As mentioned in Gut 101, billions if not trillions of gut bacteria live in the cat's bowel. There are more bacteria there than there are cells in the body, several hundred different species of bacteria in uncountable numbers. The mix of bacterial populations in the host species differs somewhat so the mix in a cat will differ from that in a dog or a human but the principles are the same.
Gut bacteria live on dietary fiber, by definition the leftovers from food digestion, what was not digested and absorbed into the cat herself upstream in the gut. The bacteria also make use of shed gut wall cells. Due to heavy use, the gut wall is built to shed and renew its cells faster than other parts of the body, to maintain its integrity when properly nourished.
Food feeds the cat. Fiber feeds the gut bacteria.
The populations of gut bacteria living in the bowel qualify as an organ, as an organized functional unit dedicated to specific purposes, and this is the most metabolically active organ in the body. Not only do gut bacteria metabolize components of dietary fiber but they stimulate and 'train' the immune system, occupy territory to prevent foreign invaders from setting up housekeeping, synthesize vitamins, and play a key role in preventing constipation since their work factors in the creation of normal stools
Important research about the role of fiber in the diet of the cat continues:
What does all that mean? It means that even though dietary fiber does not feed the cat herself, fiber is nonetheless essential for the health of the gut and therefore the health of the cat. There needs to be something in the diet that is not digestible and is not absorbable, that flows on to the gut bacteria waiting in the large intestine (aka the large bowel).
The phrase 'mucosal barrier functions' describes the necessary integrity of the gut wall, keeping it healthy and strong so what is being digested in the gut stays in the gut until it is ready to be absorbed into the body and so what does not belong stays out of the body and passes on out in the poop. The gut wall keeps gut bacteria in their place. We want those bacteria working in the bowel, outside the body itself, not invading or translocating into the body to cause septicemia, a toxic bacterial systemic infection.
Why do we care? Aren't bacteria bad news? Not necessarily. We live in the real world and the real world is not a sterile place, nor can we render ourselves sterile no matter how much we wash, no matter how many antiseptic products we use. We are all colonized starting at birth with trillions of bacteria on the outsides of ourselves and, as we learned, the gut is technically outside the body. Rather than try to keep bacteria away, alliances and truces were developed eons ago so that under normal circumstances, peace exists. Our familiar bacteria, those that live all over the skin and inside the gut help fend off and control less beneficial bacteria and other microscopic organisms. Our own bacteria depend on the host, in this case the cat, for their sustenance and, in turn, provide benefit to the cat. But rather than trust the alliances completely, the gut wall actively works to keep bacteria in their work place and not invade the body and well kept bacteria contribute to maintaining the gut wall. This activity is the mucosal barrier function and proper fiber choices in the cat's diet help keep the desirable gut bacteria well fed and content in their assigned place so they continue to fulfill their end of the bargain, as we see below. A healthy gut wall performs good traffic control.
Food feeds the cat. Fiber feeds the gut bacteria. And the gut bacteria play an important role in helping to prevent constipation.
Since the gut wall barrier forms a significant part of the immune system, the gut bacteria are key players in maintaining healthy immunity as well.
Fiber is the parts of foods that cannot be digested or absorbed and what we call dietary fiber comes from plant material. Fiber can be insoluble (does not dissolve in water) or soluble (does dissolve). Fiber can be fermentable or unfermentable. Fermentable means there are gut bacteria present in the crowd who can utilize or 'eat' or ferment that fiber source. A particular fiber is not fermentable unless someone is around to ferment it.
Insoluble fiber is considerably less fermentable than soluble fiber; it mainly acts as bulk to increase stool volume since it holds water like a spongle without dissolving in that water.
Few fiber sources and choices are all one or all the other, they are a mix of soluble and insoluble in varying percentages. So a single fiber source can provide both bulk, with its water holding capacity, and provide food for the gut bacteria, who also influence the water retained in the stool and whose sheer numbers contribute to stool volume.
There are beneficial bacteria and pathogenic bacteria in the bowel as there are everywhere. A proper choice of fermentable fiber preferentially feeds the good guys and keeps their numbers higher. The good bacteria are those who provide benefit to the host, in this case the cat. The by-products of the undesirable or pathogenic gut bacteria are irritating and toxic to the gut wall and the enteric nervous system.
If beneficial bacterial numbers are reduced by antibiotics or by lack of species-appropriate fermentable fiber and the less desirable bacteria flourish because their competition is reduced, a cat may experience bowel problems. Longer term, damage to the gut wall and the enteric nervous system could result.
Fiber is not visible, it is not like the strings on celery. In the human diet, a banana is rich in fiber even though its flesh is smooth. Fiber consists of certain nondigestible and nonabsorbable complex sugars present mainly in plant material, sugar complexes which digestive enzymes cannot digest (split apart), leaving them too large to be absorbed upstream. Fiber is present in plants and in small amounts in whole prey such as mice. A cat consuming a whole mouse in the wild eats all of the prey plus the contents of the prey's digestive tract which include plant material. Cats tend to get most of their plant material indirectly. The prey eats the plants, the cat eats the prey.
A side note here about cats and plant material. Plants are unable to run away from danger or protect themselves by socking someone in the nose. Plants depend on physical protection such as a thick shell or rind, and/or chemical warfare. Plants are remarkable chemical factories, even our familiar veggies. All of those chemicals need to be properly metabolized by the eater to be safe for the eater, to avoid harm to the digestive system. Humans, as omnivores who eat a wide variety of foods, evolved with liver enzymes that can detoxify many plants chemicals. So did most of the cat's prey species like the mouse. But our carnivorous cats did not, they lack certain liver detoxification pathways that we take for granted. Since cats ate very little plant material directly, instead running most of their plant material through their prey first and then eating the prey, we need to make appropriate plant fiber choices for cats.
Because many of us think of the body in terms of mechanical function, we often think of fiber only as a bulking agent, bulking up the stool by holding water like a sponge to make the stool softer. I hope this chapter expands the understanding of fiber. Water, as we learned, is what makes the difference between a softer stool and a harder stool but bulking fiber is not the only way to achieve that end and not a desirable approach if it starves the gut bacteria.
The natural diet of cats does not contain much bulk; a cat's digestive tract is not designed for large bulky stools such as that of a herbivore. A normal stool for a cat is smallish in diameter and firmish but not rock hard. As a carnivore, a cat naturally produces a segmented stool, not pellets or patties.
The stool should be soft enough to mold when passing through the anus which is slightly smaller than the expanded rectum. But the stool should be firm enough to respond well to pressure from the gut wall when pushing the stool out. Poop that is too soft is like pushing a chain and may require extra effort. It also does not properly stimulate the anal glands of a cat which can lead to anal gland impaction.
A particular fiber in the diet is fermentable if there is a bacterial species present in the bowel population who is able to ferment it. Different species of bacteria ferment different fibers, just as different mammal or bird species eat different diets. Of course there is also overlap, just as the species of humans and cats alike eat turkey.
Food feeds the cat. Fiber feeds the gut bacteria. And the gut bacteria play an important role in helping to prevent constipation as well as maintaining the health and integrity of the gut wall.
Bacteria obviously digest their food sources outside their bodies, they lack an internalized gut. Bacteria 'digest' or ferment the dietary fiber they can make use of, using enzymes just as we do to split large molecules into smaller molecules, and in this process they produce short-chain fatty acids (SCFAs) which provide various benefits to the host (the cat).
One type of SCFA, butyrate (butyric acid), is the chief fuel for the cells of the gut wall. That fuel is important because the cells of the gut wall have a high turnover rate, they are renewed about every three days. All this cell renewal requires energy. As the barrier between the outside world and the inside of the body, it is essential that the intestinal wall has the fuel necessary to replace shed gut wall cells with new ones, to remain in good repair. The gut wall sees a lot of activity.
Another SCFA produced by gut bacteria is acetate (acetic acid) which plays an important role in liver fatty acid metabolism. Remember, although the inside of the digestive tract is outside of the body, the digestive system includes the liver as well as the gall bladder and pancreas so it should not surprise us that something produced by the gut bacteria in the large bowel could provide benefit to the liver.
There are other SCFAs produced by the gut bacteria which are important and which are under research so we will all know more in the future. What we know to date is that SCFAs are important and cats can only benefit as we learn more.
In addition, and this is especially important for a constipated cat, the SCFAs produced by the beneficial gut bacteria help maintain the pH of the stool in the slightly acidic range which helps to retain water in the stool. Water retention in the stool is a function of chemistry. That means that simply adding water to the cat may not do the job.
This slightly acidic range also discourages pathogenic bacteria from setting up housekeeping and reproducing. Keeping the beneficial bacteria populations high means they can convert the nitrogenous (protein) waste products into themselves, into more friendly bacterial bodies made of protein, which prevents these more toxic protein waste products from harming the gut wall, turns lead into gold as it were. Harm to the gut wall can result in a higher cellular turnover rate, as the body struggles to heal, which then means even more nitrogenous waste products for pathogenic bacteria, a vicious circle. The more beneficial gut bacterial numbers, the more of their beneficial by-products produced.
Gut bacteria also produce some B vitamins and Vitamin K. It is not clear yet whether those B and K vitamins are absorbed or whether they only benefit locally in the bowel.
Properly fed to keep their numbers higher, beneficial bacteria reduce the amount of physical space left for pathogenic bacteria. The beneficial bacteria who live directly on the gut wall actively work to protect their turf from invaders with ill intent.
This is a remarkable arrangement. The bacteria have a home and the host receives a multitude of benefits.
Please note that SFCAs, short-chain fatty acids, are not the same as essential fatty acids (EFAs) which most of us recognize as Omega 3s in salmon and other cold-water fish. The EFAs, the essential fatty acids DHA and EPA, are dietary components which the cat herself can digest and absorb. The SFCAs are chemical by-products the gut bacteria produce which benefit the digestive system and the poop.
Fermentable fiber in the diet is essential, not fiber just for 'bulk' in the stool as we are used to thinking, but fiber as food for the gut bacteria to keep the bowel healthy. Half or more of the dry matter weight of poop consists of gut bacteria, which leads us into the next chapter ...
Poop follows . . . . . .
“Cats require purity and simplicity.” – SEM