DIGEST THIS: Kibble May Actually Digest Faster Than Raw

DIGEST THIS: Kibble May Actually Digest Faster Than Raw

As raw feeders, we’ve all heard the ominous warning to “never mix kibble and raw because kibble digests slower than raw.” Where did this sage piece of advice originate, and is there any truth to it? Why are dogs so commonly advised to suffer through a “detox phase” consisting of raging diarrhea and vomiting, if they could be slowly transitioned from kibble to raw in the same manner that is recommended to switch between kibble varieties? The Raw Feeding Community decided to put this myth to the test, and the results of our “experiment” may surprise you…

The Usual Advice
Scanning the top holistic and raw feeding groups and forums, one will find the same advice and explanation listed time and time again. A search of “switching from kibble to raw” on the Dogs Naturally website brings up the follow information:

“I recommend you feed dry or canned food separately from dehydrated or raw meals because they digest at different rates.” (1)

The first time I came across this tip, I remember thinking to myself, “Why does this matter?”, so I read on. This statement is usually followed up with an explanation along the lines of, “raw food digests faster than kibble and mixing them together will cause raw food to sit in the stomach too long. This makes the body more likely to contract bacteria from the raw food.”

Does raw really digest faster than kibble? We posted a thread asking the members of The Raw Feeding Community how long it takes kibble to digest, and received some interesting, although extremely inconsistent, answers. Some claimed kibble took as little as 2 to 3 hours to digest, yet there were a surprising amount of answers that claimed kibble took as much as 16 to 18 hours!

However, even if the digestion rates of kibble and raw differ, this explanation still doesn’t make much sense from a scientific standpoint. That just isn’t how the digestive system works; especially the short, acidic digestive system of a carnivore. And isn’t part of the argument for raw feeding that our carnivore’s bodies are designed to handle the large amounts of bacteria associated with consuming raw animal products? What about raw food would make it more likely to digest faster than kibble? Why would gastrointestinal transit time affect bacterial susceptibility? We know that candy goes through the body faster than a strawberry; aren’t manufactured products supposed to digest more quickly than their raw, uncooked counterparts?

Many people have successfully fed raw and kibble together for years without issue. In fact, feeding raw and kibble together is very common for sled dog teams (here is a cool video of what an Iditarod sled team is fed during the race). It seems to be only relatively recent advise to avoid this kind of feeding. Why should owners be advised to avoid mixing the two? Wouldn’t some raw be better than no raw at all? The Raw Feeding Community decided to find out where this advice could’ve possibly originated from, and if there were any amount of truth to it.

Our Test Dog
For our experiment, we used a 16 month old, intact male, raw fed borzoi who was weaned to partial raw and has been kept on a raw diet throughout his life (prey model raw with occasional supplements). A barium series was performed on December 18, 2014 feeding a kibble meal, and on December 27, 2014 feeding a raw meal. Barium is a radiopaque liquid which can be mixed with food and used to enhance visibility of the gastrointestinal tract on X-ray.

Our test dog
Our test dog, Jaeger.

The Kibble Used
For the kibble study, 2 cups of Science Diet Large Breed Adult kibble was mixed with ½ cup of Science Diet I/D canned and 20mls of barium liquid. Approximately 1 ½ cups of kibble and ½ can of food were consumed over a span of 5 minutes. Thirty minutes later, cranial (top half of the GI tract) and caudal (bottom half of the GI tract) abdominal and lateral (laying on the side) abdominal radiographs were performed. Radiographs were repeated at 1hr, 2hrs, 3hrs, and 4hrs until the stomach was empty (2).

The Raw Used
For the raw meal, 1 cup of bison and green tripe grind (3), 1 cup of chicken gizzards, and 1 chicken drumstick mixed with 20mls of liquid barium were offered over 5  minutes. Approximately ½ of the meal was consumed (including only ½ of the drumstick.) Thirty minutes post-feeding, cranial and caudal abdominal and lateral abdominal radiographs were performed. Radiographs were repeated at 1hr, 2hrs, 3hrs, 4hrs, and 5hrs until the stomach was empty.

Results
Going into this case study, I expected raw to digest more quickly than kibble, simply because I expected there to be some shred of truth to all of the information provided by knowledgeable raw feeders. I personally hypothesized that there would not be a significant (greater than 3 hour) difference between the time it took the food to move from the stomach into the large intestines.

Digestive Anatomy and Physiology Review
Before going into detailed results, it helps to have a basic orientation of what you are looking at and what exactly is happening as food moves through the body. Below are cranial and caudal lateral abdominal x-rays with the major body parts labeled.

Figure 1. Labeled to show the major organs and structures as viewed radiographically. These x-rays were taken 30 minutes after feeding a kibble meal (mixed with barium).
Figure 1. Labeled to show the major organs and structures as viewed radiographically. These x-rays were taken 30 minutes after feeding a kibble meal (mixed with barium).

Carnivores have the shortest gastrointestinal tracts of any mammal. Food breakdown in all mammalian species begins in the mouth where salivary enzymes are released and mechanical digestion is initiated by the tearing and grinding of food. With carnivores, the majority of food breakdown is accomplished by the stomach and intestines as opposed to omnivores and herbivores who rely on more “intense” mastication to “break down” their food.  Food travels from the mouth, into the esophagus, and reaches the stomach in a matter of seconds.

Once in the stomach, the real work begins. Chemical digestion of proteins is initiated by enzymes like hydrochloric acid, pepsin, and lipase and food is liquidized. Hydrochloric acid is the enzyme responsible for denaturing proteins, eliminating bacteria, and converting other enzymes. Pepsin is solely responsible for protein digestion and lipase, which is only found in the stomach of carnivores, begins digesting fats (carbohydrate digestion begins in the mouth with the addition of the enzyme amylase) (4).

After being turned into liquid, food begins to leave the stomach via a sphincter in a region called the pylorus. The first segment of the small intestines, the duodenum, contains the openings of the pancreatic duct and the common bile duct (gallbladder). Bile and pancreatic enzymes are critical for the absorption of fats, carbohydrates, and proteins. The duodenum is the shortest segment of the intestines, but due to the addition of these critical enzymes, it is where the majority of chemical digestion takes place.

The next segment of the small intestines, the jejunum, is also the longest, and is where the majority of nutrient absorption and continued chemical digestion takes place. The surface of the jejunum is covered in small, finger-like projections called villi, which serve to increase the surface area for food absorption (5). The final section of the small intestines, the ileum, serves as a final place for digestion – in herbivores, the ileum contains a high-functioning cecum and plays a much more critical role.

Finally, food moves into the large intestines (colon). In carnivores, this is where water is absorbed, bacterial fermentation takes place, and feces are formed.  Now that we have visually seen the anatomy and followed food on its path through the gastrointestinal system, we will move on to the actual results of our “experiment.”

Thirty Minutes Post Feeding

  Figure 2. Cranial view of abdomen 30 minutes post feeding a kibble (left) and a raw (right) meal.
Figure 2. Cranial view of abdomen 30 minutes post feeding a kibble (left) and a raw (right) meal.
Figure 3. Caudal view of abdomen 30 minutes post feeding a kibble (left) and a raw (right) meal.
Figure 3. Caudal view of abdomen 30 minutes post feeding a kibble (left) and a raw (right) meal.

After 30 minutes, we see both stomachs moderately distended with food and barium. With the kibble meal, the food and barium has settled into the pylorus (bottom of the stomach) and is moving into the small intestines. There is negligible amounts of barium moving into the small intestines with the raw meal because there is likely a large portion of meat preventing the barium from filling the entire stomach and moving out.

 One Hour Post Feeding

Figure 4. Cranial view of abdomen 1 hour post feeding a kibble (left) and a raw (right) meal.
Figure 4. Cranial view of abdomen 1 hour post feeding a kibble (left) and a raw (right) meal.
Figure 5. Caudal view of abdomen 1 hour post feeding a kibble (left) and a raw (right) meal.
Figure 5. Caudal view of abdomen 1 hour post feeding a kibble (left) and a raw (right) meal.

One hour post feeding, when can see the diameter of the stomach starting to decrease and the small intestines are filling with barium. Movement from the stomach into the small intestines is much more dramatic with the kibble meal than the raw.  In both cases, we can begin to see feces (not highlighted with barium) being formed in the large intestines – remains from his previous meal.

Two Hours Post Feeding

Figure 6. Cranial view of abdomen 2 hours post feeding a kibble (left) and a raw (right) meal.
Figure 7. Caudal view of abdomen 2 hours post feeding a kibble (left) and raw (right) meal.
Figure 7. Caudal view of abdomen 2 hours post feeding a kibble (left) and raw (right) meal.

At 2 hours post feeding, the difference in the rate at which food has left the stomach and entered the small intestines (jejunum) between the kibble and raw fed meals is undeniable. The kibble meal is moving into the intestines faster than the raw meal. Formed feces in the colon are becoming more obvious.

Three Hours Post Feeding

Figure 8. Cranial view of abdomen 3 hours post feeding a kibble (left) and a raw (right) meal.
Figure 8. Cranial view of abdomen 3 hours post feeding a kibble (left) and a raw (right) meal.

Three hours post feeding our kibble meal has mostly left the stomach, filled the small intestines, and is moving into the large intestines. It is safe to say that kibble has completely left the stomach by 3 hours – a far cry from what is usually claimed. Movement with the raw meal is definitely slower, but we can see decreased opacity in the stomach and definite movement of feces through the colon. Unfortunately, the caudal kibble radiograph was somehow lost so it is not available for comparison.

Four Hours Post Feeding

Figure 9. Cranial view of abdomen 4 hours post feeding a kibble (left) and a raw (right) meal.
Figure 9. Cranial view of abdomen 4 hours post feeding a kibble (left) and a raw (right) meal.
Figure 10. Caudal view of abdomen 4 hours post feeding a kibble (left) and a raw (right) meal.
Figure 10. Caudal view of abdomen 4 hours post feeding a kibble (left) and a raw (right) meal.

At four hours post feeding, you begin to notice a drastic difference in the amount of ingesta left in the kibble verses raw photos. Although the same amount “went in” with both meals, there is significant less food in the intestines with the raw meal. I’ll talk about why that difference is important in the discussion.

Five Hours Post Feeding

Figure 11. Cranial and caudal view of abdomen 5 hours post feeding a raw meal.
Figure 11. Cranial and caudal view of abdomen 5 hours post feeding a raw meal.

For the raw meal, I had time to take a final radiograph 5 ½ hours post feeding. The stomach, surprisingly, still had the bone fragment floating around, but the majority of ingesta was in the colon, as expected.

Discussion
In conclusion, the raw meal appears to have digested slightly slower than the kibble diet. When you really think about it, it makes total sense – and it is great news for our dogs, and for us raw feeders! Here is why:

  • The first reason is obvious – this means that little credence can be given to the claim that mixing raw and kibble is harmful to your pet because raw sits in the stomach longer. Rarely, a dog does not tolerate a mixed kibble and raw meal, but it has nothing to do with digestion rates. Any kind of sudden diet change typically results in digestive upset; it is not exclusive to feeding raw and kibble together.
  • Also worth mentioning is the bone fragment that stayed behind more than 5 hours after feeding, implying that whole bone takes much longer still to digest. This implies that raw meaty bones will take even longer to digest than the ground raw with bone that was used in this experiment. This makes sense, as a whole bone will obviously take more time to break down than ground bone, which was already broken down before the dog ate it. The ground raw that this dog ate also only contained 10% bone, while the chicken drumstick would have a much higher bone content.
  • By mixing raw and kibble during the transition period, your dog is less likely to experience the diarrhea and vomiting associated with “detox.” It is commonly recommended to slowly transition (over a period of 10-14 days) between kibble brands. I personally recommend doing the same with raw to avoid causing diarrhea. What many call “detox” is a normal reaction to a sudden change in diet (dietary intolerance). The pH and flora of the stomach change when a dog is fed a raw diet, so slowly transitioning gives the body time to adjust. The way I recommend transitioning is to slowly start with small pieces of raw (chicken is often best) while decreasing the amount of kibble over a period of two weeks. After your pet is fully transitioned onto raw, you may then begin adding more components (organs and other proteins) to the diet. You can see our beginner files on The Raw Feeding Community facebook group for more information.
  • If food remains in the intestines longer, more water is absorbed, thus leading to the beloved small raw feces. The reason for big voluminous kibble feces is because there is so much left undigested and unused.
  • Most importantly, remaining in the GI tract for longer periods of time means that the body is able to absorb and utilize more nutrients from the food. Why do raw fed dogs often have shinier coats, less skin issues, and seem to experience a decreased incidence of disease? Because their bodies are better able to utilize the good stuff in raw! Not only do they get the benefit of taking up the maximum amount of nutrients, studies have shown that eating whole foods burn more calories which in turn leads to a fitter animal (6).

Although one animal is by no means enough to declare conclusively that raw digests slower than kibble in every instance, the results strongly suggest that the broad blanket statement that “raw digests faster than kibble” is false. And even if one does digest quicker than the other, it would still have no effect on whether or not they would be safe to feed together.

By conducting this experiment and publishing these results, all we mean to do is encourage people to question everything they are told online, and look for proof rather than anecdotal evidence. There is an unfortunate lack of peer reviewed studies when it comes to raw diets. A lot of what is claimed online tends to be simply hear-say or anecdotal evidence. As raw feeders, we should strive to always continue learning more about what we are feeding our pets, even if what we are learning happens to go against the grain of what most raw feeding Facebook groups preach.

References

  1. http://www.dogsnaturallymagazine.com/switching-dogs-kibble-raw/
  2. http://www.dogfoodadvisor.com/dog-food-reviews/hills-science-diet-dog-food-adult-dry/
  3. Grind produced by Texas Tripe. Made with bison (meat, organs, and bones). Contains approximately 10% fat, 10% bone. texastripe.com
  4. http://www.ncbi.nlm.nih.gov/pubmed/2036471
  5. http://www.peteducation.com/article.cfm?c=2+2083&aid=512
  6. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2897733/
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