Unusual nutrient requirements of cats trace to specific enzyme changes

Why cats were worth asking about#

Most successful mammals are dietary generalists. Rodents, for instance, cope with changing food supplies by adjusting their metabolism to whatever protein level is on offer. Cats, by contrast, evolved as strict carnivores living on a single type of food: animal tissue. The paper asks how such narrow specialists managed to survive and spread, given that meat alone leaves cells short of a properly balanced nutrient mix.

Morris proposes that the answer lies in adaptation at the enzyme level. If a strict meat diet reliably supplied certain nutrients in abundance, natural selection would have had no reason to keep the enzymes that synthesise those nutrients from scratch, and keeping unused enzymes running costs energy. Conversely, where meat was abundant in a nutrient, the enzymes that process or regulate it might have been left running at high, inflexible rates, because there was never a selection pressure to turn them down. The review works through five such cases involving the nutrient requirements of cats: protein and nitrogen handling, arginine, taurine, and the vitamins A, D and niacin.

Tracing feline ancestry and domestication#

The paper opens with a short account of cat evolution drawn from molecular and fossil evidence. Animals with feline traits appeared around 30 million years before the present, but recognisable ancestors of modern cats did not appear until the mid-Miocene, some 15 to 20 million years ago, with dental and other features suggesting true carnivory by at least 15 million years ago. DNA studies cited in the paper indicate that the 37 species of modern cats arose from around eight separate lineages going back no more than 10 to 15 million years, with most fossil records of living species less than 2 million years old and some species appearing less than 100 000 years ago.

Domestic cats, the paper notes, arose quite recently in evolutionary terms from a wild African ancestor, Felis sylvestris libyca, over roughly the last four thousand years. Comparative genome mapping between cats and humans showed the domestic cat genome is closely conserved relative to the human genome, and among the non-primate species examined, cats showed the fewest chromosomal changes relative to humans. The domestic cat remains the only felid species whose nutrition has been studied in detail, though the paper notes that the limited data available suggest other felids share similar requirements.

A high protein requirement without a matching amino acid need#

Growing kittens need about 1.5 times the dietary protein of chicks or pigs, even though those species deposit far more of their dietary nitrogen into tissue. For adult maintenance the gap is larger again, with cats needing about 2 to 3 times the protein of adult non-carnivores. Measuring the amino acid requirements of growing kittens, Morris and colleagues found these matched other mammals, with one exception, ruling out a simple explanation based on one or more amino acids being unusually limiting.

The real driver, the paper argues, is nitrogen itself. Most animals reduce the activity of aminotransferase enzymes and urea cycle enzymes when protein intake is low, conserving nitrogen for making non-essential amino acids. Cats given diets of 70% and 17% crude protein showed little or no such adaptation in either enzyme system. Ammonia from amino acid breakdown is converted straight to urea and lost rather than recycled, meaning cats need a steady, generous protein supply simply to meet their nitrogen turnover, not because any particular amino acid is deficient. One practical side effect noted in the paper is that this lack of regulation helps cats maintain blood glucose during starvation better than omnivorous animals can.

The arginine requirement behind feline nutrient requirements of cats#

Among all the amino acids tested in kittens, removing arginine from the diet produced a uniquely severe reaction: rapid overnight weight loss followed by salivation, neurological changes, vomiting, and a collapse into coma and tetany, with death following quickly after eating as little as 5 g of an otherwise complete but arginine-free diet. In several other mammals, arginine is not essential because the gut and kidneys cooperate to make it from glutamate and glutamine via citrulline.

The paper traces the feline problem to two enzymes along that intestinal pathway. Ornithine aminotransferase, needed to make citrulline from ornithine, is low in cats, and pyroline-5 carboxylase synthase was measured at only 5% of the rat's activity on a body weight basis. Because both enzymes sit in the same pathway, their combined low activity has what the paper calls a multiplicative effect, reducing citrulline production to almost nothing. Supplementing an arginine-free diet with citrulline prevented the dangerous ammonia build-up and supported normal growth, confirming the pathway. Ornithine supplementation prevented the ammonia spike but not growth, because it could not be converted to citrulline. A diet based solely on casein also caused ammonia build-up and vomiting in cats, while a 50:50 mix of soy protein and casein supplied enough arginine to prevent it.

Taurine, bile acids and the cat's low synthetic capacity#

Early work by Scott and colleagues found that cats fed a casein-based diet developed retinal degeneration that vitamin A supplementation could not fix, while cats on a meat-based diet stayed normal. Later research linked this to taurine deficiency, which was also tied to impaired reproduction, developmental problems in kittens, and a reversible form of dilated heart disease.

Cats make taurine from cysteine, but two enzymes in that pathway, cysteine dioxygenase and cysteinesulphinic acid decarboxylase, have low activity, again producing a multiplicative bottleneck. Compounding this, the liver enzyme that attaches bile acids to either glycine or taurine has a very low affinity for glycine in cats and dogs, so taurine is used almost exclusively and lost from the body. Measurements showed over 80% of cysteine is instead broken down by a direct desulphydration pathway rather than the route that would make taurine. The paper also found that diet composition changes taurine needs: diets high in indigestible protein raise cholecystokinin secretion and favour gut bacteria that degrade taurine, meaning canned diets need almost twice the taurine concentration of expanded diets to keep plasma levels normal.

Vitamin A, vitamin D and niacin: enzymes removed or overactive#

Cats cannot convert carotenoids to vitamin A because the cleavage enzymes involved appear to have been lost from the relevant tissues, making preformed vitamin A an obligatory dietary requirement, a peculiarity first identified by Gershoff and colleagues. A long-term study on excess dietary vitamin A found malformation rates of 1.5%, 1.6% and 9.1% that were fatal, and 0%, 1.1% and 5.2% that were non-fatal, across three groups of queens on rising vitamin A intakes, figures the paper describes as low compared with other species such as rats, where a similar dose produced an 80% cleft palate rate against 2.9% in the highest-dose cat group.

Vitamin D tells a similar story from the opposite direction. Despite cats sunning their sparsely-haired bellies, shaved and unshaved kittens exposed to direct sunlight could not make vitamin D, because an enzyme that converts the skin precursor onward to cholesterol is too active, leaving too little substrate. Blocking that enzyme with an inhibitor allowed vitamin D synthesis to proceed under UV light. In the wild, the paper suggests, prey species such as rodents and birds supply enough vitamin D in their tissues to make this unnecessary. Niacin follows the same logic: cats have the highest measured activity of an enzyme called picolinic carboxylase, which diverts the tryptophan breakdown pathway away from niacin production, making dietary niacin obligatory even though all the synthetic enzymes are present.

Coat colour, fatty acids and the limits of the evidence#

An unexpected finding involved coat colour. A black cat's coat turned reddish brown on a gelatin-based diet low in phenylalanine and tyrosine, and switching to casein-lactalbumin protein reversed the change. Further work found that preventing this colour shift required 16.5 g to 24 g of combined phenylalanine and tyrosine, well above the 8.5 g previously set as the requirement for growth and nitrogen balance, a gap the paper describes as unusually large compared with other secondary functions in any species studied.

On fatty acids, the paper notes cats have low desaturase activity in the liver, limiting their ability to make arachidonate from linoleate, yet queens fed only vegetable fat could still produce up to two litters, and toms reproduced normally on vegetable fat diets containing linoleate alone. How much of the longer-chain omega-3 fatty acids cats actually need was not pinned down in the work reviewed. Given that Morris frames the whole paper as reasoning from present-day enzyme patterns back to evolutionary pressures, it is worth treating the adaptive story as a plausible interpretation of association rather than as proven cause and effect, which the paper itself acknowledges explicitly. This review sits within the broader Recent Advances in Animal Nutrition series, alongside other contributions gathered in the RAAN Conference Proceedings collection held on this index, and readers browsing further feline nutrition topics can also see the record for chronic renal failure management in cats.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Sheep Genetics (LAMBPLAN and MERINOSELECT): which runs LAMBPLAN and MERINOSELECT
• Animal Genetics and Breeding Unit

Questions#

— The Livestock Library team