused by feeding an acidifying diet may not occur in healthy cats.

Calcium oxalate urolith formation occurs when urine is supersaturated with calcium and oxalate.1 Hypercalciuria has not been well-defined in normocalcemic cats with calcium oxalate uroliths but may occur. Metabolic acidosis promotes hypercalciuria by promoting bone turnover (release of calcium with buffers from bone), which increases serum ionized calcium concentration, resulting in increased urinary calcium excretion and decreased renal tubular reabsorption of calcium. Consumption by cats of diets supplemented with the urinary acidifier ammonium chloride is associated with increased urinary calcium excretion and decreased bone density.2 A urine pH < 6.2 may represent a risk factor for calcium oxalate formation because of acidemia and hypercalciuria in addition to altering function and concentration of crystal inhibitors. In humans, low urine pH decreases urine citrate concentration by increasing renal proximal tubular citrate reabsorption and decreases the crystallization inhibitory capacity of citrate through decreased calcium-citrate complex formation.3–5 Acidic urine is known to impair the function of macromolecular protein inhibitors.

Epidemiologically, feeding acidifying diets and aciduria has been identified as increasing the risk for calcium oxalate urolith formation in cats.6,7 Urinary saturation with calcium oxalate increases as the degree of aciduria increases; however, the relationship is sometimes unpredictable.8 For this reason, an alkalinizing agent, typically potassium citrate, is included in dietary formulations for feline calcium oxalate preventative diets; however, the effect of potassium citrate on the risk of calcium oxalate formation has not been evaluated in cats. In dogs, supplemental potassium citrate has limited influence on urinary variables associated with calcium oxalate formation.9

The purpose of the study reported here was to evaluate the influence of diet-induced aciduria, a neutral urine pH, and alkaluria on URSS with calcium oxalate or struvite, acid-base status, and bone mineral density in healthy cats. The hypothesis was that induction of alkaluria would decrease URSS with calcium oxalate and increase URSS with struvite, decrease 24-hour urinary excretion of calcium and oxalic acid, increase 24-hour urinary excretion of citrate, and be associated with increased bone mineral content and density, compared with induction of aciduria in healthy adult cats.

Materials and Methods

Cats—Six castrated male and 6 spayed female cats (age, 1 to 2 years old) were studied. Cats were judged to be healthy on the basis of results of physical examination, CBC, serum biochemical analyses, complete urinalysis, aerobic bacteriologic culture of urine obtained by cystocentesis, and survey abdominal radiography. Cats were housed in individual cages under conditions of controlled lighting and temperature according to the principles outlined in the National Institutes of Health Guide for the Care and Use of Laboratory Animals.10 Approval for this study was obtained from the University of Tennessee Institutional Animal Care and Use Committee.

Diet composition—Single batches of dry formulations of the following diets were used (Table 1): a diet formulated to induce a urine pH of 6.0 to 6.4 (acidifying diet), a diet formulated to induce a urine pH of 6.4 to 6.8 (neutral diet), and a diet formulated to induce a urine pH of 6.8 to 7.2 (alkaline diet).a Diets were formulated to be similar to a feline adult maintenance diet and to contain the same ingredients except for those ingredients required to induce the desired urine pH. All diets contained corn, corn gluten meal, poultry meal, animal fat, natural flavors, calcium sulfate, choline chloride, potassium chloride, a vitamin and mineral premix, taurine, and ethoxyquin; the acidifying diet also contained calcium chloride and the alkalinizing diet also contained potassium citrate.

Table 1—

Nutrient analysis (% [as-fed basis]) for acidifying, neutral, and alkalinizing diets fed to 4 healthy cats/diet group.

NutrientAcidifying dietNeutral dietAlkalinizing diet
Protein34.033.134.1
Fat18.019.217.9
Ash4.74.64.6
Calcium0.680.680.71
Phosphorus0.670.640.66
Magnesium0.090.090.09
Sodium0.180.170.18
Potassium0.800.770.80
Chloride0.640.580.44
Crude fiber1.21.41.4
Moisture6.67.16.2

Feeding protocol—The amount of food fed was determined on the basis of estimated daily maintenance caloric requirements determined by body weight (70 kcal/kg of body weight).11 Food intake was monitored daily, and cats were weighed weekly; the amount of food was adjusted so that cats maintained a body weight within 10% of their body weight at the initiation of the study. Fresh tap water was available at all times.

Experimental design—Cats were fed a commercially available dry formulated adult maintenance dietb for 1 month before baseline data were collected (CBC, serum biochemical analyses, urinalysis, aerobic bacteriologic culture of a urine sample obtained by cystocentesis, venous blood gas analysis, blood ionized calcium concentration, and bone density determined by DEXA) to determine health status. Male and female cats were then randomly assigned to 1 of 3 groups (acidifying diet, neutral diet, and alkalinizing diet) by use of a random numbers table so that there were 2 males and 2 females in each group. Diets were fed in a double-masked manner so that neither the investigator nor the caregivers were informed as to which diet was fed to which cat. Cats were transitioned from the adult maintenance diet to the study diet over 1 week, and food intake was adjusted to maintain body weight within 10% of their body weight at the time of randomization for 3 weeks, to establish food intake. Diets were fed for 12 additional months, and at 6 and 12 months, information on 24-hour urinary excretion of ammonia, calcium, chloride, citric acid, creatinine, magnesium, oxalic acid, phosphorus, potassium, sodium, and uric acid was collected and estimation of urinary saturation with calcium oxalate monohydrate and struvite was determined by relative supersaturation12; baseline data were also collected. During the 12 months, diets were adjusted to maintain body weight within 10% of body weight after transition to the study diet.

Blood, serum, and plasma acquisition and assay—Blood samples were collected from the jugular vein. Blood was divided and analyzed for blood gases, CBC, serum biochemical analyses, and ionized calcium concentration by the Clinical Pathology Laboratory, College of Veterinary Medicine, University of Tennessee.

Urine collection and analysis—Cats were housed in individual cages for collection of urine samples. Modified litter boxes allowing for collection and separation of urine and feces from individual cats were used.13,c The modified litter pan neither emits nor absorbs minerals or electrolytes, and their use usually obviates the need to sedate cats for collection of urine samples. Urine was collected in receptacles containing thymol placed beneath the modified litter pans.14 The urinary bladder of each was emptied by manual expression, if required, at the beginning and end of the 24-hour period. Every 6 hours during the collection period, urine in collection receptacles, if present, was transferred to individual closed containers kept at 38°C in a shaker bath.

Urine collected over 24 hours was combined, mixed, and then separated into aliquots for analyte determination. Urine pH of pooled 24-hour samples was measured with a combination pH electrode.d,e One-milliliter aliquots of 24-hour urine were stored at 4°C for approximately 5 days, at which time sodium, potassium, chloride, phosphorus, magnesium, calcium, and creatinine were measured with an automated analyzer. Five-milliliter aliquots of 24-hour urine were stored at −20°C for approximately 7 days, at which time ammonia was measured with an ion-selective electrode.15,f One-milliliter aliquots of 24-hour urine were mixed with 19 mL of deionized water and stored at −20°C for approximately 5 days, at which time uric acid was measured with high-performance liquid chromatography.16,17 One-milliliter aliquots of 24-hour urine were stored at −70°C for approximately 14 days, at which time citric acid was measured by means of ion chromatography.18 Urine was acidified by adding 1 mL of 1M hydrochloric acid to 1.5 mL of 24-hour urine and stored at −70°C for approximately 14 days, at which time oxalic acid was measured with ion chromatography.19

URSS with calcium oxalate and struvite—Molar concentrations of urinary analytes measured previously were entered into a microcomputer-based program.g The relative supersaturation for lithogenic compounds was calculated.20

DEXA—Whole-body DEXA was performed after the 1-month baseline period and again at 6 months and 12 months. Following premedication with ketamineh (2 to 10 mg/kg), atropine sulfatei (0.05 mg/kg), and acepromazine maleatej (0.05 mg/kg) administered IM, an IV catheter was inserted and cats were anesthetized by IV injection of a bolus of thiopental sodiumk (10 to 16 mg/kg). A light plane of anesthesia was maintained by use of boluses of thiopental administered IV as needed. Endotracheal intubation was performed, and oxygen was supplied. Cats were positioned in sternal recumbency, and bone density was determined by use of DEXA.21,l

Statistical analysis—Data were analyzed with a statistical software packagem on a desktop computer. Analysis of variance was used to test the hypothesis that diet had no effect on mean results for each analyte excreted during the 24-hour collection period, urine volume, and pH of 24-hour urine samples, blood cell counts, serum biochemical analyses, blood gas values, plasma ionized calcium concentration, and bone mineral content and density. A value of P < 0.05 was considered significant. When differences in a single variable were attributed to diet, comparison among mean results of diet groups for that variable was performed by use of the Bonferonni-Dunn procedure. For this post hoc procedure, P < 0.017 (0.05/3 groups) was considered significant.

Results

Food intake and body weight—There was no difference in body weight (P = 0.7) or food intake (P = 0.8) among dietary groups over the study period (Table 2). In the acidifying diet group, food intakes after transition to the study diet and at 6 and 12 months were (mean ± SD) 75.7 ± 8.67 g/d, 75.5 ± 8.20 g/d, and 76.3 ± 8.44 g/d, respectively. In the neutral diet group, food intakes were 68.3 ± 11.8 g/d, 70.0 ± 12.8 g/d, and 71.1 ± 11.0 g/d, respectively. In the alkalinizing diet group, food intakes were 70.8 ± 10.3 g/d, 69.9 ± 8.94 g/d, and 70.4 ± 11.3 g/d, respectively.

Table 2—

Values (mean ± SD) for variables measured at 6 months and 12 months in healthy cats (4/diet group) fed an acidifying, neutral, or alkalinizing diet.

 Acidifying dietNeutral dietAlkalinizing diet 
Variable6 months12 months6 months12 months6 months12 monthsP value
Body weight (kg)5.40 ± 0.595.45 ± 0.605.00 ± 0.915.08 ± 0.794.99 ± 0.645.023 ± 0.810.7
Volume* (mL/kg/d)8.57 ± 2.059.71 ± 2.609.27 ± 2.2610.4 ± 2.159.16 ± 0.3010.1 ± 1.480.8
pH*6.19 ± 0.12a6.21 ± 0.08a7.02 ± 0.28b6.61 ± 0.12b7.22 ± 0.35c7.03 ± 0.38c< 0.001
Ammonia (mM/kg/d)*0.089 ± 0.0600.121 ± 0.1030.360 ± 0.3850.330 ± 0.2740.116 ± 0.1060.118 ± 0.0870.045
Calcium (mg/kg/d)*0.334 ± 0.2000.302 ± 0.3540.319 ± 0.1600.201 ± 0.0360.273 ± 0.1060.196 ± 0.0470.6
Chloride (mEq/kg/d)*2.06 ± 0.422.13 ± 0.472.14 ± 0.412.03 ± 0.241.54 ± 0.252.09 ± 0.300.2
Citrate (μg/kg/d)*236 ± 216288 ± 207239 ± 216434 ± 216209 ± 222458 ± 1790.8
Creatinine (mg/kg/d)*30.3 ± 2.2434.8 ± 10.832.8 ± 5.4234.9 ± 2.4033.3 ± 5.8038.5 ± 5.200.6
Magnesium (mg/kg/d)*0.77 ± 0.490.31 ± 0.200.62 ± 0.720.30 ± 0.190.52 ± 0.670.26 ± 0.170.8
Oxalate (μM/kg/d)*3.25 ± 1.343.09 ± 1.521.90 ± 1.483.53 ± 2.576.54 ± 3.562.73 ± 0.4020.3
Phosphorus (mg/kg/d)*11.5 ± 1.2811.5 ± 2.6414.0 ± 4.7314.6 ± 3.2413.7 ± 2.3116.3 ± 5.090.3
Potassium (mEq/kg/d)*1.33 ± 0.402.52 ± 0.811.33 ± 0.373.06 ± 0.821.39 ± 0.143.11 ± 0.690.7
Sodium (mEq/kg/d)*0.21 ± 0.060.61 ± 0.210.18 ± 0.150.65 ± 0.230.20 ± 0.141.12 ± 0.970.5
Uric acid (mg/kg/d)*24.6 ± 9.8541.6 ± 9.5236.3 ± 18.044.9 ± 11.628.3 ± 22.047.2 ± 6.910.6
Ccrendogenous (mL/min/kg)*4.57 ± 0.544.08 ± 0.714.73 ± 0.174.50 ± 0.244.05 ± 0.383.86 ± 0.570.07
URSS com*2.47 ± 1.38a1.40 ± 1.24a0.42 ± 0.36b0.59 ± 0.16b0.18 ± 0.19c0.29 ± 0.19c0.003
URSS map*0.13 ± 0.110.06 ± 0.031.32 ± 0.920.41 ± 0.110.97 ± 0.720.80 ± 1.080.09
Plasma iCa (mM/L)1.42 ± 0.071.39 ± 0.021.36 ± 0.031.37 ± 0.031.37 ± 0.051.38 ± 0.040.1

Value measured in urine.

Ccrendogenous = Endogenous creatinine clearance. Plasma iCa = Plasma ionized calcium concentration. URSS com = Urine relative superaturation for calcium oxalate monohydrate. URSS map = Urine relative superaturation for magnesium ammonium phosphate (struvite).

At each time point, values with different superscript letters differ significantly (P < 0.05) among diet groups.

CBCs, serum biochemical analyses, and plasma ionized calcium—Results of CBCs were within reference ranges for all dietary groups at all time points, and there was no significant difference among dietary groups or time points. Results of serum biochemical analyses and plasma ionized calcium concentration (Table 2) were within reference ranges for all dietary groups at all time points, and there was no significant difference among dietary groups or time points.

Twenty-four-hour urine variables—Urine pH values were significantly different among diet groups (Table 2) with the lowest urine pH values observed with the acidifying diet (6.2 ± 0.1) and the highest values with the alkalinizing diet (7.1 ± 0.3). Differences were not observed in other variables.

URSS with calcium oxalate and struvite—There was a significant difference in URSS of calcium oxalate among the 3 diets with the highest saturation in cats consuming the acidifying diet and the lowest saturation in cats consuming the alkalinizing diet (Table 2). Urinary saturation with struvite was not different among groups.

DEXA—No significant differences among diet groups were detected for bone mineral content, density, or calcium content (Table 3). No significant changes over time occurred.

Table 3—

Results (mean ± SD) of DEXA measurements obtained at baseline, 6 months, and 12 months in healthy cats (4/diet group) fed an acidifying, neutral, or alkalinizing diet.

 Acidifying dietNeutral dietAlkalinizing 
VariableBaseline6 months12 monthsBaseline6 months12 monthsBaseline6 months12 monthsP value
Bone mineral density (g/cm2)0.58 ± 0.020.62 ± 0.010.63 ± 0.010.60 ± 0.020.62 ± 0.020.64 ± 0.020.58 ± 0.010.61 ± 0.020.64 ± 0.050.9
Total bone calcium (g)39.8 ± 7.148.8 ± 5.949.8 ± 5.341.8 ± 11.446.3 ± 10.452.8 ± 11.538.5 ± 3.144.5 ± 5.847.8 ± 8.20.7

Discussion

Urolith formation can occur when urine is supersaturated with calculogenic minerals in association with decreased crystallization and calculogenic inhibitors, increased crystallization and calculogenic promoters, or a pH that decreases mineral solubility. Urine pH is known to influence the potential for urolith formation for several minerals. Aciduria directly increases the risk of urate and cystine urolith formation,22–24 and alkaluria directly increases the risk for struvite urolith formation.9,25–27

Diets used in the present study were similar except for their alkalinizing or acidifying potential, and results supported a pH effect on urinary saturation with calcium oxalate in healthy cats. The highest urine pH was associated with the lowest urinary saturation with calcium oxalate, and the lowest urine pH was associated with the highest urinary saturation with calcium oxalate; the neutral urine pH was associated with an intermediate saturation with calcium oxalate. This effect of urine pH on urinary saturation with calcium oxalate occurred despite a lack of significant difference in urinary excretion of calcium, oxalate, and citrate, although urinary calcium excretion was highest with the acidifying diet and lowest with the alkalinizing diet. Calcium oxalate solubility increases with neutral to alkaline urine pH in humans.28 On the basis of the results of the present study, inducing a urine pH greater than approximately 7.0 appears beneficial in decreasing urinary saturation with calcium oxalate, which may decrease the risk of calcium oxalate urolith formation.

The alkalinizing diet contained potassium citrate, whereas the acidifying and neutral diets did not. Potassium citrate, a urinary alkalinizing agent, is used in humans with calcium oxalate uroliths, particularly those with hypocitraturia29 because citrate is an inhibitor of calcium oxalate crystal formation.30–32 Potassium may also have a role in calcium oxalate urolith formation, and dietary potassium depletion in rats decreases urinary citrate concentrations and increases urinary saturation for calcium oxalate.33 There have been no reported studies evaluating potassium citrate in cats and only 2 studies34,35 in dogs; the latter did not reveal a significant influence on urinary variables in healthy dogs, although 3 Miniature Schnauzers in one of the studies,34 a breed predisposed to calcium oxalate urolith formation, had lower values of urinary saturation with calcium oxalate when receiving potassium citrate. Despite the alkalinizing diet that contained potassium citrate, excretion of urinary citrate was not different among diet groups, although it was lowest in the acidifying diet group.

Although not significantly different, urinary saturation for struvite was lowest with aciduria and highest with alkaluria. Struvite solubility decreases in urine with pH > 6.8.36,37 Despite alkaluria, urinary saturation with struvite was < 1.0 in most cats, which is consistent with undersaturation of urine with struvite. The computer program used to estimate urinary saturation may overestimate urinary saturation with struvite in dogs and cats; therefore, urine from the cats in the present study may have been less saturated with struvite than the values implied.38 Additionally, there was a great degree of variability in the data and a small number of cats in each diet group, which may have lowered the probability of detecting a significant difference.

A potential mechanism for hypercalciuria and increased risk of calcium oxalate urolith formation is chronic mild metabolic acidosis resulting in increased calcium mobilization from bone.2,39,40 This effect was not observed in the present study; bone mineral density, estimated bone calcium content, and 24-hour urinary calcium excretion were not different among dietary groups over the 12-month period. Dual energy x-ray absorptiometry has been validated for use in cats for determining body composition,21,41–43 and bone mineral content and bone calcium content value are significantly correlated with values obtained by chemical analysis.21 It is possible that the degree of acid ingestion was not sufficient to induce metabolic acidosis and calcium mobilization, although blood pH was not determined to evaluate this; that this effect is minimal in cats; or that 12 months was not a sufficient period to observe this phenomenon.

The present study had limitations. The sample size was small, with 4 cats (2 females and 2 males) in each dietary group. Although no change in bone mineral density was found during the 12-month study, a longer study might reveal a change in bone mineral density. Systemic acid-base status was not evaluated. Additionally, young adult cats were evaluated and the risk for calcium oxalate urolith formation7 and urine saturation with calcium oxalate increases with increasing age.44 Future studies should include older cats and cats that form calcium oxalate uroliths.

Another limitation was that baseline urinary saturation data were not collected from cats, although laboratory evaluation and body composition determination were performed. Because the study was designed to evaluate the influence of urine pH on urinary saturation as well as other biochemical variables, comparison with baseline data obtained when cats were consuming an adult maintenance food was deemed unnecessary. The 3 study diets differed from the adult maintenance diet in both formulation and ingredients; therefore, comparison with data collected when cats were consuming the adult maintenance food would be difficult to interpret because of confounding factors, although it may have yielded interesting information. The objective of the study was to compare the influence of diets that induced different urine pH values but that did not differ in other aspects on urinary saturation, biochemical variables, and body composition variables; thus, comparison with a basal diet was not performed.

Urinary saturation for calcium oxalate was influenced by urine pH in young non–urolith-forming adult cats, with the highest solubility occurring with alkaluria and the lowest solubility occurring with aciduria. Results of this study provided some support for formulation of a diet that decreases urinary saturation with calcium oxalate without inducing supersaturation with struvite at a urine pH of approximately 7.0. On the basis of the results of this study, inducing a neutral to slightly alkaline urine pH has the potential to aid in preventing calcium oxalate urolith formation in cats.

ABBREVIATIONS

DEXA

Dual energy x-ray absorptiometry

URSS

Urine relative supersaturation

a.

Science and Technology Center, Hill's Pet Nutrition Inc, Topeka, Kan.

b.

Science Diet Feline adult maintenance, Hill's Pet Nutrition Inc, Topeka, Kan.

c.

Quad cages equipped with metabolic pans, BH Inc, Wheatland, Wyo.

d.

pH Meter 245, Corning Glass Works Science Products Division, Corning, NY.

e.

Model SA-720 pH/ISE Meter, Orion Research Inc, Boston, Mass.

f.

Model 95–12 ammonia electrode, Orion Research Inc, Boston, Mass.

g.

EQUIL 89d, Department of Biochemistry and Molecular Biology, College of Medicine, University of Florida, Gainesville, Fla.

h.

Ketaset, Fort Dodge Animal Health Inc, Fort Dodge, Iowa.

i.

Atropine for injection, Fort Dodge Animal Health Inc, Fort Dodge, Iowa.

j.

Promace, Fort Dodge Animal Health Inc, Fort Dodge, Iowa.

k.

Pentothal, Ceva Animal Health Ltd, Overland Park, Kan.

l.

Lunar DPX, Small Animal Software, version 3.65. Lunar Corp, Madison, Wis.

m.

Statview, version 4.0, Brainpower Inc, Calabassas, Calif.

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