Sodium is the predominant electrolyte in the extracellular fluid (ECF) and plays a major role in the maintenance of membrane potentials, osmotic pressure, and the ECF volume of the body. Sodium homeostasis in mammals is primarily regulated by the renin-angiotensin-aldosterone system, the terminal hormone of the cascade being aldosterone secreted by the zona glomerulosa of the adrenal cortex (Koeppen and Stanton 1996).

The minimum sodium requirement of kittens for growth has not been experimentally defined. On the basis of very limited data and the requirements of other small mammals, the National Research Council (1986) proposed a sodium requirement of 0.5 g Na/kg diet for both kittens for growth and adult cats for maintenance. The objective of the present study was to define the sodium requirement of kittens for growth. Two experiments were conducted: Experiment 1 (pilot experiment) to measure the changes of variables associated with sodium deficiency in kittens and Experiment 2 (main experiment) to measure the minimum sodium requirement of kittens for growth.

MATERIALS AND METHODS

Table 1. Ingredients of experimental diets [View Table]

Table 2. Body weight, food, water and sodium intakes, urine production, urinary specific gravity and sodium output, and plasma sodium concentration in kittens during sodium depletion and repletion (Experiment 1)1 [View Table]

In Experiment 1, one-way ANOVA was used to test diet effects followed by Tukey test to compare significant differences of variables among d 0, 12 and 26 if significance was detected by the one-way ANOVA. Water intake, the ratio of water intake to food intake, sodium intake, urine production and urinary aldosterone output data were logarithmically transformed before the data were tested by one-way ANOVA because of unequal variances among groups (Bartlett's test). Plasma aldosterone concentration and urinary sodium output data were tested by the Kruskal-Wallis test followed by the Mann-Whitney U test if the Kruskal-Wallis test showed significant differences.

Data of Experiment 2 were subject to three-way ANOVA with diet, period and sex as main effects. Pooled sex data for each variable are given if there was no sex effect; otherwise, both pooled and separate sex data are presented. Aldosterone concentration in plasma, body weight gain, water intake, the ratio of water intake to food intake, sodium intake, and apparent sodium absorption (mmol/d) data were logarithmically transformed before the three-way ANOVA test because of heterogeneous variances (Bartlett's test). The Kruskal-Wallis test was used for urine production, urinary sodium output, apparent sodium absorption (percentage of intake) and sodium retention data. Break points of selected variables were calculated using a nonlinear least square method (Robbins 1986) if diet effect was significant (three-way ANOVA).

RESULTS

Table 3. Food intake, body weight, and plasma sodium concentration of kittens fed purified diets with various levels of sodium1 [View Table]

Both absolute water intake (Fig. 3, panel A) and urine production (Fig. 3, panel C) were elevated and urinary specific gravity (Fig. 3, panel D) was reduced in kittens fed diets containing less than 1.5 g Na/kg diet (P < 0.05, ANOVA for water intake and urinary specific gravity, and Kruskal-Wallis test for urine production). However, kittens fed the diet containing 0.6 g Na/kg had significantly higher absolute water intake (P = 0.034) and urine production (P = 0.020) and lower urinary specific gravity (P = 0.043) compared with those fed the diet of higher dietary sodium concentration (2.1 g Na/kg diet) (Student's t test). Both water intake (Fig. 3, panel A) and urine production (Fig. 3, panel C) of male kittens were significantly higher than those of female kittens. The diet containing 0.6 g Na/kg significantly elevated (P = 0.033, ANOVA) the ratio of water intake to food intake (Fig. 3, panel B).

The sodium balance values are presented in Table 4. Although sodium intake directly reflected the sodium concentrations in the diets in a linear fashion, sodium output in feces (approximately 0.6 mmol/d) was constant in kittens fed diets containing 0.6 to 2.1 g Na/kg diet. Apparent sodium absorption (Na intake  Fecal Na loss) and urinary sodium excretion increased with the increase of sodium intake. However, the increase in apparent sodium absorption (percentage of Na intake) reached a plateau after dietary sodium concentration reached 1.8 g Na/kg diet. Sodium retention (Na intake  Fecal Na loss  Urinary Na loss) of pooled sexes reached a plateau when dietary sodium concentration was more than 0.9 g Na/kg diet (Fig. 4). Sodium intake, fecal loss (Table 4) and retention (Fig. 4) of male kittens were significantly higher than those of female kittens.

Table 4. Sodium balance of kittens fed purified diets with various levels of sodium1 [View Table]

Both plasma aldosterone concentration (Fig. 5, panel A) and packed cell volume (Fig. 5, panel B) were significantly elevated when dietary sodium concentrations were lower than 1.5 and 1.2 g Na/kg diet, respectively. Sex did not affect aldosterone concentration in plasma or packed cell volume. Plasma aldosterone concentration in kittens fed a diet containing 1.2 g Na/kg diet was significantly elevated compared with that in kittens fed a diet containing 1.5 g Na/kg diet (P = 0.012, Mann-Whitney U test) (Fig. 5, panel A).

Using selected variables, we estimated the minimum sodium requirement of kittens for growth by means of a broken line technique, with break points calculated by nonlinear least square analysis (Robbins 1986). These estimated minimum sodium requirements of kittens for growth are summarized in Table 5. The minimum sodium requirements ranged from 0.80 to 1.35 g Na/kg diet, depending on the variables used.

Table 5. Estimated minimum sodium requirement of kittens for growth using a break point constructed with selected variables1 [View Table]

DISCUSSION

Because male kittens grow faster than female kittens, males had higher body weights, rates of body weight gain, and food intake. As a result, male kittens also had a higher mean water intake, urine production, sodium intake and retention, and fecal sodium loss. The other measured variables were not affected by sex.

Cats fed dry cat food drink approximately 1.5-2.0 mL of water per gram of food consumed (Burger et al. 1980, Kane et al. 1981). Kittens fed the sodium-deficient diet had a slightly elevated absolute water intake (g/d) but a greatly augmented relative water intake (g water/g food intake). Reduced urinary specific gravity resulted from a lower solute load from food accompanied by a higher urine production that resulted from the enhanced water intake.

Polydypsia and polyuria in sodium-deficient kittens are the consequence of reduced total sodium in the ECF. The reduced total amount of sodium in the ECF induces a decreased volume and pressure of ECF, which stimulate the thirst center of the brain, causing an increase in water intake (Koeppen and Stanton 1996). A decreased amount of sodium delivered to the macula densa of the kidney increases aldosterone secretion via the renin-angiotensin-aldosterone system (Koeppen and Stanton 1996). Elevated concentration of angiotensin II has a thirst-inducing effect (McKinley et al. 1992), which in turn increases water intake and results in hypoosmolality of the ECF that stimulates osmoreceptors in the hypothalamus to inhibit antidiuretic hormone secretion, leading to polyuria (Koeppen and Stanton 1996).

Plasma sodium concentration is a poor indicator of sodium status of kittens and was unchanged even when the plasma aldosterone was elevated markedly (26 times the initial level in kittens fed the sodium-deficient diet for about 2 wk). The sodium-deficient kittens maintained plasma sodium concentration constant, presumably at the expense of a reduced ECF volume as reflected in the hemoconcentration of these kittens.

Aldosterone concentration in plasma and urinary aldosterone output at the end of depletion period were 26 and 18 times the initial levels, respectively. Aldosterone concentration in plasma responded rapidly to sodium intake and doubled after the kittens were fed the sodium-deficient diet (0.1 g Na/kg diet) for only 1 d (data not shown). These observations indicate that plasma aldosterone concentration is a sensitive and reliable indicator of sodium status of kittens. Because measurement of plasma aldosterone concentration is simpler than the measurement of aldosterone output in urine, which requires quantitative urine collection, only plasma aldosterone was measured in Experiment 2. 

The broken line technique has been widely used to measure nutrient requirements of animals (Hammer 1996, Morris and Peterson 1975, Robbins et al. 1979). The assumption of the broken line method is that the physiological response measured is linearly related to the added nutrient under study until a point (break point) where the requirement is met. The break point can be estimated objectively by using nonlinear least square analysis (Robbins 1986).

The National Research Council (1986) suggested a sodium requirement of 0.5 g Na/kg diet for both kittens and cats. In our pretrial, we found that this recommendation was too low for growing kittens (data not shown). Thus, the lowest dietary sodium concentration we tested in the main experiment (Experiment 2) was 0.6 g Na/kg diet, which was inadequate for growing kittens.

The minimum sodium requirement of growing kittens ranged from 0.80 to 1.35 g Na/kg diet, depending on the variable selected (Table 5). The sodium requirement was met when dietary sodium concentration was 1.0 g Na/kg diet, according to most of the variables (body weight gain, feed utilization efficiency, sodium retention, and water intake/food intake) but not according to packed cell volume and plasma aldosterone concentration. The curves of plasma aldosterone concentration and packed cell volume reached a plateau when dietary sodium concentration was 1.5 g Na/kg diet.

Aldosterone controls sodium reabsorption in the kidney (Earley and Daugharty 1969, Koeppen and Stanton 1996) and induces antidiuresis and retention of sodium in isolated perfused cat kidneys (Lockett 1967). For a marginal sodium intake, sodium balance can be achieved by an elevated aldosterone secretion that reduces sodium loss in urine. It seems that sodium requirement for body weight gain of the kittens was met at the expense of enhanced aldosterone secretion.

In a separate study, we measured plasma aldosterone concentration in 148 apparently healthy specific-pathogen-free cats (age ranged from 8 wk to 3 y) fed either purified diets (containing adequate amounts of all nutrients for cats) or commercial dry cat foods with sodium concentrations ranging from 2 to 6 g Na/kg diet. We found that the median plasma aldosterone concentration was 0.2 nmol/L (SD = ± 0.2) with a 95th percentile of 0.7 nmol/L. There were no sex or age effects on aldosterone concentration in plasma.

In Experiment 2, plasma aldosterone concentration in kittens fed the diet containing 1.5 g Na/kg diet was lower than 0.7 nmol/L, whereas in kittens fed the diet containing 1.2 g Na/kg diet the value was higher (Fig. 5, panel A) than 0.7 nmol/L, suggesting the sodium requirement of growing kittens was higher than 1.2 g Na/kg diet.

Packed cell volume was significantly greater in kittens fed diets with sodium concentrations lower than 1.2 g Na/kg diet (Fig. 5, panel B). A similar change in packed cell volume was observed in the sodium-deficient broiler chickens (Walicka et al. 1979). In sodium deficiency, a reduction in total sodium content is accompanied by a decreased ECF volume so that constant osmolality can be maintained. Decreased total ECF volume in sodium-deficient kittens was reflected in an increase in packed cell volume.

It seems that in cats, as in other animals, sodium balance is primarily regulated by sodium excretion in the kidney, rather than by changes in the efficiency of sodium absorption in the intestine. Urinary sodium output increased with an increase in sodium intake, whereas fecal sodium output was maintained at about 0.6 mmol/d and was independent of sodium intake. These fecal losses of sodium are less than those described by Finco et al. (1989), who reported fecal sodium losses of adult cats of 1 and 1.5 mmol/d when the sodium intakes were 17 and 39 mmol/d, respectively. The reason for the difference in measured fecal loss is not apparent. When dietary sodium concentration was 1.5 g Na/kg diet or greater, the apparent sodium absorption was about 86% of intake, similar to that reported by Ching et al. (1989) in adult cats ingesting 3.46 mmol Na per day per kilogram of body weight.

In addition to hormonal effects on sodium homeostasis, other factors such as total fecal sodium loss and anion-cation balance also alter sodium metabolism and its requirement. A cereal-based diet containing 2.4 g Na/kg diet vs. a purified diet containing 2.7 g Na/kg diet reduced apparent sodium absorption by 17% in growing pigs (Partridge 1975a), and a high cellulose diet lowered sodium content in ileal digesta of growing pigs (Partridge 1975b). Ammonium chloride, a urine-acidifying reagent used in therapeutic cat foods, increases sodium excretion in the urine of cats (Ching et al. 1989). High temperature could increase the sodium requirement of kittens because of an extra sodium loss in saliva caused by panting and in the sweat secreted from paws (Dobson and Slegers 1971), but these losses are minor when compared with those in urine and feces.

Because many factors affect the sodium requirement of growing kittens, we suggest a minimum sodium requirement of kittens for growth of 1.6 g Na/kg diet, which was derived from the upper asymptotic 95% confidence interval level of the break point generated from the curve of plasma aldosterone concentration as a function of dietary sodium concentration (Fig. 5, panel A, and Table 5). The calculated metabolizable energy (ME) density of our experimental diets was 22 kJ ME/g diet, based on the modified Atwater values of 16.7, 37.7 and 16.7 kJ/g for protein, fat and carbohydrate, respectively. The minimum sodium requirement of growing kittens on an energetic basis is 0.07 mg Na/kJ ME, or 34 mg Na·kg body wt¹·d¹ [Food intake × 1.6 mg Na/g diet·body weight in kg), where food intake is the mean food intake of kittens fed the diets containing 1.5-2.1 g Na/kg diet and body weight is the mean body weight of kittens fed the same diet at d 14 and 21] on the basis of sodium intake.

An estimation of the minimum sodium requirement of adult cats for maintenance can be calculated from that of growing kittens by subtracting sodium needed for growth. Spray and Widdowson (1950) reported that sodium concentration in fat-free body tissue of cats was highest at birth (about 104 mmol/kg), declined rapidly to 74 mmol/kg at 120 d of age, and continuously decreased to 65 mmol/kg until maturity of cats. If we assume that sodium concentration in whole body was about 74 mmol/kg in kittens of 11-22 wk of age with a body weight gain of 16.4 g/d (mean for kittens fed diets containing 1.5-2.1 g Na/kg diet, Fig. 2), sodium need for the body weight gain will be 28 mg/d. The sodium requirement of cats for maintenance can be estimated from 76 mg/d (the sodium requirement for kittens) minus 28 mg/d (sodium for body weight gain of growing kittens), which is equivalent to 21 mg Na·kg body wt¹·d¹.