Gastric pH in Dogs & how kibble versus a raw diet may affect digestion

January 10, 2021


Since educating myself on raw feeding as it pertains to canines (an ongoing journey), I have often seen debates surrounding the "effects kibble & raw food have" on a dog's gastric pH; as it is always referenced when someone poses the question, "Is it okay to mix a bit of fresh food with my dog's kibble?"

A common response I have seen is, "stomach acid fluctuates depending on what's being digested and different foods digest at different rates, therefore it is best to not mix kibble & raw food."
 
While the latter is in fact true, foods do digest at different rates, the assertion of gastric acid pH fluctuating significantly enough to warrant omitting fresh foods from a bowl of kibble (or vice versa) is factually incorrect. Let me explain...

One thing to remember throughout this discussion is that nearly every process in physiology is dynamic, yet controlled. Meaning, these processes will constantly fluctuate, however, they are not fluctuating with high variance

This is why we refer to narrow ranges rather than single values, as your “normals”, for important physiological values such as internal temperature, blood pressure, and the pH of your stomach acid for example. 

Our bodies have many “checks and balances” to maintain these values at their normal ranges, rapidly and unconsciously. Controlling these values is inherently important for survival that our brains have evolved around these basic functions and engineered them into automatic neurophysiological processes, performed by structures of the midbrain and hindbrain. 

With that being said, let’s discuss gastric acid in particular, its role in digestion, and how it pertains to a dog’s dietary choices.

Gastric Acid and Digestive Enzymes

At the molecular level, the primary role of hydrochloric acid (aqueous HCl), the major component of gastric acid, is to catalyse the breakdown of quaternary, tertiary, and secondary polypeptide structures by creating an extremely oxidative environment. 

A polypeptide is a protein polymer, or a complex structure using repeating versions of a base molecule- amino acids in this case. Muscles, such as a chicken breast, have high amounts of complex structural protein structures, which we need to break down into their simplest single-molecule amino acids, in order to use as an energy source for metabolic processes. In order for the digestive enzymes that target proteins to reach the specific peptide bonds they are engineered to recognise, your gastric acid first needs to “unfold” the proteins by breaking down disulfide bridges, esther linkages, and other hydrophobic molecular interactions that give the protein its form. The unfolding renders the protein inactive and exposes the peptide bonds- that digestive enzymes will then break and result in single amino acid molecules.

The image below illustrates how a complex quaternary protein structure (right) is progressively unfolded as it becomes denatured (left). Extreme pH and temperatures are the most common ways to denature proteins.


The highly acidic environment of the lumen of the stomach, where gastric acid resides, also has the function of activating digestive proenzymes such as pepsinogen, trypsinogen, and chymotrypsinogen. 

These precursors to protein-digesting enzymes, or proteases, are inactive until they reach the pH range of the gastric acid. This is important because if they were activated prior to reaching the digestive environment, these proteases would “attack” your body’s own structural proteins and, thus, damage the internal tissues/organs that secrete them.

This delayed activation is important for enzymes that specifically cleave proteins, as opposed to lipid-digesting enzymes. It also illustrates further, how important it is for the stomach to have a very well controlled pH range for optimal activity of these enzymes (which are adapted to function at very low pH values). 

In humans and comparable mammals, such as dogs, gastric pH is kept within an average range (~1-3) using sensory and secretory cells in the stomach lining. The hormone gastrin is secreted during digestion in order to control the flux of H+ ions into gastric acid, in order to maintain the normal pH range. Bicarbonate is produced as a buffer to further control the pH of gastric acid by increasing pH if it drops below normal range.

The stomach dynamically keeps the pH within range in response to changes incurred by ingested food or compounds. 

Since most foods we eat have neutral pH’s, the main change of the stomach during digestion is to increase its volume by expanding its smooth-muscle walls in order to accommodate the amount of food in the cavity. Additionally, the stomach secretes more gastric acid in anticipation and during digestion in order to appropriately dissolve the amount of food ingested. 

Ingesting too much food within a short period of time can lead to acid reflux, or heartburn, as the stomach fills with food and additional gastric acid, faster than it can empty its dissolved contents into the duodenum of the small intestine, through the pyloric sphincter. Secreting this extra amount of gastric acid during digestion does not necessarily mean you are decreasing gastric pH, or creating a more acidic environment, as the pH is balanced at the molecular level by proton pump action and bicarbonate buffer secretion. 

The pH may decrease initially as the increase in gastric acid volume occurs, and then proceed to balance out shortly after, since the proton pumps of the stomach are hormonally activated by gastrin. Hormone signalling cascades reach their desired effects within minutes, but not instantly.

Research Analysis

We can see this dynamic process illustrated, specifically in canine physiology, by a 2012 study that accurately measured the gastric pH of dogs before, during, and after a meal across several experimental conditions (source). 

This study found evidence that gastric pH did not change significantly, both, throughout digestion and across experimental conditions such as the dogs being in a fed or fasting state prior to the meal. Interestingly, the researchers noted that the lack of significant increase in pH during digestion of the dogs differed from the pH variation that studies have typically seen in humans and monkeys during digestion.

“While the fasted gastric pH in the dogs was similar to the monkey and human, the dog lacks the steep and sustained elevation in postprandial pH, as seen in the human and monkey. One explanation may be due to a higher acid output in the dog, which prevents a buffering effect by the food consumed.” (Mahar et al, 2012).

Another study from 2009, using telemetric technology to measure pH, similarly found a lack of significant buffering effect of dry dog food in measuring the gastric pH of fasting versus fed beagles, rather the opposite (source).

“The mean gastric pH in the fasted dogs was 2.03. The gastric pH in fed dogs was 1.08 and 1.26 for 10g and 200g fed, respectively. These values were significantly lower than in fasted dogs. This is unpredicted and counterintuitive as the gastric pH is reported to be higher in fed stomach in human due to the buffering effect of food. However, in dogs, it is reported that the initial buffering effect of food is not observed and there is no trend in pH over the first postprandial hour likely due to a higher peak acid output in fed dogs.” (Sagawa et al, 2009).

The findings of these studies compile evidence towards the conclusion that dogs have even less fluctuation in gastric pH before and after the start of digestion than expected. This is perhaps due to their physiology through increased gastric acid output, or maybe due to differences in the buffering ability of human versus "dog food." Maybe a bit of both. 

But there is certainly something to be said about the availability of alkaline salts in human food, which are common chemical buffers, however, the researchers in these studies also pointed to the acid-secreting parietal cells being concentrated in the bottom of the stomach, and how they found the posterior aspect of the cavity to be lower in pH than the anterior aspect as a result. 

Could the physical output of gastric acid be facilitated by the stomach’s horizontal position in four-legged animals like dogs versus vertical position in humans and monkeys with respect to dispersing effects of gravity? Investigating the reasons for these findings could be worthwhile to further understanding the interaction of diet and physiology with gastric pH.

Conclusion

As it stands, the evidence in studies, using recent telemetry technology, shows that gastric pH remains largely unaffected by traditional dog food (kibble), which is closer in its potential acid buffering characteristics, to processed foods (for human consumption), than it is to a raw food diet. 

Inorganic minerals/salts that act as significant acid buffers, such as bicarbonates, are not found in raw meats or virtually any unprocessed foods. These compounds are isolated from mineral sources, purified, and used for consumption when baking (w/ baking soda) for example. 

Studies that specifically compare dry dog food to a raw diet would be worthwhile in order to definitively determine the relationship between diet type and acid buffering capability.

Lastly, there is also a distinction between statistical significance in pH changes with different diets, and physiological significance. 

Some of the findings in the studies illustrated significant differences across certain experimental conditions, which have been previously pointed out in articles on the topic, however, we need to remember that we maintain a normal pH range for a reason. This goes back to my point about mammalian evolution and the dynamic yet controlled nature of physiology. Fluctuations are expected and remain balanced under normal conditions. 

For example, acidic change in pH from 2.5 to 1.5 may be found to be statistically significant with respect to p-values and other measures of statistical analysis, however, such a change remains within the normal gastric pH range of a dog and is therefore an insignificant change physiologically. If the pH change were to be from 2.5 to 4.5, that would be significant on both fronts and would negatively affect digestion, if sustained for more than a very short period of time.

Switching from dry food (kibble) to a raw fed diet is no small transition, but rather inconsequential with respect to effects on gastric acid (the same can be said for those who feed kibble & raw in the same meal). Time of digestion, type/diversity of gut microbiota, and other aspects of the digestive process may change. In a normal and healthy canine, these potential changes are normal and should not be cause for concern.

Please consult physicians/veterinarians, and/or other trustworthy science-based sources for advice on human and animal dietary questions.


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xoxo


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