(Click on the boxes in the chart to the left to navigate through digestive system comments)
A raw diet is a species appropriate diet based upon feeding raw meat, vegetables and fruit. Dogs and cats are carnivores, which lends the question of why some manufacturers base their pet foods on carbohydrate matter, namely grain. Mountain Dog Food makes a raw frozen pet food that contains no grains, preservatives, or fillers. We provide variety by offering chicken (bone-in - available with vegetables and fruit), turkey (bone-in - available with vegetables and fruit), beef; as well as chicken livers, chicken gizzards, tripe and an assortment of bones -- all from inspected human grade sources.
Normally a dogs stomach pH is very acidic - at a pH of 1 to 2. Foods, like dry dog foods, that are high in carbohydrates shift the stomach pH to a more neutral environment (close to pH of 6). A raw diet is very low in carbohydrates (only provided by fruits and vegetables) and allows the stomach to stay within the optimal pH range (closer to a pH of 3). This is more easily illustrated in the figure below. Both diets were tested using a solution of hydrochloric acid at a pH equal to that of a dog's stomach and compared for the changes that occurred during digestion.
Figure 1. The pH change seen during digestion of raw and dry dog food.
Reduced Survival of Bacteria
Most bacteria, such as Salmonella and E.coli., cannot survive at a pH lower than 4 and optimally require a pH range of 5-7 to grow. Since raw food allows the stomach to stay within its naturally acidic pH range the bacteria are killed before they reach the small intestine. However, caution should be taken if a dog's immune system is compromised and medications or diets affect this process, or if mixing dry dog food (including grain based treats) with raw -- this should be avoided whenever possible. This is due to the use of bicarbonate products used to blend the nutrient profile in dry food.
Optimum Protein Digestion
In the stomach pepsin is the enzyme that is responsible for the digestion of protein. Pepsin is activated by the stomach acids and functions best at an acidic pH (starts at a pH of 6 and works best at less than a pH of 4), meaning that when the stomach contents are acidic protein digestion occurs at a high rate. However if the stomach contents become more neutral pepsin is degraded and protein digestion slows considerably. The pH of the stomach can greatly effect the amount of protein digestion that occurs before the food reaches the small intestine and can ultimately affect the time necessary to digest the meal. This becomes more of a concern when discussing a dry dog food, as will be seen later.
Reduced Pancreatic enzymes needed - minimal carbohydrates
As the food moves from the stomach into the small intestine the food is digested by the enzymes that are released from the pancreas and from the cells of the small intestine. At the normally acidic pH of the stomach pepsin begins the digestion of protein, breaking it down into smaller peptide chains and amino acids. The resultant products of this process require fewer enzymes from the pancreas to further digest the smaller peptides and amino acids. Because a raw diet does not contain high concentrations of grain there is less stress on the digestive system to produce the volume of enzymes needed for complete carbohydrate digestion.
Digestion/Absorption of carbohydrates, protein and fat
The digestion and absorption of all major macro nutrients occurs before the food leaves the small intestine. Stimulated by food entering the small intestine the pancreas releases enzymes to digest the protein, fat and carbohydrate matter. At the same time bile is released from the gallbladder to aid in the digestion of the fat and fat-soluble vitamins. This is achieved by creating an emulsification, thereby allowing the water-soluble enzymes to interact with the fat. A raw diet provides fats that have not been over processed and contains a higher percentage of essential fatty acids and as a result is more easily digested and absorbed.
Absorption and re-circulation of bile
Bile is manufactured by the liver and stored in the gallbladder until needed. During digestion the volume of bile needed requires that it be re-circulated many times throughout the meal. To facilitate this bile is reabsorbed in the small intestine and sent through the circulation back to the liver. From there it is sent to the gallbladder to be concentrated and stored for further use.
Absorption of water and electrolytes
The function of the large intestine is to absorb water along with electrolytes such as sodium, chloride and potassium. The natural micro flora that exist in the large intestine also aid the host by producing needed biotin, some of B-vitamins and vitamin K. These microbes also function to break down fiber producing short-chain fatty acids, an energy source for the cells of the large intestine. Fluctuations in the system at this point can lead to diarrhea, causing dehydration and malabsorption in the animal.
Commercial dog food as we know it today began as an industry merely 60 -100 years ago and was developed as a product of convenience, both for the manufacturers and consumers. Its emergence into the market gave producers an outlet for the secondary byproducts from the manufacturing of food for humans. As a result dog food was produced using an abundant and cheap source of protein - grain. Animal protein and fat are added in minimal amounts in order to reach the required guidelines while maintaining a low cost. Because it is a cooked product certain vitamins, minerals and essential fatty acids have to be added either in excess (to ensure they are present after processing) or in chemical form. With the use of chemical additives and preservatives to maintain and reach nutrient profiles the resultant availability is questioned. Also questioned is the associated problems of storage and evacuation of built up unused volumes of these nutrients (i.e.: struvite stones). The composition of the diet (high carbohydrates content) and the fact that it is heat extruded brings us to question the dogs ability to completely digest such a product. Plant protein sources are also not as complete and balanced when comparing their amino acid profiles to that of animal proteins leading to the addition of chemical based nutrients.
As mentioned earlier dry dog food can change the pH of the stomach contents to that which is more neutral, in part because of its high carbohydrate content and bicarbonate based additives. This can have a domino effect upon the functioning of the digestive tract, altering the rate of digestion and could possibly lead to problems such as diarrhea or vomiting.
Increased Bacterial Survival Rate
Secondary to the changes in digestion the neutral pH environment created by the dry diet can leave the dogs system more susceptible to bacterial infections. At a pH of 6 bacteria can still survive and make their way into the small intestine. Once in this normally neutral environment the bacteria can more easily proliferate.
Reduced Action of Pepsin
Pepsin is responsible for beginning the digestion of protein in the stomach. It is pH dependent and does not become activated until the pH drops below 4. If dry food were altering the stomach pH, making it more neutral, this would resultantly effect the action of pepsin. As the environment of the stomach becomes neutral pepsin becomes denatured and its ability to digest protein is greatly minimized.
Reduced Protein Digestion
As pepsin is denatured the rate of protein digestion slows considerably. This results in an increase in the amount of undigested protein matter reaching the small intestine. To compensate the pancreas would have to increase the volume of proteolytic enzymes it releases in order to completely digest the food within the confines of the small intestine.
Dry dog food provides a high proportion of its energy as carbohydrates supplied by grains and plant proteins. As carnivores dogs have no obligate need for carbohydrates in their diet. Unlike humans they lack salivary amylase to begin early digestion as well as the necessary time to fully complete the digestion (dogs have shorter digestive tracts than humans). The increased proportion of grain matter in dry dog food also correlates to an increase in the amount of insoluble fiber entering the dogs system.
Fibre binds bile (increased losses, increased production in liver)
One of the consequences of this increase in insoluble fiber is that bile becomes bound while in the small intestine, preventing its reabsorption. When bile is lost from the system the liver must increase its rate of production in order to maintain the pool of bile. If dry dog food is being fed on a long-term basis the dogs system is continually producing bile. This puts the dogs' liver under constant stress to maintain the levels of bile that are necessary for proper digestion. Under normal physiological conditions the bile is recycled with up to 95% efficiency.
Increased Pancreatic enzymes (digest carbohydrates, protein and fat)
The high concentration of carbohydrate matter combined with the stomach's reduced efficiency for protein digestion requires the pancreas to compensate and increase its output of enzymes. This puts the pancreas under increased stress to continually secrete the volume of enzymes necessary to digest the food that is entering the system.
Increased transit time
A dry diet essentially shifts the start of digestion to the small intestine, thereby placing higher emphasis on the role of the pancreas to produce the necessary enzymes. In order for the body to absorb all of the nutrients the food must remain in the small intestine until it has sufficiently been broken down, thereby increasing the transit time.
The digestive tract of the dog is much shorter than that which is seen in humans. With increased emphasis being placed on the role of the pancreas it seems unlikely that every dog would be able to completely digest and absorb all of the nutrients within the confines of the small intestine. If this were the case then a higher percentage of undigested nutrients would reach the large intestine. Since dry dog food has a high concentration of carbohydrates it stands to reason that it would represent the highest proportion of undigested matter.
During transition between diets the dogs system may be unable to completely digest and absorb the nutrients from its food. This of course is dependent upon how the system adjusts to the pH shift that results when switching from dry dog food to raw. If the pancreas does not produce the volume of bicarbonate necessary to neutralize the acidity of the stomach contents when entering the small intestine inflammation or irritation may occur. This inflammation could potentially alter the release of the enzymes and bile and over time could stress the pancreas to the point of malfunction. Given the pancreas may need time to shift back to the normal rates of bicarbonate production a series of smaller meals during the day is in order when making the transition to the raw food diet. Production of bicarbonate in the pancreas is a function of the acidity of the material moving into the small intestine. Prolonged ingestion of food high in bicarbonate based nutrients may lead to reduction of this pancreatic function causing production as well as absorption/evacuation problems in other organs (i.e. urinary tract).
Diarrhea (Osmotic Shift) + Bacterial Overgrowth
The presence of undigested nutrients in the large intestine causes a shift in the osmotic gradient and draws water back into the intestine. Coincidentally carbohydrates have the most drastic effect upon this gradient. The result of such a situation is diarrhea, which is often combined with overgrowth of bacterial colonies within the large intestine. If the underlying cause is left untreated the dog will suffer from dehydration due to their inability to retain water and/or electrolytes. Another common outcome seen is the elimination of large malformed stools.
Dry dog foods have a tendency to have high concentrations of salt in their formulations. It is known that sodium is involved in either the digestion or absorption of fat, protein and carbohydrates. Interestingly in the absence of sodium the absorption rate of glucose (final end product of carbohydrate digestion) almost comes to a halt. It is intriguing then that a food that derives its highest percentage of energy from carbohydrates also supplies a high concentration of salt. While salt is not the only available form of sodium, it does raise the question of whether the concentration of salt is present to ensure high absorption rates of carbohydrate. This is particularly curious for an animal that is not designed to survive almost solely on a grain-based diet. During transition to a raw diet the remnants of the high salt concentrations from the dry food may have an effect upon how the dogs system is able to adjust to the new diet. As a secondary consequence the dog must also increase their intake of water to compensate for the excess sodium to facilitate in its loss from the system. Diets high in salt concentration also cause the excretion of excess calcium in the urine, effectively altering the calcium balance. This again affects the normal functioning of the organs and their ability to regulate the absorption/evacuation function.
As bile enters the small intestine it is neutralized by sodium to create bile salts. These bile salts can then interact with the fat and create micelles, allowing for digestion of fat to occur. The amount of salt (sodium) required is a function of bile production and sodium recovery.
It is also important to question how the conversion process of bile acids to bile salts changes when the diet is switched to raw. When initially fed a dry diet the liver is forced to continually produce bile in order to replace that which is lost when bound by the insoluble fiber. Additional sodium is required due to high losses in the bile bound in the fiber. A raw diet is lower in sodium thereby limiting the possibility for large quantity of bile acids to be neutralized and converted to bile salts. This leads to a situation where a reduction in the amount bile acids is required in the small intestines. Given the dry diet makes the liver accustomed to production of large quantities of bile, it is possible that the excess bile acids can in part perpetuate inflammation of the digestive tract during transition. Again a series of smaller meals during the day will give the organs time to adjust back to a normal state.
Increased Fat Digestion -- Increased Blood Lipid Levels
With high concentrations of bile salts there is more opportunity for the interaction of the fats with lipase and the enzymes that are excreted by the cells of the small intestine. This interaction would allow for an increase in the rate of digestion and absorption of fat from the diet and result in a rise in the blood lipid levels. Large volumes of saturated fats, as seen in dry dog food, require that more bile be available for digestion as they are not readily absorbed by the small intestine. This is especially a concern for a dog that is making the transition to raw. The reason is raw food contains large quantities of short and medium chain triglycerides, significant because these fats do not unequivocally require bile salts to be absorbed. Because the fats are not heat processed in raw food they are more easily absorbed without large volumes of bile.
Given that the dogs fed dry food have had their digestive systems altered from the evolved baseline caution during transition is warranted. With all types of fat being absorbed at an increased rate the dog is at risk for high blood lipid levels. As the blood lipid levels rise the body may respond by closing off the sphincter for the bile duct to minimize the amount of fat that is absorbed. Due to the close proximity this action may also affect the pancreatic duct and alter the release of lipase along with the enzymes that are needed to digest the protein and carbohydrate matter. This could contribute to the incomplete digestion of some of the major macro nutrients as well as alter the release of bicarbonate. The lack of bicarbonate being released changes the pH balance of the small intestine thereby leading to further inflammation and possible pancreatitis or diarrhea. A series of smaller meals during transition will mitigate problems in the transition back to a normal function.
Mixed (Dry to Raw)
When transitioning between any diet it is common for a dogs system to react to the change, generally seen as diarrhea. This is because their body is not accustom to producing the necessary enzymes, bicarbonate or bile to deal with the variation in the diet. The biochemical changes that we feel result from years of feeding a dry diet may predispose some dogs to complications when changing them to any new diet. This is especially true when making the transition to a diet, such as raw, that has a completely different formulation than dry dog food.
As mentioned earlier it is best to avoid feeding dry dog food simultaneously with a raw diet. This includes giving "biscuit-type" treats that are essentially composed of the same ingredients as dry commercial dog foods to a dog that is fed a raw diet. This could potentially recreate the conditions necessary to increase the blood fat levels (increase in [salt] therefore increase [bile salts]) and potentially increase the risk of developing pancreatitis.
High to Low PH
Changing from a dry dog food diet to one that is based on feeding raw meat, vegetables and fruit will invariably have an effect upon the pH of the stomach. As their system adjusts to the change in diet the stomach contents will move closer to the low pH that is seen normally in dogs.
Compensating Bicarbonate Production
When feeding a dry diet, the food leaving the stomach is generally more neutral, and therefore does not require as much bicarbonate be produced by the pancreas to reach the desired pH of the small intestine. Switching to a raw diet would cause the stomach contents to return to their naturally acidic state. Initially the pancreas may not be able to readily compensate by producing the necessary volume of bicarbonate.
This could affect the action of pepsin that, along with the food, is being released from the stomach. Normally pepsin becomes deactivated upon entry of the small intestine because of the neutral environment it encounters. With the lack of bicarbonate to facilitate this shift pepsin may remain active and continue to digest protein matter. Unlike the stomach, the cells of the small intestine are not protected by a mucosal lining and as a result may be irritated by the presence of pepsin in its active state. Smaller meals over time allows compensation to occur and return to the normal state.
Compensation - Neutral PH
If the pancreas responds to the acidic nature of the stomach contents and is able to produce enough bicarbonate to counteract and achieve a neutral pH digestion will be carried out as usual. The food will then be broken down by the pancreatic enzymes allowing for the absorption and utilization of the nutrients and energy.
Inflammation of small intestine - lack of adjustment to acidity
The dogs system, however, may need a period of adjustment to compensate for the shift to normal pH. If during this time the pancreas does not secrete enough bicarbonate, due to long term consumption of dry dog food, that neutralizes the environment of the small intestine it stands to reason that the sensitive tissue would be affected by the acidic pH. This could lead to inflammation or irritation of the tissues of the small intestine - including the sphincters of the pancreatic and bile ducts. Compounding this problem is that pepsin remains active in this acidic environment, thereby causing further irritation of the cells of the small intestine.
Altered Release of Bile, Bicarbonate and Enzymes
Altered states of function, due to diet, over long periods cause the animals body to shift function on a more permanent basis. Moving back to a normal function may take some effort such as the the use of supplemental fiber and multiple smaller meals. Intestinal inflammation can cause the body to alter the function of the pancreatic or bile ducts, ultimately affecting the release of bile and/or enzymes into the small intestine. This alters the ability of the body to properly digest the nutrients within the food and could lead to diarrhea. The buildup of enzymes within the pancreas can potentially trigger an autoimmune response.
In an attempt to counteract the absorption rate of fat and subsequent rise in blood lipid levels the body may shut down the sphincter controlling the release of bile (as mentioned earlier). Unfortunately this may also affect the functioning of the pancreas due to the close proximity of the ducts at the point of entry to the small intestine. As a result the pancreas cannot release enzymes and/or bicarbonate to counteract the pH of the stomach acids. This only acts to increase the inflammation of the small intestine and leads to larger volumes of undigested material reaching the large intestine, thereby perpetuating the vicious circle.
While in this discussion we are focused on the rise in lipid levels due to the ingestion of unsaturated fats in the raw diet. The same could be said of high sodium levels in dry food contributing to a similar imbalance and causal effect.
Pancreatitis is an autoimmune disease that essentially causes inflammation of the pancreas along with the destruction of pancreatic cells. Much of the information regarding pancreatitis is hypothesized based upon reports of cases seen to date. Little is known about what initially causes pancreatitis and why some dogs are more susceptible than others.
One of the factors thought to lead to the development of pancreatitis is hyperlipidemia, or high blood lipid levels. Questions remain whether hyperlipidemia causes pancreatitis or if it is the result of the disease, although low fat diets are recommended for dogs that are predisposed to this disease state.
The inflammation of the pancreatic duct may block the flow of enzymes into the small intestine. Whether this inflammation would be a trigger for pancreatitis is unknown, although the buildup of enzymes within the pancreas could lead to auto-digestion. Because of the close proximity of the common bile duct the release of bile may be affected as well. As the dogs system adjusts to the naturally acidic stomach contents seen with raw feeding their system will produce the necessary bicarbonate to prevent such a reaction. It is also relevant to question the daily stress that a dry diet puts on the pancreas to digest the large volumes of carbohydrate. Does this make their systems more susceptible to developing a pancreatic (pancreatitis, diabetes) condition regardless of diet?