Paleontology for veterinarians: The evolution of cats and carbohydrates
Tom Rothwell, DVM, PhD, is medical director of Paris Hill Cat Hospital in Sauquoit, New York.
Cats are clearly obligate carnivores. So, why are cat foods so high in carbohydrates, and what should be done about it?
The story of cat nutrition is written in the fossil record. As both a paleontologist and a feline veterinarian, I have studied these fossils and they reveal that cats have been meat-eating obligate carnivores for an exceedingly long time. Felis catus, the domestic cat, has a long evolutionary story, a cat’s tale that should greatly influence the practice of veterinary medicine today.
Fossils of the earliest-known cat species are from rocks found in southern France that date back 29 million years. Named Proailurus, of the family Felidae, these early cat fossils contain teeth and bones with all the characteristics of a carnivoran, a meat-eating member of the mammalian order Carnivora. Clearly evident in these specimens are the elongated canines and fully developed carnassial meat-eating dental apparatus that defines the order Carnivora: The upper premolar teeth (#108, #208) shear like a scissor blade with the lower molars (#309, #409). The skull and postcranial bones of these earliest known cats are almost identical to the felids of today, including the domestic cat. It is clear to me, after examining thousands of these fossils, that cats have been hypercarnivorous, ambush predators for at least 29 million years.
Why do we care about cat evolution?
We care because cats are determined to be different. They are not close relatives to the more omnivorous dog. The evolution of dogs and cats diverged 50 million years ago. In fact, cats are more closely related to hyenas than to dogs. This means that their anatomy, biochemistry, physiology, and behavior have been evolving separately from dogs for at least 50 million years.
Today, there are 30 closely related species within the family Felidae. None of these cats evolved to digest food with more than 10% carbohydrates, not the lion in the zoo or the cat in the kitchen. The wild ancestor of the domestic cat, Felis silvestris, is still found today in Eurasia and Africa. Its diet consists primarily of small rodents and birds, nourishment that is close to 20% protein and less than 10% carbohydrates. As a cat veterinarian and a feline paleontologist, I ask: If all members of the family Felidae evolved with a diet low in carbohydrates and high in protein, why do we feed excessive carbohydrates to the species that we are most empowered to care for?
The chronology of cat nutrition
The explanation relies in part on international geopolitical conflict. The cat food carbohydrate story has its beginning during World War ll when the pet food industry was forced to stop using metal to package pet food in cans. By 1946, dry cat food accounted for 85% of the market share, but the ingredients were somewhat difficult and expensive to bake and convert into kibble. In 1950, the dry food extrusion process, which required adding starch, increased the efficiency and profitability of putting dry pet food into a bag. Dough could be fed into a machine and cooked under pressure before being extruded (pushed out) through small holes to create the kibble. Supermarket shelves subsequently were stocked with inexpensive, convenient dry cat food that had a 20% to 30% carbohydrate content. After 29 million years of nutritional evolution, pet cats were experiencing a new diet.
At the same time, the pet care industry was evolving. Earlier generations of veterinarians were largely ambulatory. They made farm calls, where they were not necessarily involved in the diets of the dogs and cats they encountered. A new type of veterinarian was emerging. Small animal clinics began to open in cities and suburbs specializing in the care of the increasing number of pet dogs and cats. In the 1960s, small animal clinics began to sell dog and cat food, including some from the veterinarian-created company Hill’s Pet Nutrition.
The next big moment in the chronology of cat nutrition occurred in 1976 when Colgate-Palmolive bought Hill’s, which by this time included Prescription Diet and Science Diet products for both dogs and cats. All of the feline diets contained more than 10% carbohydrates. Because of their huge success in marketing toothpaste through dentists, Colgate-Palmolive began retailing Hill’s Prescription Diet products through veterinarians. Suddenly, many small animal veterinarians were selling these popular therapeutic diet formulas. Clients demanded them. Veterinarians entered the high-carb cat food business.
For the past 70 years, most pet cats have been eating food much different than what they have evolved to digest. Have there been consequences? One notorious repercussion was discovered in 1987. Because numerous commercial cat foods did not include muscle meat and its naturally occurring taurine, some cats developed dilated cardiomyopathy. Collectively, veterinarians and the pet food industry had failed to understand that cats are unable to synthesize and conserve sufficient taurine as dogs can. Cats are different.
Could there be other medical or surgical effects? Perhaps dry cat food with excessive carbohydrates contributes to other common cat maladies such as liver disorders, inflammatory bowel disease, obesity, or lower urinary tract issues. Carbohydrate percentage certainly influences diabetes mellitus. What can we do? What should we do? As veterinarians, we are the caretakers. Perhaps, it is time for us to change.
Tom Rothwell, DVM, PhD, is medical director of Paris Hill Cat Hospital in Sauquoit, New York. He can be reached via email at firstname.lastname@example.org.