Plant vs Meat - The Protein Feud

Answers: Plant vs. Meat – The Protein Feud

I have read that plant proteins are inferior for cats, that they need meat protein. What's the difference? Why does it matter where the protein came from?
Not all proteins are equal. Proteins are made up of amino acid chains and there are a myriad of different combinations that serve many functions in the body. Proteins are structural and tissue components in the body, enzymes and antibodies and serve messenger and transport functions. Ingested proteins will vary considerably in how well they are utilised by the body. Plant-based proteins do not have the same amino acid profile as meat-based proteins and these differences are crucial to cats. The natural diet of cats in the wild is a meat-based diet of rodents and birds. Cats are metabolically adapted to preferentially use protein and fat as an energy source, not carbohydrates. There is a substantial difference in protein requirements between our cats as obligate carnivores and other carnivorous species, such as the dog. Adult cats require two to three times more protein than adults of any omnivorous species, such as humans.
Cats have a limited ability to adjust protein utilization to the amount of protein in their diets. In other words, they need to burn protein for energy even if they aren't getting enough in their diet. Cats have lost a number of metabolic pathways that omnivores still utilise to metabolize plant-based foods. They have lost these pathways because in their natural habitat they hunt prey species that are high in meat-based protein, so they no longer needed the ability to efficiently metabolize plant material.¹
Cats have an increased need for specific amino acids in their diet compared to other species. Some of these important amino acids are: taurine, arginine, methionine and cysteine. Their natural diet contains an abundance of each of these amino acids in the form of meat-based proteins.
Taurine is essential in cats because they can't synthesise it from the typical precursors of methionine and cysteine. Taurine is found in animal-sourced proteins but not in vegetable-sourced proteins and must be supplemented in most commercial cooked pet foods. Taurine deficiency can result in blindness, reproductive failure and dilated cardiomyopathy.
Arginine is another essential amino acid which cats cannot synthesise and must have in their diet. Arginine is abundant in the tissues of their prey but is deficient in plant-origin protein sources. Arginine deficiency results in salivation, neurological abnormalities, hyperaesthesia, emesis, tetany and coma.
Cats have high requirements for methionine and cysteine. These amino acids are important as an energy source for cats. Cats will use protein for the maintenance of blood glucose levels even when sources of protein in the diet are limited. Deficiencies can develop in cats fed plant-source proteins. Methionine and cysteine are converted to important anti-oxidants and free radical scavengers. They are also used in the production of felinine, which is important in territorial marking. A deficiency can be seen as poor growth in kittens, or crusting skin issues in adult cats.
Tyrosine is another important amino acid in cats. It plays an important role in the synthesis and homeostasis of melanin, the black hair and skin pigment. A deficiency is most commonly observed in black cats whose hair becomes reddish brown. This change can be reversed by feeding cats increased levels of tyrosine through diets high in animal-source proteins.
In commercial cat food, a product's stated protein percentage can ignore two very important factors:
  • How packed full of essential amino acids is that particular protein source?
  • What is the biological value of that protein to the cat?
The biological value of the protein tells us how beneficial that protein is to our cats – how readily that protein can be utilised by the cat. In other words, the higher the biological value, the more usable it is to the cat.
Chicken eggs are used as a benchmark by which all other proteins are judged. They have a biological value of 100 – due to their near perfect usability. The biological value of chicken is 79, beef is 80. The biological value of wheat is 60, and of corn is 54.² It is easy to conclude that meat proteins will have a higher digestibility for cats than plant proteins.
Cats are strict carnivores that rely on nutrients in animal tissues to meet their specific and unique nutritional requirements. The limitations of substituting animal-origin protein with plant-origin protein in food formulated for cats is being increasingly recognised. Simply looking at the overall protein content of a food is not enough to determine whether the food is a good choice for your feline companion.
Note: Feline Nutrition provides feline health and nutrition information as a public service. Diagnosis and treatment of specific conditions should always be in consultation with your own veterinarian. Feline Nutrition disclaims all warranties and liability related to the veterinary advice and information provided on this site.
Dr. Lyn Thomson trained at the University of Bristol in England and is studying with the Australasian College of Nutritional and Environmental Medicine. A dedicated and experienced advocate of bio-appropriate nutrition, Lyn practices in Auckland, New Zealand. Her Raw Essentials stores have grown to seven retail locations, providing a variety of raw diet products for cats and dogs.
  1. JG Morris, "Idiosyncratic Nutrient Requirements of Cats Appear to be Diet-induced Evolutionary Adaptations," Nutrition Research Reviews 15, no. 1, June 2002, 153-68.
  2. Figures based on the Relative Utilization scale. For a comprehensive listing of biological values using the Percentage Utilization scale, see the "Amino-Acid Content of Foods and Biological Data on Proteins" report by the Food Policy and Food Science Service, Nutrition Division of the FAO.

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