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The domestic cat is believed to have evolved on a diet that consisted mainly of animal tissue. The relatively constant composition and the high and balanced nutrient content of such a diet is likely to have been the reason for the unique metabolic adaptations in the cat.

Examples of these adaptations are the inability of the cat to regulate hepatic amino acid catabolic enzymes, to synthesize niacin from tryptophan, and to synthesize several urea cycle intermediates and sufficient taurine to meet requirements.

The cat, furthermore, cannot use carotene to synthesize vitamin A, and excretes several unusual sulfur-containing amino acids such as felinine (Hendriks et al., 1995) in high quantity in the urine. Although to date no peculiarities in the metabolism of essential minerals have been found, it is well recognized that domestic cats have an increased risk of urinary stone formation.

Shields (1998) recently updated the role of vitamins and minerals in the nutrition of cats and dogs, and discussed health benefits of some minerals for these companion animals. This contribution aims to summarize current knowledge on the requirements for essential minerals in cats and identify gaps in current knowledge. Furthermore, the role of minerals in the most common condition to afflict cats, struvite urolithiasis, is reviewed.



Mineral requirements of domestic cats

REQUIREMENTS DURING THE SUCKLING PERIOD

During early life (up to 4 weeks of age), kittens rely solely on the milk from the queen for nutrition. At approximately 5 weeks of age the kittens start to ingest small amounts of solid food, the intake of which will increase until weaning at 8 to 10 weeks of age.

Milk intake in conjunction with the mineral composition of the milk can be used to estimate mineral intake during the suckling period, which may serve as a basis for estimating mineral requirements during this period. The mineral composition of queen’s milk throughout lactation has been determined by Keen et al. (1986) and Adkins et al. (1997).

Zottman et al. (1997) estimated the milk yield of queens nursing litters of various sizes throughout the suckling period. Table 1 shows the calculated mineral intakes of kittens raised in litters of either 1 to 2 or 3 to 4 kittens during week 1 and weeks 2 to 4 of the suckling period, respectively.

The intake of calcium, phosphorus, magnesium, zinc and manganese of kittens during the suckling period increases as lactation proceeds. The intake of iron and copper, however, remains relatively constant. In the period from 1 to 3 weeks of age, the body weight of kittens increases from approximately 150 to 350 g (Loveridge, 1987).

The iron and copper intake per kg metabolic body weight, therefore, decreases dramatically during the first 4 weeks of the kitten’s life. Table 1, furthermore, shows that the daily mineral intake of kittens nursed in a litter of 3 to 4 is higher than the mineral intake of kittens nursed in a litter of 1 to 2.


Table 1. Daily intake of minerals and metabolizable energy (ME) in kittens raised in litters of different size during the suckling period.*


* Calculated from data of Keen et al. (1986), Adkins et al. (1997) and Zottman et al. (1997) with kittens nursed by a 3.8 kg queen.



MINIMUM MINERAL REQUIREMENTS DURING GROWTH

In 1962 the cat was included in the National Research Council’s (NRC) publication on the ‘Nutrient Requirements of Laboratory Animals’ (NRC, 1962). It was stated in this publication that there have been no studies made on the quantitative requirements of cats for any mineral elements.

In 1978 the NRC published the ‘Nutrient Requirements of Cats’ and made recommendations on the nutrient allowances for cats that included 11 minerals.

The NRC reviewed the literature again in 1986 and started the section on minerals with the following statement: ‘That cats require minerals is indisputable. However, there is a scarcity of data on both the qualitative and quantitative requirements for this class of nutrients.’

In this publication the minimum requirements of growing kittens for several essential minerals were presented. Estimates of the minimum requirements for phosphorus and manganese were based on the criterion of satisfactory performance, while the recommended minimum dietary requirement for sodium, chloride, iodine, copper and selenium were extrapolated from other species.

It was, furthermore, noted that there were no published data on the requirements for the minerals sulfur, fluorine, molybdenum, tin, silicon, nickel, vanadium and chromium in cats.

Over the past 15 years a number of studies have been conducted to estimate the minimum requirements of several minerals for growing kittens. Table 2 presents the minimum requirements for eight essential minerals in growing kittens and the mineral source used in these studies to establish the requirements.

The latter information is important as the availability of minerals may differ among mineral sources (Morris and Rogers, 1994; Fascetti et al., 1997).

The minimum requirements presented in Table 2, therefore, relate to the mineral sources listed.

To the author’s knowledge no work has been conducted to advance the qualitative and minimum quantitative requirements of sulfur, fluorine, molybdenum, tin, silicon, nickel, vanadium and chromium in the cat over the past decade.

Comparison of the minimum mineral requirements of growing kittens as listed in Table 2 with those of dogs (NRC, 1985) and rats (NRC, 1972) shows that the quantitative requirements for essential minerals are of the same order of magnitude in these three species. The requirements for calcium and phosphorus in the dog and rat are about twice that of the cat while the requirements for sodium of the dog and rat are about half the value of that of cats.

The growing kitten, therefore, seems to have requirements for essential minerals similar to those of other animal species. Apparently, the cat does not seem to have idiosyncrasies in the metabolism of minerals. However, comparison of the quantitative requirements for minerals among the cat, dog and rat does not preclude the possibility of the cat having unusual transport mechanisms or peculiarities in the storage of minerals (MacDonald et al., 1984).


Table 2. Minimum dietary requirements of several minerals for growing kittens, and the mineral source and response criterion used to obtain the estimates.


*mg/MJ metabolizable energy.
†Lower value relates to diets with a low protein content, high value relates to diets with a high protein content.



Furthermore, the essentiality and quantitative requirements of several minerals have not yet been determined in the cat; and differences with other animals with respect to these minerals may be found.



MINIMUM MINERAL REQUIREMENTS FOR MAINTENANCE

The NRC (1986) stated that the minimum mineral requirements of adult cats are not known, but that it is likely that the requirements for maintenance are lower than the requirements for growing kittens. Very little research has been conducted over the last decade on the mineral requirements of adult cats.

Table 3 presents the minimum dietary requirements for calcium, sodium and magnesium for adult cats. The estimates for calcium and magnesium were found to maintain calcium and magnesium balance in adult cats, respectively.

The requirement for sodium was calculated based on the maintenance requirements of kittens.


Table 3. Minimum dietary requirements of several minerals in the cat for maintenance and the mineral source used to obtain the estimates.


*mg/MJ metabolizable energy.



Bioavailability of mineral sources in the cat

Knowledge on the availability of individual essential nutrients is required for the accurate formulation of adequate and nutritionally balanced diets for companion animals. Availability of a nutrient can be defined as the proportion of a dietary nutrient being absorbed across the gastrointestinal epithelium which can potentially be utilized by the animal. In most cases the availability of a nutrient is taken to be equal to the digestibility, i.e., the fraction of dietary nutrient absorbed.

The NRC (1986) lists 40 common mineral sources for cats. However, the availability of each singular mineral constituent from these sources may vary considerably. Little research has been conducted to determine the availability of minerals in diets for cats.

Hintz and Schryver (1986) compared the utilization of calcium in limestone, dicalcium phosphate and bone meal in cats and found that the availability of calcium in these three sources was 39, 36, and 38%, respectively. Pastoor (1993) supplemented diets with calcium carbonate and found that the absorption of calcium from the gastrointestinal tract was in the order of 20%. Pastoor et al. (1995b), furthermore, found that magnesium in the form of magnesium carbonate has an availability of ~60%.

Yu and Morris (1997) determined the sodium requirements of growing kittens and reported an average apparent absorption for sodium from sodium chloride of 70 to 80% with an increase in apparent absorption as the inclusion level of sodium chloride in the diet increased. Pastoor et al. (1995a) reported an apparent sodium absorption coefficient from sodium phosphate dihydrate (NaH₂PO₄·2H₂O) between 53 and 77% depending on the calcium content of the diet.

Recently, Fascetti et al. (1997) reported that cupric oxide, which is commonly used in commercial cat diets, does not present an available form of copper for cats. It is apparent that there is a paucity of information regarding the availability of various mineral sources for cats.

One mineral of current interest due to its immune modulating effects is selenium. This mineral may be more important in carnivores compared to other animals due to the high fat containing diets commonly ingested by this group of animals. To this end we, at the Monogastric Research Centre, are currently evaluating the availability of selenium from different sources (organic Se: Sel-Plex 50 selenium yeast, inorganic Se: sodium selenite) in adult cats.

Indices of selenium status (whole blood selenium and glutathione peroxidase levels) and liver function are measured.



The role of minerals in feline urological syndrome

Minerals have long been recognized to be involved in the most important disorder affecting the lower part of the urinary system of cats and one of the most common conditions to afflict cats, urolithiasis or feline urological syndrome (FUS). In 1985, an overall incidence of lower urinary tract disease of 0.85% was reported in the United States cat population (Lawler et al., 1985).

The problem of uroliths in cats is not unique. Uroliths can be found in other animals such as dogs, horses, cattle, sheep, goats, mink, foxes and pigs (Leoschke and Elvehjem, 1954; Nguyen et al., 1979; Long, 1984; Osborne et al., 1989) but the incidence of urinary stones seems to be higher in the mammalian carnivores, the cat, mink and fox. Naturally occurring feline uroliths may be composed of magnesium ammonium phosphate hexahydrate (struvite), calcium oxalate, ammonium phosphate, calcium phosphate, cystine and xanthine with struvite being the most predominant form (Osborne et al., 1989).

For complex salts to precipitate, the dietary mineral content requiring urinary excretion, urinary volume and pH are important factors, and many studies have been conducted to establish the relative importance of these factors in the formation of struvite in cats.

Urinary pH (Taton et al., 1984; Tarttelin, 1986) and dietary mineral content (Rich et al., 1974; Buffington et al., 1990), in particular dietary magnesium, have been shown to greatly influence struvite crystallization. Urine volume depends on water intake (dietary water, drinking water and metabolic water), and insensible and fecal water losses. In the cat, urine volume has been shown to decrease significantly when fed a dry (~10% water) diet compared to a moist (~75% water) diet (Gaskell, 1989), thereby increasing the risk of the formation of uroliths.

Strategies for preventing struvite formation in cats through dietary manipulation either target urinary pH or the excretion of magnesium in the urine. Lowering urinary pH to 6.0–6.4 by the addition of urinary acidifiers to diets is one of the most effective measures to prevent the formation of struvite crystals, and has become normal practice in commercial diets.

Dry diets especially, which are more prone to forming struvite in the cat, are formulated to include urinary acidifiers such as phosphoric acid, ammonium chloride, calcium chloride and methionine. In conjunction with urinary acidifiers, diets can be formulated to contain low magnesium levels, which is the primary mineral found in the struvite crystal.

Early studies (Rich et al., 1974; Lewis et al., 1978) ‘demonstrated’ that increased dietary magnesium and consequently an increased urinary magnesium excretion predisposed cats to urolithiasis. This led to the conclusion that magnesium concentrations commonly found in commercial diets are the major dietary factor contributing to this disease.

However, Buffington et al. (1985) showed that previous studies claiming that ‘excessive’ dietary magnesium causes struvite urolithiasis were the result of an error in the experimental design as the magnesium salt was added in a form which increased urinary pH. The importance of urinary magnesium concentration in the prevention of struvite formation in the cat, therefore, may be overemphasized.

The efforts by the pet food industry in preventing struvite formation through the inclusion of urine acidifiers in diets or the formulation of low magnesium diets have resulted in a significant decrease in the presence of struvite in feline uroliths.

However, at the same time the presence of calcium oxalate in feline uroliths has increased markedly and this is now the most predominant form of uroliths found in cat urine (Osborne et al., 1996). In this respect it is interesting to note that increased consumption of magnesium is often recommended to humans for the prevention of calcium oxalate uroliths (Shields, 1998).

Omnivores have evolved on diets varying in the composition of minerals.

The cat and other true carnivores such as the mink and fox, however, have evolved on a diet consisting mainly of animal tissue, which has a relatively consistent mineral composition. The cat is likely to have adapted anatomically and metabolically to the forms in which the minerals are present in animal tissues.

When presented with a diet containing minerals in a form not commonly found in their natural diet, cats may be unable to cope adequately with the mineral challenges caused by such a diet. For example, the source of calcium in natural diets of cats mainly originates from bone.

By adding calcium to a diet in the form of calcium chloride, the normal fixed molar ratio of calcium to phosphorus absorption is disturbed due to an increased gastrointestinal absorption of chloride. This leads to an increased rate of urinary acid and chloride excretion, a reduced urinary excretion of phosphorus and may, in turn, lead to metabolic acidosis (Pastoor et al., 1993).

In a colloquium on mammalian urology held at the University of California/Davis in 1995 (Anonymous, 1997), Q. Rogers made the following statement: ‘I think the struvite problem is going to be with us forever: it is a result of the anatomy and metabolism of the cat. I am not so sure about calcium oxalate. Maybe we are doing something wrong in formulating diets that’s causing it.’

In a future perspective, I think it would be interesting to study the prevalence of naturally occurring uroliths in wild or feral cats to see whether this disease is common to cats ingesting normal prey. In this way an indication could be obtained whether the way commercial diets for cats are currently formulated and manufactured contributes to the problem of urinary stone formation, or they are the consequence of the unique anatomy, physiology and metabolism of the cat. It is difficult to believe that evolution has made cats as susceptible to urinary stone formation as that shown by the current incidence rate found in the domestic cat.



Summary

The domestic cat is believed to have evolved on a diet mainly consisting of animal tissues and as a result has developed a unique metabolism. Although there is a large body of information on the requirements for amino acids, fatty acids and vitamins in the cat, data on the requirements for minerals are scarce.

This contribution provides a review of the literature on the mineral intake of suckling kittens and the minimum requirements of growing and adult cats.

It is concluded that the minimum mineral requirements of cats are similar to those of dogs and rats although the calcium and phosphorus requirements for growth are somewhat lower. The cat, therefore, does not seem to have a peculiar mineral metabolism similar to that seen in its protein, fatty acid and vitamin metabolism.

Minerals are involved in the most important disorder affecting the lower part of the urinary system of cats and one of the most common conditions to afflict cats, urolithiasis. It is well recognized that domestic cats and other mammalian carnivores such as the mink and fox have an increased risk of urinary stone formation.

Feline uroliths may be composed of magnesium ammonium phosphate hexahydrate (struvite), calcium oxalate, ammonium phosphate, calcium phosphate, cystine and xanthine with struvite being the most predominant form.
The efforts by the pet food industry in preventing struvite formation through the inclusion of urine acidifiers in diets or the formulation of low magnesium diets have resulted in a significant decrease in the presence of struvite in feline uroliths since 1984. However, at the same time the presence of calcium oxalate in feline uroliths has increased markedly and this is now the most predominant form of uroliths found in cat urine.



Acknowledgement

The author wishes to thank Dr S. Wamberg for his valuable comments and discussions.



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