Written by Bonnie Edkin

The term “obligate carnivore” comes up repeatedly in online social media cat groups. There are a lot of misunderstandings about what the term means. Here is some clarification regarding what an obligate carnivore is and what that means as far as feline nutrition.

An internet search for “obligate carnivore” results in a lot of overly simplified definitions of the term. Many of the online dictionaries include a phrase something like ‘an obligate carnivore is an animal that cannot digest plant foods.’ First, let me be clear: that definition is not accurate. This has given rise to a misunderstanding of what an obligate carnivore really is and what a cat can and cannot digest.

Let’s break down the term.

First, what is a carnivore?

Simply put, a carnivore is a plant or animal that eats meat. The word carnivore comes from a Latin word meaning “meat” or “flesh.” The venus fly trap is one example of a carnivorous plant. Chili con carne, for example, is a spicy soup that contains beans along with meat (con carne). Chili sans carne is a spicy soup with beans, but without meat (sans carne).

Here’s the definition of a carnivore from the Encyclopedia Britannica (1):

“Carnivores have digestive enzymes that are able to break down muscle protein into amino acids, which can then diffuse through the walls of the small intestine. Therefore, carnivores have no need for any special development of the gut that allows for fermentation. Carnivores are also able to utilize animal fat. If their prey is small, they can chew and swallow bones, which serve as a source of calcium.”

The digestive tract of an herbivore efficiently breaks down raw plant matter by means of fermentation in the gut. Microbes and enzymes break down the plant matter and convert the nutrients to forms that are bioavailable for the animal. Bovines are an example of an herbivore. By contrast, the enzymes in the digestive tract of a carnivore break down the proteins in meat, thereby making the amino acids available to the animal. There is a small amount of fermentation of fibers that takes place in the large intestine of carnivores and this does result in the conversion of some nutrients to a form that cats can use. However, cats do not have the kind of digestive tract that is efficient at fermenting raw plant matter like an herbivore’s digestive tract.

What about the word “obligate?”

The word means “by necessity. Definitions from medical dictionaries are along the lines of ‘lifeforms characterized by the ability to survive only in a particular environment.’ (2) The word is often used in biology to indicate what is necessary for a plant or animal. For example, an “obligate aerobe” is an organism that cannot survive without oxygen (aerobic = with oxygen). The opposite of that is an “obligate anaerobe” – an organism that cannot survive in the presence of oxygen (anaerobic = without oxygen). (3)

What then, is an obligate carnivore?

Here’s the definition from the Encyclopedia Britannica (1):

“Some carnivores, particularly cats (family Felidae), are obligate carnivores, meaning they cannot obtain all the nutrients that they need from the plant kingdom and bacteria. In particular, obligate carnivores lack the enzyme needed to split carotene, obtained from plants, into vitamin A. Instead, these animals obtain vitamin A from the liver of their prey. Obligate carnivores are similarly unable to synthesize some essential very-long-chain, highly unsaturated fatty acids that other animals can make from shorter fatty acids found in plants.”

So, an obligate carnivore is an animal that needs to (is obligated to) obtain some nutrients by eating the flesh of other animals (or those nutrients would have to be supplied in a form the cat can absorb by means of supplementation). Cats do not have the long digestive tracts of ruminants to efficiently ferment raw starch or fiber, but that doesn’t mean they cannot digest plant matter at all. It also does not mean they are unable to obtain any nutrients from plants or from fiber. However, being an obligate carnivore does mean that their diet MUST provide certain nutrients from the flesh of other animals or from supplementation. What are some of these nutrients?

Vitamin A is one example the Encyclopedia Britannica mentioned. Plants do not contain vitamin A. They do, however, contain beta carotene. Beta carotene is actually two molecules of vitamin A joined together by a chemical bond. It takes an enzyme called beta-carotenoid-15,15′-dioxygenase to break the bond between the two molecules of beta carotene. Humans produce this enzyme. So, we can eat a carrot or some other orange-colored vegetable and our body can convert each beta carotene molecule into two molecules of vitamin A. Cats do not make this enzyme. So, while they can eat and even digest carrots or pumpkin, they are not able to derive any vitamin A from vegetables. They must obtain their vitamin A needs by ingesting something that has actual vitamin A molecule in it – either animal flesh or a vitamin A supplement.

Although cats cannot convert beta carotene to vitamin A, that does not mean that beta carotene is of absolutely no use to them. Beta carotene is an antioxidant. It has been found to be useful for cats with sun damage on their ears, for example. Cats with white ears are prone to this because the skin under the white fur is pink (which offers little protection from the sun) and the fur on the ears is not thick like it is on the rest of a cat’s body. One of the treatments for sun damage to the ears is beta carotene. The antioxidant effect of beta carotene helps heal the damaged skin. So, even though a cat cannot convert beta carotene to vitamin A, the antioxidant property of beta carotene can be helpful for some health issues.

The above quote from the Encyclopedia Britannica also pointed out that cats are not able to make all the long-chain fatty acids they need. The fatty acids that they need but cannot synthesize are called “essential fatty acids” and the diet must supply them. Whereas other animals may be able to synthesize some long-chain fatty acids that they need from other fatty acids the diet provides, cats cannot manufacture all the long-chain fatty acids they need. There are some long-chain fatty acids that they must obtain from meat or fish. Some people have assumed this means cats cannot digest short-chain fatty acids, or, that cats cannot benefit from fatty acids obtained from plants. Neither of those ideas are true. Cats cannot convert some fatty acids to other fatty acids that are essential to their health. For example, cats cannot convert the linoleic acid (LA) to the fatty acids found in oily fish, such as EPA or DHA. However, they do have a requirement for LA and are perfectly capable of utilizing the LA from canola, corn or safflower oils. Similarly, they are not able to convert the gamma-linolenic acid (GLA) in borage seed oil to alpha-linolenic acid (ALA), but they can benefit from the wonderful anti-inflammatory properties of the GLA in borage seed oil even though they have no established requirement for GLA.

Taurine is another important factor when it comes to nutrition for cats. Cats need taurine to manufacture bile acids. It is extremely important for heart health, for their vision and it’s even a key nutrient for them in growing their fur. While our bodies are able to make taurine from the amino acids cysteine and methionine, cats cannot do this. Thus, cats must consume foods that contain taurine, and that is why it is called an “essential amino acid” for cats – because they lack the enzymes needed to tear apart other amino acids and reconstruct those amino acids into taurine. Taurine is not found in land vegetables. (It is found in small amounts in some sea vegetables.) So, for cats, meat is important. If the diet does not supply sufficient taurine, it must be supplemented.

Can cats digest carbohydrates?

Yes. Some confusion over this may have arisen because, unlike humans, cats do not produce salivary amylase. So, the breakdown of carbohydrates does not begin in their mouth. However, their pancreas produces amylase. The Merck Veterinary Manual explains another reason some people think that cats cannot digest carbohydrates. “Cats lack the enzyme glucokinase, which unfortunately has led some to believe that cats cannot digest dietary carbohydrates. Cats produce the enzyme hexakinase, which allows them to digest and use properly processed dietary carbohydrates.” (4)

All animals have a metabolic need for carbohydrates in the form of blood glucose. This requirement can be supplied by the diet or by metabolic pathways in the liver and kidneys that produce glucose from other nutrients. The latter process is called gluconeogenesis. Cats are in a constant state of gluconeogenesis and so do not have a need for dietary carbohydrates to meet their need for blood glucose. However, according to Linda P. Case, who taught companion animal science at the University of Illinois for 20 years, they can actually handle up to 43% of their kcal intake coming from carbohydrates. “The fact that cats do not require carbohydrates from their diets is usually immaterial . . . Although raw starch is poorly digested, cooked starch is very efficiently digested and absorbed by cats . . . Although dietary fiber is not a required nutrient per se, the inclusion of optimal amounts of fiber in the diets of cats is necessary for the normal functioning of the gastrointestinal tract. Insoluble fiber functions to increase the bulk of the diet, contributes to satiety, and maintains normal intestinal transit time and gastrointestinal motility. Soluble fiber delays gastric emptying, and, when fermented by colonic bacteria, produces short-chain fatty acids that are important energy sources for colonocytes.” (5)

What are colonocytes? They are the epithelial cells that line the colon. These cells are important because they maintain the conditions in the gut to support the bacterial organisms that consume dietary fiber and produce short-chain fatty acids that are beneficial to a healthy microbiome. If there is a change in colonocyte metabolism, the epithelium can become oxygenated. This encourages the growth of pathogenic bacteria and can result in an imbalance between the types of organisms present in the microbiome. A healthy microbiome of diverse organisms plays an important role in immune function.

To Summarize

->An obligate carnivore is an animal that must obtain certain nutrients from animal flesh, or from supplements added to the diet. Some examples of these nutrients are actual vitamin A (not just a precursor to vitamin A such as beta carotene), some long-chain fatty acids found in meat, and taurine.
->Cats cannot convert some fatty acids to other fatty acids. For example, they cannot convert LA or ALA to EPA or DHA. However, they can still utilize and benefit from fatty acids from plant sources, including LA, ALA and GLA.
->Cats produce amylase and hexokinase and so are able to digest carbohydrates and obtain nutrients, including amino acids, from plant matter. While they are not efficient at digesting raw plant starch, cooked starches are efficiently digested and absorbed by cats.
->Although there is no official requirement for fiber, it promotes the normal functioning of the gastrointestinal tract. Fermentation of fiber in the colon is an important energy source for colonocytes.

References
  1. Encyclopedia Britannica. “Nutrition.” Encyclopedia Britannica, Inc.  2018 Jul 12. 2018 Oct 15. https://www.britannica.com/science/nutrition/Herbivores#ref843396  
  2. obligate.” Collins Dictionary of Biology, 3rd ed.. 2005. W. G. Hale, V. A. Saunders, J. P. Margham. 2018 Oct 15. https://medical-dictionary.thefreedictionary.com/obligate 
  3. “obligate anaerobe.” Saunders Comprehensive Veterinary Dictionary, 3 ed.. 2007. Elsevier, Inc. 2018 Oct 15. https://medical-dictionary.thefreedictionary.com/obligate+anaerobe 
  4. Merck Veterinary Manual. “Nutritional Requirements & Related Diseases of Small Animals.” 2020. 2020 Oct 3. https://www.merckvetmanual.com/management-and-nutrition/nutrition-small-animals/nutritional-requirements-and-related-diseases-of-small-animals
  5. Linda P Case. “The Cat: Its Behavior, Nutrition & Health.” Blackwell Publishing Professional. Ames Iowa. 2003. pp. 296-300.