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Article

Sodium Requirement of Adult Cats for Maintenance Based on Plasma Aldosterone Concentration

Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616

	ABSTRACT

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The sodium requirement of adult cats for maintenance was determined using a randomized block design of eight dietary sodium treatments (0.1, 0.4, 0.5, 0.66, 0.8, 1.2, 1.6 or 2.0 g Na/kg in a casein–lactalbumin-based purified diet) administered for periods of 4 wk. A total of 35 adult specific-pathogen-free domestic shorthaired cats (26 males and 9 females, 1.5–3 y of age) was given an equilibration diet (2 g Na/kg) for 14 d before assignment (or reassignment) to the treatments. A total of 12 cats (8 males, 4 females) was randomly assigned to the lowest six levels of sodium, and four cats to the highest two sodium levels. Cats consuming the diet containing 0.1 g Na/kg had significantly elevated aldosterone concentration in plasma, and packed cell volume. In addition, these cats exhibited anorexia, body weight loss, reduced urinary specific gravity and sodium excretion, and had a negative sodium balance. However, adult cats did not develop polydypsia and polyuria reported in sodium-deficient kittens. Cats given the diet containing 0.66 g Na/kg did not have an increased packed cell volume, but aldosterone concentration in the plasma was significantly elevated. However, cats given diets containing 0.8 g Na/kg had plasma aldosterone concentrations 0.7 nmol/L (reference value for sodium-replete cats) and normal packed-cell volumes. A minimal sodium requirement of adult cats for maintenance of 0.8 g Na/kg diet (energy density = 22 kJ/g diet) or 0.4 mmol Na · kg body weight^-1 · d^-1 is proposed.

KEY WORDS: • cats • sodium • aldosterone • requirement

	INTRODUCTION

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In the absence of data from cats, a sodium requirement based on other mammals of 0.5 Na /kg diet was proposed by the National Research Council (1986) for growing kittens. Yu and Morris (1997) reported that this dietary concentration was inadequate and proposed a minimal sodium requirement of 1.6 g Na/kg diet (energy density = 22 kJ ME/g diet) for growing kittens, a value about three times that suggested by the National Research Council. The objective of this study was to determine the minimal sodium requirement of adult cats for maintenance based on changes in physiological measurements including aldosterone concentration in plasma.

	MATERIALS AND METHODS

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The experimental protocol adhered to the Guide for the Care and Use of Laboratory Animals (National Research Council 1985 ) and was approved by the Animal Use and Care Administrative Advisory Committee of the University of California at Davis.

Animals and their management.

A total of 35 (26 males and 9 females) specific-pathogen-free domestic short-hair adult cats (1.5–3 yr of age) from the Feline Nutrition and Pet Care Center of the University of California, Davis, was used. The cats were individually housed in stainless steel metabolic cages (60 x 60 x 60 cm) in temperature-controlled rooms (21 ± 2°C) with a light/dark cycle of 14:10. They had free access to the experimental diets (provided in stainless steel food bowls) and deionized water in plastic bottles.

Diets.

Experimental diets were made by adding various amounts of sodium (as NaCl that does not alter acid-base status of cats) to a casein-lactalbumin-based purified basal diet at the expense of dextrose. The basal mixture contained (g/kg): casein (New Zealand Milk Products, Petaluma, CA), 222.5; lactalbumin (New Zealand Milk Products), 222.5; rendered animal tallow (Florin Tallow, Dixon, CA), 300; sucrose, 100; corn starch (Melojel, Bridgewater, NJ), 90.5; taurine (Ajinomoto USA, Raleigh, NC), 1.5; choline chloride (International Mineral and Chemical, Terre Haute, IN), 3.0; vitamin mixture, 10 (Williams et al. 1987 ); and mineral mixture, 42.577 (containing CaHPO₄, 19.529; MgSO₄, 2.25; KCl, 10; K₂HPO₄, 4.5; CaCO₃, 5.50; MnSO₄ · H₂O, 0.019; FeSO₄ · 7H₂O, 0.47; NaF, 0.007; Kl, 0.0015; ZnSO₄ · 7H₂O, 0.223; CuSO₄, 0.04; SnSO₄, 0.005; NaSeO₃, 0.0015; (NH₄)₆Mo₇O₂₄ · 4H₂O, 0.002; CrCl₃ · 6H₂O, 0.01) dextrose anhydrous 7.347. The basal diet contained all nutrients (including chlorine) in amounts sufficient to meet the requirements of adult cats, with the exception of sodium. Experimental diets containing 0.10, 0.40, 0.50, 0.66, 0.80, 1.20, 1.60 and 2.00 g Na/kg were made by the substitution of the dextrose with 0.076, 0.839, 1.093, 1.507, 1.856, 2.873, 3.890 and 4.906 g NaCl/kg diet. Dietary sodium concentrations were verified by atomic absorption spectrophotometry. Diets were stored at 4°C after preparation until they were fed.

Experimental design.

A stratified random design with sex and body weight as criteria of classification was employed. A total of 12 cats (8 males and 4 females) was tested at each of the six lowest dietary sodium concentrations, and 4 cats at each of the two highest concentrations of sodium (2 males and 2 females for the diet containing 1.6 g Na/kg diet, and 3 males and 1 female for dietary sodium concentration of 2.0 g Na/kg diet). Because 80 adult cats and metabolism cages were not available, the 35 cats in this study received more than one dietary treatment. As cats entered the study or came from a previous test diet, they underwent an equilibration period of 14 d, during which time they were given a sodium-replete purified diet (2 g Na/kg diet) and tap water. At the end of this period (d 0 of the experiment), the cats were randomly assigned to a dietary treatment. The duration of all dietary treatment was 28 d. A broken-line method (Robbins 1986 ) was used to estimate the minimal sodium requirement.

Sample collection.

Food and water intakes were measured daily, and body weight was monitored weekly. Heparinized blood samples were taken from the jugular vein of unanesthetized cats at d 0 and 21 (for the measurement of aldosterone in the plasma and packed-cell volume) and d 28 of the experiment (for all measurements on blood). Blood samples were centrifuged at 1100 x g for 20 min and plasma was separated. Complete urine and fecal collections were made during the last 7 d of the experimental period for the measurement of sodium balance. All samples were stored at -20°C until analysis.

Sample analysis.

Aldosterone in plasma was assayed using a commercially available radioimmunoassay kit (Coat-A-Count, Diagnostic Products Corporation, Los Angeles, CA) and a counter (COBRA, Packard Instrument Company, Downers Grove, IL). Urine specific gravity was measured with a hydrometer (Baxter Diagnostics Inc., McGaw Park, IL). Packed cell volume was measured in a microhematocrit capillary tube centrifuged for 5 min (Robertshaw Lux, Waterbury, CT). An atomic absorption spectrophotometer (Model 3030B; Perkin-Elmer, Clay Adams, NJ) was used to analyze sodium concentrations in plasma, urine, feces and diets. Sodium concentrations in plasma and urine were measured directly after dilution of the samples with deionized water. Preparatory to analysis, feces were dried in a vacuum oven at 80°C for 48 h and ground. Diet samples and dried feces were digested in 16 mol/L HNO₃ at 120°C for 2 h, then diluted with deionized water, and the sodium was measured by an atomic absorption spectrophotometer.

Statistical analysis.

All statistical analyses were performed according to Steel and Torrie (1980) , using SPSS/PC+, Version 2.0 (SPSS Inc., Chicago, IL) and PC-SAS (for broken-line analysis), Version 6.03 (SAS, Cary, NC). The data were subject to a two-way ANOVA with diet and sex as main effects. Aldosterone concentration in plasma (d 28), fecal sodium output, urinary sodium excretion and sodium retention data were logarithmically transformed before the two-way ANOVA test because of heterogeneous variances (Bartlett's test). Kruskal-Wallis test was used in the analysis of water intake (g/d at wk 0 and g/g food intake at wk 4), sodium intake and apparent sodium absorption (fraction of intake and mmol/d). Paired Student's t tests were used to compare the data between the beginning (d 0 or wk 0) and the end (d 28 or wk 4) of the experiment for each dietary treatment. To estimate the minimal sodium requirement, break points from the broken lines constructed using plasma aldosterone concentration (d 28) and packed cell volume (d 28) as a function of dietary sodium concentration were calculated using a nonlinear least square method (Robbins 1986 ). Pooled SEM for each variable is given if the variances between dietary groups were homogeneous. Otherwise, a separate SEM is presented for each dietary group. Probability levels P < 0.05 were considered significant for all tests.

	RESULTS

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Body weight, food and water intakes.

Cats lost an average of 0.1 kg body weight over a 4-wk period when given the diet containing 0.1 g Na/kg diet, but maintained body weight during the experimental period when the dietary sodium concentration was 0.4 g Na/kg diet (Table 1 ).Food intake was significantly reduced when dietary sodium concentration was 0.4 g Na/kg diet, while both absolute water intake and the ratio of water intake to food intake were not affected by the sodium concentrations in the diets tested (Table 1) . As expected, female cats had lower body weights and food and water intakes than male cats (separate data for male and female cats are not shown).

At the end of the experiment, dietary sodium concentration did not affect the quantity of urine produced, but urinary specific gravity was significantly lower in the adult cats consuming diets containing 0.4 g Na/kg (Table 1) . The cats fed the diet containing 0.5 g Na/kg diet had a lower urine production and greater urinary specific gravity at d 28 when compared to d 0 (Table 1) .

Sodium and aldosterone concentrations in the plasma and packed cell volume.

Sodium concentration in the plasma was not affected by the dietary treatment (Table 1) . However, both aldosterone concentration in the plasma (Fig. 1 )and packed cell volume (Fig. 2 )were significantly elevated in cats at d 28 when dietary sodium concentration was 0.5 g Na/kg diet. There was no difference in packed cell volume between d 21 and 28 for any groups. Plasma aldosterone concentrations of cats given diets containing 0.5 g Na/kg diet were not different at d 21 and 28 for each dietary treatment, indicating that these cats were stable in aldosterone secretion. However aldosterone concentration tended (P > 0.05 to be higher at d 28 than d 21 when dietary sodium concentrations were 0.4 and 0.1 g Na/kg diet (Fig. 1) , indicating an exacerbation of sodium deficiency. Calculated break points for plasma aldosterone concentration (d 28) and packed cell volume (d 28) were 0.57 and 0.69 g Na/kg diet, respectively.

View larger version (19K): [in this window] [in a new window]   Figure 1. Plasma aldosterone concentration of adult cats given purified diets with various levels of sodium for 0, 21 and 28 d. Each point is a mean with a vertical bar representing SEM for cats in each dietary group (see Table 1 for the number of cats in each dietary group). There was a significant diet effect at d 28 (P < 0.05, ANOVA). Break point for d 28 was 0.57 g of Na/kg diet (asymptotic SEM = 0.012; 95% CI 0.54, 0.59). *: Significantly different from d 0 (P < 0.05, paired Student's t test).

View larger version (20K): [in this window] [in a new window]   Figure 2. Packed cell volume of adult cats given purified diets with various levels of sodium for 0, 21 and 28 d. Each point is a mean with a vertical bar representing SEM for cats in each dietary group (see Table 1 for the number of cats in each dietary group). There was a significant diet effect at d 28 (P < 0.05, ANOVA). Break point for d 28 was 0.69 g of Na/kg diet (asymptotic SEM <0.001; 95% Cl 0.69, 0.69). *: Significantly different from d 0 (P < 0.05, paired Student's t test).

Sodium output in feces (about 0.5 mmol/d) was not significantly affected by dietary sodium concentrations ranging from 0.1 to 2.0 g Na/kg diet. (Table 2 ).Sodium output in urine was directly related to dietary sodium concentration. Cats given diets containing 0.4 g Na/kg had negative sodium balances [sodium intake - (fecal sodium output + urinary sodium output)].

	DISCUSSION

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Plasma aldosterone and packed cell volume significantly increased in cats when the concentration of sodium in the diet decreased and were the most sensitive indices of sodium status. These measurements were also the most sensitive indices of sodium deficiency in growing kittens (Yu and Morris 1997 ). Sodium-deficient adult cats also exhibited anorexia, body weight loss, hyponatriuria, decreased urinary specific gravity and negative sodium balance, but not the polydypsia and polyuria observed in growing kittens.

Water intake is controlled by the thirst center in the brain that responds to volume and pressure changes of the extracellular fluid (Koeppen and Stanton 1996 ), and is stimulated by elevated angiotensin II concentration in the plasma (McKinley et al. 1992 ). Water intake of most mammals is closely correlated with food intake. Cats consuming dry cat food drink about 1.5–2.0 mL of water per gram of food consumed (Burger et al. 1980 , Kane et al. 1981 ). Water intake was about 1.7 g/g of food intake for cats given the diet containing 0.5 g Na/kg even though plasma aldosterone concentration and packed cell volume were significantly elevated. Increased aldosterone concentration was probably the result of activation of the renin–angiotensin–aldosterone system (Koeppen and Stanton 1996 ) while elevated packed cell volume probably reflected decreased volume of plasma and extracellular fluid though we did not measure plasma osmolality. Apparently, the reduced volume of extracellular fluid of cats fed the sodium-deficient diet (0.1 g Na/kg) did not stimulate water intake.

Independent of sodium intakes, adult cats had fecal sodium output of about 0.5 mmol/d which was not significantly different due to treatment (Table 2) . These losses were similar to the fecal sodium losses of growing kittens (0.6 mmol/d) fed similar purified diets (Yu and Morris 1997 ), suggesting the obligate fecal sodium loss of cats fed the purified diets. Finco et al. (1989) reported fecal sodium losses of 1 and 1.5 mmol/d in male adult cats fed commercial dry cat food, when sodium intakes were 17 and 39 mmol/d, respectively. Compared to purified diets, commercial dry cat food diets usually have higher amounts of indigestible matter that resulting in greater fecal volume, fecal water and hence fecal sodium excretion, resulting in reduced apparent sodium absorption (Partridge 1975 ).

Definition of a nutrient requirement requires that it be based on one or more physiological responses, and generally, the response with the highest requirement determines the requirement. Of the measurements taken, plasma aldosterone concentration and packed cell volume were the most responsive to dietary sodium concentration. Similar packed cell volumes and plasma aldosterone concentrations between d 21 and 28 for each dietary treatment (0.5 g Na/kg diet for aldosterone) suggested stabilized responses to dietary sodium concentrations. These variables were used to construct broken lines as a function of dietary sodium concentration. The estimated break points for plasma aldosterone concentration and packed cell volume at d 28 were 0.57g Na/kg (asymptotic SEM = 0.012; 95% Cl 0.54, 0.59) and 0.69 g Na/kg diet (asymptotic SEM <0.001; 95% Cl 0.69, 0.69), respectively.

Packed cell volume of cats given the diet containing 0.66 g of Na/kg was similar to those of cats consuming dietary sodium concentrations above 0.66 g of Na/kg diet, suggesting that sodium requirement of adult cats based on packed cell volume can be met at 0.66 g of Na/kg diet (energy density = 22 kJ of ME/g diet, calculated according to the modified Atwater values of 16.7, 33.7 and 16.7 kJ/g for protein, fat and carbohydrate, respectively). However, plasma aldosterone concentration of cats given the diet containing 0.66 g of Na/kg was 1.12 ± 0.2 nmol/L, which is above the reference level we reported in sodium-replete adult cats (0.7 nmol/L, n = 148). Only when cats were given diets containing 0.8 g of Na/kg, were plasma aldosterone concentrations 0.7 nmol/L. Therefore, if plasma aldosterone concentration is taken as the criterion of sodium adequacy, a minimal dietary sodium concentration of 0.8 g of Na/kg diet (energy density = 22 kJ/g diet) is necessary. As the mean body weight and food intake of cats fed diets containing 0.8 g Na/kg were 3.85 kg (d 28) and 42 g/d (wk 4, Table 1 ), the minimal sodium requirement of adult cats for maintenance is equivalent to 0.4 mmol Na · kg body weight^-1 · d^-1.

Cats have a higher sodium requirement for maintenance than the value of 0.5 g of Na/kg diet proposed by the National Research Council (1986) based on the requirements of other mammals. As both sodium-depleted and repleted cats do not exhibit sodium appetite (Yu et al. 1997 ) and will not select foods on the basis of their sodium content to correct a deficiency, it is essential that feline diets contain adequate concentrations of sodium.

	ACKNOWLEDGMENTS

 
We acknowledge the donation of the vitamin mixture by Hoffmann-La Roche, Inc., Nutley, NJ and the assistance of Yulin Wu in the care of the cats and analysis of samples.

	FOOTNOTES

 
¹ To whom correspondence should be addressed.

¹ Supported in part by Hills Pet Nutrition, Inc., Topeka, KS.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked ''advertisement'' in accordance with 18 USC section 1734 solely to indicate this fact.

Manuscript received August 14, 1998. Initial review completed October 13, 1998. Revision accepted November 4, 1998.

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