Dietary Anatomy & Physiology of the Carnivore

The Digestive System of the Dog and Cat

The first thing to note about the digestive system of all carnivores is that they are remarkably similar and the simplest of all animal species.  Because carnivorous animals come in different sizes, the overall lengths of carnivores’ digestive tracts differ however; in general they are all rather short (about six times the length of the animal’s body).


Carnivores have a large, simple (single-chambered) stomach. The stomach volume of a carnivore represents 60-70% of the total capacity of the digestive system.  Because meat is relatively easily digested, their small intestine (where absorption of food molecules takes place) is short. Since wild carnivores average a kill only about once a week, a large stomach volume is advantageous because it allows them to quickly gorge themselves, taking in as much meat as possible at one time, which can then be digested later while resting. Additionally, the ability of the carnivore stomach to secrete hydrochloric acid is exceptional. Carnivores are able to keep their gastric ph. down around 1-2 (similar to industrial strength hydrochloric acid) even with food present. This is necessary to facilitate protein breakdown and to kill the abundant dangerous bacteria often found in decaying flesh foods.  Dogs hold chewed food in their stomachs for 4 to 8 hours after ingestion. Only a little food at a time is released into the intestine, which passes through rapidly. This gives any bacteria that may live through the repeated acid baths little time to colonize and produce gastrointestinal distress.

Small Intestine

The small intestine is vitally important. Without it, no digestion could take place and the animal could not survive. The dissolved food, called ‘chyme’ at this stage, leaves the stomach in a series of spurts, controlled by a valve, the pylorus, and enters the small intestine. It is in the small intestine where food is digested and ultimately enters the bloodstream. After a few inches, two ducts connect from the pancreas and the liver to the small intestine. These two organs supply and deliver the enzymes needed to break down the fats and proteins into their component fatty acids and amino acids. Only in this form can they pass through the gut wall into the bloodstream. These enzymes are vitally important to the carnivore. Those from the pancreas immediately start to break down the chyme into its basic components and continue to do this throughout its passage along the small intestine.

The chyme is a watery mixture but fat will not mix with water so it requires some special handling. This is where bile comes in. Bile is manufactured in the liver and stored in the gall bladder until such time as it is needed. When fat is detected in the small intestine, this triggers the release of the stored bile, which enters the intestine through the bile duct. Bile acts just like a detergent in that it emulsifies the fat to make it soluble in water. This action makes fat susceptible to digestion by the digestive enzymes.

In the carnivore there are large amounts of fat in diet on occasion and, as bile is so important, its waste is not allowed. The liver makes bile continuously, the excess being diverted to the gall bladder to be saved and concentrated until it is needed (for the next meal). When a hormone in the upper gut signals that fat is again present in the gut, the stored bile is forcibly ejected to perform its function.

Digestion of food in a carnivore is performed by enzymes produced by glands in the animal’s own body and all the absorption of nutrients in that food is through the wall of the small intestine.  The digestion of protein and fat, with little or no carbohydrate, in the carnivore’s gut is remarkably efficient. Experiments which have measured the amounts of various nutrients eaten and compared these with the amounts passed in the animal’s excreta have shown that a healthy animal loses no more than four percent of its fat intake and only a trace of the protein.

Carnivores lack the enzymes needed for breaking down the cell walls of plant matter. As a result, little to no digestion of carbohydrates can usually take place.

Large Intestine

The large intestine (colon) of carnivores is simple and very short, as its only purposes are to absorb salt and water and allow stool matter to form.   It is approximately the same diameter as the small intestine and, consequently, has a limited capacity to function as a reservoir. The colon is short and non-pouched. The muscle is distributed throughout the wall, giving the colon a smooth cylindrical appearance. Although a bacterial population is present in the colon of carnivores, its activities are essentially putrefactive.

Gut Flora

Practically the whole of the gastrointestinal tract of a carnivore is sterile.  Studies show that the majority of probiotic bacteria in the dog are Lactobacillus and Bifidobacterium species.  The same can be noted in the cat although there are fewer Bifidobacterium normally found. The hydrochloric acid in the stomach ensures that most potentially harmful bacteria and other micro-organisms swallowed in food are killed. Those that escape the stomach are rarely able to survive the digestive processes as bacteria are primarily protein.  The colon is the exception. This, where no further digestive processes occur, does tend to harbor a variety of organisms that form certain vitamins such as pyridoxine, vitamin B-12, biotin, vitamin K and folic acid; but, as these are not absorbed through the wall of the colon, they are of little account. These micro-organisms thrive in an alkaline environment and are of the putrefactive type.


Nutrient Digestion & Utilization  

Digestion involves a combination of mechanical, chemical and microbial events, all contributing to the sequential degradation of food components.  Many animal factors must be considered when evaluating digestibility. These include breed, age, gender, activity level and physiological state.

Gastric secretion is influenced by the amount of protein in a meal, the volume of the meal and by hormones that indirectly affect the acidity of the stomach contents.

Most of the enzymatic digestion of food occurs in the small intestine. It is also where microbial breakdown of food occurs. An important function of the micro-flora (good bacteria) of the small intestine is preventing the colonization of pathogenic organisms (bad bacteria). Gut micro-flora accomplish this by competing for available nutrients, controlling oxygen concentrations, and producing antibacterial substances directly affecting pet nutrition.

The primary role of the large intestine of dogs and cats is to absorb electrolytes and water and to a very minor extent serve as an environment for microbial fermentation of nutrients that escape digestion and absorption in the small intestine.

Matching the Diet to an animal’s Anatomy and Physiology

History tells us that the dog & cat evolved as a carnivore that fed on the flesh, blood, bones and organs of other animals.

Living conditions included fresh air, lots of exercise, sunlight, with grass and soils rich in trace minerals.  Their digestive system developed in respect to efficiently utilizing the prey it consumed.  It should be stated that the dog and cat have the same digestive systems today as their ancestors of yesteryear.

Domestication slowly imposed a different set of conditions that our dogs & cats had to adapt to:

  • Reduced exercise
  • Concrete and carpet instead of grass and soil
  • Fresh air and sunshine have been replaced by polluted environments
  • Man selected a new diet for his friend

We should not make the mistake of assuming that all is well with our pets just because they continue to survive in spite of these new conditions and diet.  For species to survive in nature they must be able to adapt.  Something our dogs and cats have managed to achieve; however, at what cost to their health?

Let’s take a quick look at how well our pets have adapted from a health standpoint.  The following are personal observations from many years of pet ownership and 40 years of veterinary medical experience.

Today’s pets have many of the following characteristics that depict a not so smooth adaptation to their new environment and diet:

  • Low energy levels
  • Overweight and unfit
  • Conformational & genetic defects abound
  • Skin & hair coat problems
  • Reproductive problems
  • Behavioral Problems
  • Multiple health problems
  • Relatively short lives

When applying well known scientific principles to determine what exactly our pets should be fed, we need to consider the anatomy and physiology of the animal we are feeding.

The percentages below represent the relative capacities of each section of the digestive tract for the three basic classifications of animals.

Carnivore (dog & cat) Omnivore Herbivore
Stomach 62.3% 29.2% 8.5%
Small Intestine 23.3% 33.5% 30.2%
Colon 14.4% 37.3% 61.3%

Very simply, protein is primarily broken down in the stomach; fats and carbohydrate primarily digested in the small intestine and water absorption occurs in the large intestine and colon to allow for fecal formation.

From this, we can predict that the carnivore diet should be balanced accordingly:

  • High in Protein
  • Low in Fat and Carbohydrate
  • Highly concentrated
  • Highly digestible
  • Low in fiber and other plant residues

Let’s examine the primary diet of the animals our canine and feline predecessors evolved on:

  • Herbivore meat sources (mice to deer) provide approximately 55% protein on a dry matter basis
  • These wild herbivores do not contain excess fat.  Healthy animals usually carry up to 14% of their body weight as fat.
  • Carbohydrate is only found as glycogen stores in the muscle of herbivores. Stomach contents would contain abundant quantities of carbohydrate; however, biologists that have studied the eating behavior of wolves and large cats report that meat, organs, blood and bones are consumed along with stomach linings but the stomach contents go uneaten.
  • Meat is highly concentrated in amino acids and other nutrients.  It is also easily digested by the extremely high acid content of the carnivore stomach.  Meat is also nutrient dense which makes it a perfect fit for the very short and simple digestive tract of the carnivore.
  • Fiber and fecal residue are the byproducts of plant matter (carbohydrate) which simply doesn’t work well within the carnivore digestive system that lacks the enzymes needed to break down the cell walls of most plant matter.

Example:  Man has been eating corn for over 10,000 years.  Even with their omnivore anatomy and physiology corn is not digested well.  Imagine what goes on in the digestive tract of the carnivore being fed corn.

A typical canine commercially prepared diet can be described as:

  • Low in Protein (18 – 30% protein)
  • Low to Moderate in Fat (8 – 20%)
  • High in Carbohydrate (30 – 55%)

This raises the question – what is the best diet for carnivores?

Carnivores have difficulty digesting grains and other complex carbohydrates. With the lack of digestive enzymes in the mouth, complex carbohydrates are not predigested.  They take a long time to break down in the stomach, and small intestine, if they break down at all. Most of the complex carbohydrates pass through undigested, and create larger than necessary stool volume.

Most commercial dry pet foods are still primarily cereal based, consisting of large amounts of corn, wheat, rice and soy. We are starting to see new entrees (grain free diets) emerging in popularity. After all, there are numerous studies that demonstrate the need for high-quality protein and little to no carbohydrate in our dogs and cats especially if optimum nutrition is the goal.

Dr. David Kronfeld, one of the most respected animal nutritionists, reported that carbohydrate may be useful in young animals just coming off their mother’s milk (which is around 12% carbohydrate) and for the lactating bitch and queen, who need three times the usual turnover of blood glucose for production of milk. He goes on to state that “no carbohydrate” need be provided for dogs or cats following weaning, not even for those subjected to hard work. The liver is easily able to synthesize sufficient glucose from amino acids and glycerol. He also stated that he feels the high carbohydrate content in pet foods is what contributes to coprophagy (stool eating), hypoglycemia and multiple pet disorders.

The success of today’s highly processed foods is mainly due to convenience, and good marketing. Pet food companies have convinced the public that their foods are complete and balanced. They use extensive marketing and advertising to convince the public that their diets are complex, and scientifically formulated.

Prior to the advent of commercially prepared foods, owners gave their pets scraps, raw meat, eggs, milk and bones. The digestive enzymes and bacteria found in fresh food helped pets digest food better, and built stronger immune systems. These necessary nutrients are not found in processed foods, as the processing and cooking destroys them. Pets began to develop coat, skin and allergy conditions. They also began to show dental problems, as their teeth are not designed for chewing, but for tearing and swallowing. Raw meat and bones contain enzymes and acids that help keep teeth clean, and also help to develop good musculature in the jaw and head.

Most of the fat used in processed foods can easily go rancid, and need preservatives to help maintain their integrity. Many of these preservatives (ethoxoquin, BHA and BHT) have been found to be detrimental to a pet’s health. They can inhibit the production of white blood cells, lower the immune system and block the absorption of glucose. Also lacking in processed foods are the Omega-3 fatty acids, necessary for good skin and coat health. These fats cannot withstand the long shelf life of processed foods.

Fat is crucial to carnivore nutrition for maintaining and creating energy, and to produce glycerol. While humans require carbohydrate for energy and endurance, carnivores need protein and fats to create glucose and develop stamina.

Achieving “Optimum Pet Nutrition”

Scientists teach us that food can make us healthy and strong or it can make us very sick and weak.  In 450 BC, Herodotus said the following: “All diseases to which man is subject proceed from food.”  This is also a widely held opinion among many scientists today.

As owners and veterinarians, we need to question the message provided by our commercial manufacturers and require much more of their offerings.  Nutrition is the foundation of life.  We owe it to our dogs and cats to do everything possible to get it right.

The good news – out of all of the forces of life that determine your pet’s health and well-being, Nutrition is the easiest to control.

A balanced diet contains optimal proportions of all essential nutrients, which can ultimately be utilized with maximal efficiency for any of life’s challenging experiences. Optimal amounts of each nutrient are best provided as a range rather than a single value because of the variation between an animal’s size, weight, activity level and health issues. In addition, a broad range of hair coats, body types, temperaments and environmental settings all complicate the estimation of a pet’s nutritional requirement.

Nutritional requirements are broad in undemanding situations like adult maintenance of a healthy pet in comfortable surroundings. The range narrows under more demanding situations, such as growth, reproduction, hard work, disease and stress.

Increasing a pet’s daily nutritional intake from minimal to optimum will have an immediate effect.  What does this actually mean in terms of what you will see and how fast you will see it?

Optimum nutrition consists of feeding the correct amounts of a biologically appropriate diet plus specific nutritional supplements to provide all necessary nutrients needed by the animal body during each physiological state or medical condition present.

What happens when optimum nutrition is provided?

  • Bones, ligaments and joints are strengthened and properly lubricated
  • Organs are fully nourished allowing them to function at peak performance
  • The cells of the body are fed properly giving them the ability to accomplish all of the special tasks required to create optimal health and well-being
  • Fat cells are slowly replaced with strong, lean muscle tissue.
  • The skin (the largest organ in the body) is strengthened so it can act as a complete protective barrier system
  • Hair coats will strengthen, become thicker, shinier and more colorful and shedding will be reduced
  • The dog and cat will begin to feel the effects of achieving optimal health and react accordingly with more energy, vitality and responsiveness

When optimum nutrition is at work you will be able to see firsthand that the animal feels better, looks better and acts happier.  Science has proven that a well-nourished animal is capable of remaining healthier throughout its life, healing itself and capable of living longer.

My recipe for success:

  1. Recognize that our pets are individuals with many unique and different nutritional needs throughout their lives. In other words, one food cannot meet the varying wellness needs throughout a pet’s entire life.
  2. Nutritional needs change constantly throughout life. Today’s pet diets have fixed protein -calorie ratios, which do not allow for adjustment when needed. Supplementation allows for adjustment – I call it fine tuning the diet to achieve Optimum levels of Wellness.
  3. Don’t buy into pet food advertising. Learn to read pet food labels so you can actually select a good basic pet diet that fits your pet’s nutritional needs.
  4. Feed 20% less than most pet foods recommend – daily amounts are set too high and are causing an obesity epidemic. Adjust food quantities by observing your pet and knowing when nutritional needs will increase or decrease.
  5. Use a fully balanced Nutritional Supplement to raise nutrient levels from minimum to optimum and to adjust a dog or cat’s nutritional needs throughout their entire life.
  6. Stay away from high calorie, low-quality pet treats.

Most people are aware of the pet health benefits derived from supplementing today’s highly processed foods. Yet, pet food manufacturers warn against the use of supplementation. The question is not whether you should add any supplements; but why not, what to use and how much to apply?

Most supplements today fall into the category of nutritional treats. I have a different theory about supplementation that comes from an understanding of pet health and how nutrients are digested, absorbed and utilized. Common sense dictates that if you are going to take a marginal pet diet and attempt to make it better with a supplement, the supplement must be significantly better than the food. This can only be accomplished if the supplement is totally complete – contains all of the necessary nutrients for the purpose intended; is highly concentrated so a small amount can actually create a difference; and, contains balance – which facilitates optimum absorption and utilization of all the nutrients being fed.

I have been developing nutritional supplements for the veterinary profession for over 35 years now.  During that time, I have clinically witnessed both dogs and cats achieving improved fitness and health with most living longer than non-supplemented counterparts.