ANW Apprenticeship Guide # 1
Dog and Cat Physiology and Digestion Basics
In this Guide, you will learn the basic physiology of your pet's Digestive System, from end to end, and how it pertains to your pet's dietary needs. This is a vital beginning step to understanding the nutritional needs of your dog or cat, equipping you to meet those needs, and creating a foundation of knowledge to build upon.
If you are ready to get started, let's go ahead and begin our Apprenticeship with the first lesson!
You've probably heard of "species-appropriate diets" many times. But what does it mean, exactly?
Both carnivores and herbivores have the same nutritional needs, essentially. In fact, whenever a carnivore eats an herbivore, it's getting nutrition from what the herbivore ate. In this way, carnivores make herbivores do all the work of eating the fiber, while the carnivores spend most of their time hunting.
That's the biggest difference between carnivores and herbivores. One spends most of their time eating (as one must do when eating high-fiber foods like plants), and the other spends most of their time hunting (as their meals consist of high-energy proteins and fats but the food is much harder to come by).
An herbivore is an animal that obtains its nutrients from plant sources. Herbivores will spend up to 20 hours a day eating. Carnivores, on the other hand, usually have several small meals throughout the day or one large meal, and consume mostly, or only, animal tissue.
The herbivore survives on a diet that is extremely low energy, low nutrition, and high fiber - when compared to a meat-based diet. As a result, their bodies have gone to great lengths to develop complicated systems to obtain nutrients from plant matter.
The herbivore has a large belly because it generally contains what is essentially an internal fermentation factory. Ruminants, such as cows, for example, have a series of complex stomachs - simply for eating grass and hay! That is why goats, cows, sheep, and other veggie-eating critters can look like they have big, bloated tummies.
Predators tend to have sleek, more ‘tucked in’ looks because they don’t have or need such plant-digesting factories in their guts.
The herbivore’s stomach also has very low acid content (or high pH), especially in comparison to carnivores and omnivores. This is partly due to their diets excluding the kind of pathogens you find in meat.
Some herbivores, such as rabbits and chinchillas, even have developed a system where they must re-ingest their own fecal matter (known as Caecotrophs [See-Kuh-Troughs]) in order to obtain the nutrients they missed out on during the first pass through their digestive tract. This is known as coprophagy, and while it sounds pretty gross to us humans, it’s an essential piece of nutrient absorption for many herbivores.
All in all, the takeaway here is that plants are very hard to digest!
Even in the case of true omnivores, it is most accurate to view them as carnivores that have the ability to digest some plant matter, and not as herbivores that have the ability to digest some meat. This is simply because of the incredible lengths the body must go to in order to obtain nutrients from a plant-based diet.
And this is where “species-appropriate diets” come in. The bioaccessibility of the food is the focus in these diets, and for good reason; the proportion of nutrition that is released from the food they eat, and is potentially available for digestion, in their unique gastrointestinal tracts, is what determines how nutritious and beneficial their diet is.
So pet owners that work to ensure the biology of their pet is considered in the food they choose to feed them are providing their pets with more species-appropriate diets, and as a result their pets often see many long-lasting health benefits.
These benefits are, unfortunately, mainly anecdotal. The reason most likely being that there is a severe lack of interest, and more importantly, funding, from the scientific community to research raw food for dogs and cats. Understandable - there is so much to research!
However, some studies on raw diets have been published in recent years and have shown promising results in terms of reducing cholesterol, for example, and other studies which look at the inefficient digestibility of commercial dry food diets, as well. Now, this is just the first guide, easing you into the basic biology of your dog or cat. So you won’t see a large number of references to the vast array of puzzle pieces that come together to build a nutritional picture for your pet. That will come later and over time. There is a lot to learn here!
But even with science lagging behind in giving us hard data and experimentation to tell us what nature has already made clear, we can make evidence-based dietary decisions for our pets simply by starting with the most obvious and basic clues. That means looking at their physiology and biology. Once one has learned about their pet’s digestive system and evaluated the nutritional needs of their pet, the right choices are easier to make.
If an animal’s biology tells us they are perfectly designed for ripping flesh, digesting animal tissue, and utilizing large amounts of protein while lacking an ability to use plant matter nearly as well, then before we even get the additional confirmation we would like from raw food studies by unbiased researchers, we know that animal should be eating meat - and not a carbohydrate-heavy diet.
Now, there are basically four steps in the digestion process for your pet dog or cat, and that is the mechanical, then the chemical breakdown of food, the absorption of nutrients, and then the elimination of waste and indigestible bits.
Again, the digestive tract of an animal will help us determine exactly what kind of food should be in their diet; namely, meat or vegetation.
First, 'Digestion' is the process of turning raw ingredients (the food eaten) into metabolizable and absorbable nutrients. For example, when you eat a burger or a salad, your body goes to work breaking down all the components of that burger or salad into smaller and smaller units, obtaining the nutrients from within, and disposing of the bits it couldn't (or shouldn't) digest or absorb.
When you eat food that your body is ready and capable of breaking down, you can obtain more nutrients from the food, and avoid symptoms of indigestible food - such as gas, bad breath, dry skin, brittle hair, inflamed bowels, and much more. Our pets can suffer from all of these symptoms when eating indigestible food as well. This is why, when your beloved boxer is clearing out rooms with their noxious bodily outputs, it is wise to look at a food that agrees better with their stomach as a solution.
Now, both dogs and cats are classified as carnivores. Yes, you may have heard that your dog is an omnivore, and much of the time you will want to consider the source. If you perform an internet search to ask if your dog is an omnivore, the largest players in the pet industry, including Purina, Hill's Science Diet, and Royal Canin, will tell you that yes, your dog is an Omnivore, and can digest starches just fine!
And that means you can feed your pet their starchy kibble, and it makes perfect biological sense, right?
Nope.
We know by looking at our dog’s biology that it doesn’t really fit. In fact, what makes the most sense is that these very large corporations are likely referring to a dog’s omnivorous traits, as a facultative carnivore, to sell pet owners on the idea that a kibble made up mostly of grains, or wheat, or potatoes is not only acceptable but the most healthy option.
Biology tells us otherwise.
Wizard's Note: Starch is a polysaccharide (a carbohydrate consisting of a number of sugar molecules all bonded together). This is also known as a Complex Carbohydrate, which takes longer than Simple Carbohydrates, such as table sugar and syrup, to break down because it has so many molecules of sugar bonded together. Throughout this guide, I may use the terms Starch and Carbohydrate intermittently. Examples of Starches/Complex Carbohydrates include Peas, Beans, Oatmeal, Quinoa, Barley, Potatoes, Sweet Potatoes, Whole Grains, and Vegetables.
Dogs are of the Order Carnivora, and they have the second shortest digestive tract of all mammals, the first shortest belongs to the cat. And short digestive tracts, you will learn through this guide, are not ideal for breaking down tough, fibrous plant matter such as starches (and remember, that includes everything from peas, to corn, to potatoes!)
The best way to describe your dog in terms of its dietary designation is as a Scavenger or Facultative Carnivore. And your cat, the truest of carnivores, is an Obligate Carnivore.
A Facultative Carnivore, or Scavenger, is primarily a meat eater, best equipped for a diet high in protein and fat, but which can survive on plant matter if absolutely necessary. Dogs do not thrive on a high-starch diet, but they can certainly survive on one.
An obligate carnivore, like your cat, is an animal that must eat meat to survive and will thrive only on a diet high in animal protein, moderate in fat, and very low in carbohydrates.
Once more, when you hear about "species-appropriate diets", that's also what it means. It is in reference to the Order designation of your pet, and what they are built to eat and thrive on. Or, as I like to call it, their "Physiological Food".
For both dogs and cats, that is meat.
Now let's see how they go about digesting that meat!
The function of your pet's teeth is primarily for tearing and gripping food. Adult dogs have 42 teeth, 10 of which are molars with the same function as human molars - which is to help grind foods. Adult cats have 30 teeth, none of which, including their molars, have what are called grinding surfaces (the surfaces needed to properly grind plant matter).
Both dogs and cats lack the ability to move the jaw horizontally. Horizontal jaw movement is what allows for grinding and chewing like herbivores do all day. Omnivores and herbivores also have large, flat molars that allow them to grind plant matter.
If you were to look at images of many herbivores’ mouths, you would likely see their teeth are generally very flat and wide, and that their jaws move side to side whilst chewing. This allows for the mechanical first step of breaking down fibrous plant matter.
In a carnivore’s mouth, the mechanical first step is to rip and shred meat from the bone, and this is why their teeth are sharp, and/or long and curved.
One concern many cat owners have is their cat not chewing its food. This is because cats don't really "chew" - they aren't built for it. Their teeth are not meant to chew, and their jaws are physically unable to make the correct grinding motion. Many cats will break up kibble in their mouths, however, and this is interpreted as normal chewing behavior. Some will even gnaw on treats or inappropriate objects, like your television wires. But for the majority of cats, chewing as we know it is a somewhat unnatural part of their eating process.
The cat's mouth is more adapted to a prey diet than most other carnivores. They also have a rough tongue that helps them remove flesh from the animals they eat.
Also in the mouth, of course, is saliva. Saliva is 98% water, but also includes a mixture of sodium and other minerals and phosphates as well as immunoglobulins, proteins including enzymes, and nitrogenous products, such as ammonia.
Here is an interesting difference between humans and our dogs and cats; In humans, our saliva contains an enzyme called Amylase that breaks down carbohydrates into simple sugars. For us, chemical digestion actually begins in the mouth - before food even reaches our stomach!
Most mammals, in fact, have this enzyme in their mouths. But the amount varies from species to species.
Dogs, for example, have been shown to have a very low abundance of this enzyme, especially in comparison to humans. It’s also higher in terms of pH. And both the meager amount of amylase and the difference in acidity reflect their expected diet in comparison to an omnivore such as a human. Their entire mouth is telling us that the dog requires a diet consisting primarily of meat, organs, and bones from prey.
In fact, because of this lack of enzymes to help break them down, carbohydrates in the diet help lead to plaque, tartar, and tooth decay more in dogs. As I have discussed in one of my live Q&A Sessions, kibble does not prevent Periodontal Disease (PD). The only dry diets which have been documented and clinically proven to be somewhat effective as a dietary preventative oral health measure are veterinary dental diets. But these diets are cost-prohibitive, and they still come with all the other problems that kibble entails. There is also good evidence that a natural diet protects your cat’s teeth better than a commercial diet, according to at least one study.
It is objective fact that feeding kibble is not an effective method for keeping your pet's teeth clean and healthy. Said succinctly, kibble does not ‘clean’ teeth. There is evidence that much larger kibbles have a somewhat better effect on dental health, but not all dogs and cats can eat such large kibble, and it’s not by any means enough on its own.
All this to say, it is evident simply by reviewing what we’ve learned about your dog’s and cats’ mouths alone, that starch-based kibble is not an ideal, species-appropriate diet.
Wizard’s Note: Enzymes are specialized proteins that act as catalysts. Digestive enzymes, of course, help ensure digestion occurs! But more than that, without enzymes, life itself would not be possible.
Now, this guide is meant to help you learn about what cats and dogs are adapted for eating, from tooth to tail. We’ve covered teeth. Now let’s talk a little about what lies between their mouths and their tails; the stomach.
The walls of the stomach are folded, which increases the surface areas, and it’s very muscular - mixing and blending the food we eat. The stomach also houses several glands which produce "digestive juices", mainly to break down protein and bone, and destroy bacteria (particularly salmonella spp, clostridia, campylobacter, and E Coli - all of which are naturally in the body but have the potential to become pathogenic) that are sensitive to strong acids (which is why your dog and cat are so well adapted for eating raw meat!)
The stomach will store food and liquid, mixing and digesting them in these juices, slowly emptying them into the small intestine. There is more that goes on here, but keep in mind these are just the basics!
Now, the entire Gastrointestinal Tract, or GIT, is lined with a mucus membrane that protects it; from bacteria, from sharp edges such as on bones or claws, and from the powerful enzymes coming from the upper gut. As these enzymes are designed for breaking down tissue, it's very important to keep that mucus membrane healthy, because it prevents the GIT from digesting itself!
In the small intestines, more specialized enzymes break the foods down further. Fat, carbohydrates, and proteins are broken up into usable units at different stages.
Situated away from the central point of the stomach (or distal) is the pancreas, which is both an organ and a gland duo, whose main job is to produce hormones and pancreatic juice - a digestive enzyme blend to help digest and absorb nutrients in the small intestine.
The pH of the stomach largely dictates the actions the pancreas takes in digestion, and a compound called Pancreatic Secretory Trypsin Inhibitor (or PSTI) keeps the enzymes within the pancreas inactive until they reach the intestines. These inhibitors are crucial for protecting the pancreas from being destroyed by its own enzymes - which is essentially what acute pancreatitis is, and why it's potentially fatal.
Of course, many more enzymes are found in the lining of the wall of the small intestines, that perform a variety of functions - all of these help with continuing the process of food breakdown.
The food then passes through its last stop, the large intestine (or to be more precise, the rectum, but for the purposes of this guide, we’ll skip that part). Its main role is really water regulation; it absorbs liquid to hydrate the body before the food is passed outside (i.e. feces). The large intestines also absorb electrolytes - essential for nerves and muscles to function, such as the heart - and it helps to form and propel the final waste before it leaves the body.
Interestingly, in cases of a bacterial infection or food poisoning, it can also do the opposite and add water into the feces in order to help flush the irritants out of the body. The results of which are uncomfortable, as anyone who's experienced food poisoning would know, but they do serve an important purpose.
The beneficial bacteria (or microbiota) within the large intestine, known as the Microbiome, also play a huge role here - not only in digesting food but keeping out or reducing “bad” bacteria. They also produce valuable micronutrients as a by-product of eating some of the food passing through.
Wizard’s Note: This is actually only one of the many microbiomes I will discuss throughout these guides, which are pivotal to both mental and physical health. Realizing that I’m glossing over the Microbiome for the sake of keeping this guide simple and not too overwhelming, it would be irresponsible of me not to point out that its an incredibly paramount piece of the nutritional puzzle, and one that cannot be ignored if one is to become a wizard in their pet’s health! Protecting your pet's microbiomes is the best, and really only way, to have a healthy, thriving animal. Unprocessed, biologically appropriate foods are as important for the gut microbiota as they are for your pet!
Again, the dog and cat have the shortest digestive systems of any mammals. A short digestive tract is typical of carnivores. It can take 8-9 hours for food to pass through the entire digestive system. This process is slower in puppies and kittens because their system isn't mature yet. It takes time for the gut and microbiome to become fully functional.
These numbers will vary a bit depending on where you look, and in most cases, the human intestinal length is referenced as being longer than a dog - but you get the idea.
The large intestines of both dogs and cats are far smaller than that of herbivores like horses, or even some omnivores like pigs. This allows food to pass through much more quickly.
The large intestines and hind guts of herbivores are used mainly for fermenting fiber, so they must be much longer. But remember, omnivores like pigs and humans still have an easier time digesting meat than plants. We simply don't have the complex systems in place that ruminants and all other herbivores have to obtain the most nutrients from plants.
As omnivorous primates, humans can thrive on a variety of foods; meats, organs, fish, eggs, fruits, and vegetables. And, while our systems are longer than an obligate carnivore, even we don't thrive on a diet that is heavy on starches and sugars.
Congratulations! You’ve just completed my guide on the basics of your pet's digestive system.
You should now be able to -
Easily tell the difference between an herbivore and a carnivore
Understand and explain why dogs and cats aren’t designed for eating plant matter but for eating meat and bone
Talk about basic dog and cat physiology
Thank you for reading, and remember - Nutrition is magic!