The digestive tract has several very important functions, such as:
Digesting and absorbing essential nutrients critical for health.
- Digestion breaks down food into very small particles (e.g., simple sugars, amino acids, or fatty acids) and processes them for absorption.
Protecting the dog or cat by:
- Acting as a physical barrier, just like the skin, keeping foreign and undesirable substances from entering the body.
- To gain access to the internal environment, ingested nutrients, allergens, and pathogens must first pass through the mucosal lining of the digestive tract.
Defending against bacteria and toxins via its immunological role.
- The gut contains immune cells that protect the body against invading organisms such as bacteria and viruses.
- As much as 70% of all the body’s immune cells reside in the digestive system (known as the Gut-Associated Lymphoid Tissue, GALT), making it the primary site for the body’s immune system response.
Hosting intestinal microflora or bacteria, another important defense system.
- Billions of bacteria – both beneficial and potentially harmful (or pathogenic) – live along the intestinal tract.
- The beneficial bacteria or microflora protect against infection and colonization by harmful and sometimes pathogenic bacteria.
- Digestive upsets (e.g., loose stools, diarrhea) can occur due to decreases in the number of beneficial bacteria and increases in the number of potentially harmful bacteria.
Nutrients that influence the digestive system
In its role as a digestive organ, the gastrointestinal (GI) tract plays a critical role in extracting these nutrients from the food and releasing them into the body in a usable form. Because of the extent of the digestive system’s responsibilities, it contains some of the most metabolically active tissues in the body, emphasizing the critical role of nutrition. There are several nutrients that are essential for maintaining a healthy gut. Below are 3 important nutrients to consider in your dog’s diet.
Proteins and Amino Acids
Proteins and amino acids are required by the digestive system to help the absorptive cells grow and thrive, thus enabling nutrients to be properly absorbed.
Cells in the small intestine have a rapid lifecycle (turning over every three days).
- Amino acids are used for new cell growth.
- Approximately 50% of ingested protein is used by cells of the digestive system.
The amino acids glutamine and glutamate are key energy sources for intestinal cells.
- More than 90% of glutamine and glutamate are used by intestinal cells.
- Glutamine is found abundantly in meat and other proteins and is normally made in the body from other amino acids.
- Glutamine and glutamate are precursors to glutathione, a potent antioxidant that helps protect the body from free radicals.
Proteins and amino acids are required to maintain and regenerate immune cells in the digestive tract (within the GALT) as well as produce antibodies and other substances needed to launch an effective immune response.
- Since the intestines are one of the major routes of entry for foreign substances into the body, a strong immune defense is important.
If the diet does not contain enough protein or essential amino acids, the digestive and absorptive functions of the digestive system can be impaired by:
- Atrophy (or wasting away) of absorptive cells in intestines
- Decrease in the number and size of absorptive cells
- Alterations in levels of digestive enzymes
- Compromises in immune function (remember that 70% of immune cells reside in the intestinal tract)
Fatty acids like linoleic acid (LA) and arachidonic acid (AA) are important for normal cell membrane structure, including cells in the digestive system. These fatty acids:
Influence the fluidity of cell membranes
- Cell membranes must be flexible to allow migration of molecules across them.
Influence the tight junctions between cells
- Allows cells of the intestinal wall (or mucosa) to form an effective protective barrier.
- Weakness in cell membrane integrity can result in poor protective barrier function (remember that the protective barrier in the gut works to keep foreign, unwanted substances from getting into the body).
- These fatty acids serve as precursors for the synthesis of cellular messengers and inflammatory mediators, playing a role in gut-associated immune function.
Fatty acids also play a role in inflammation
- Omega-3 fatty acids functioning to reduce or control inflammation in the gut.
Dietary Fibre and Prebiotics
Fibres are beneficial to the digestive system. Remember that fibre is a type of dietary carbohydrate that is not digested by mammalian digestive enzymes. Because fibres are not digested, they remain within the digestive tract, where they can have numerous health benefits. There are two basic types of dietary fibre – insoluble and soluble – depending upon whether it dissolves in water.
Insoluble fibres aid in normal gastrointestinal motility and function. They tend to absorb water and add bulk to the intestinal contents and faeces, which is beneficial to digestive health.
- The bulk stimulates gastrointestinal motility, thus promoting regular bowel movements.
- The sponge-like effect of absorbing water helps soften stool, aiding in passage and preventing constipation.
Soluble, fermentable fibres are also beneficial to the digestive system. They have a greater water holding capacity and can form gels and viscous solutions within the digestive tract. Soluble fibres can also bind with harmful/toxic substances in the gut to be excreted out of the body. Some soluble fibers are also prebiotic fibers. Inulin (from chicory), wheat aleurone, and beet pulp are examples of prebiotics.
Prebiotic fibers are selectively fermented by beneficial bacteria in the large intestine, producing Short-Chain Fatty Acids (SCFAs).
- SCFAs are an energy source for the cells in the large intestine (or colon), and thus help keep the cells of the digestive system healthy.
- SCFAs also help lower the pH of the intestinal contents, which inhibits the growth of potentially harmful bacteria.
By serving as a food source for the intestinal bacteria, prebiotic fibers increase the numbers of beneficial bacteria.
- Potentially harmful bacteria such as E. coli and Salmonella cannot utilize prebiotics; only beneficial bacteria can. Thus, adding prebiotics to the diet allows the beneficial bacteria to ferment the prebiotic fibers, producing SCFAs which lower the intestinal pH. A low pH is a preferential environment for beneficial bacteria to grow.
- Digestive health is maintained when there is a stable and diverse population of microflora in the intestinal tract.
Prebiotic fibers like inulin (from chicory) also have other benefits, such as:
- Enhanced absorption of dietary calcium and other minerals.
- Fermentation of prebiotic fibers results in decreased pH of the large intestine, which then increases mineral solubility and absorption.
- Increased height of the intestinal villi (e.g., finger-like projections) and increased number of epithelial cells per villi are well-known effects of prebiotic fibers. This results in passive absorption of minerals.
- These villi increase the surface area of the intestinal wall, meaning more space is available for absorbing nutrients.
Normal microbiota function in the gut
The term “microbiota” is the technically correct term for all of the microorganisms residing in the gut and is often used interchangeably with the term “microflora” (Bacteria used to be classified in the plant kingdom and were thus “flora”).
Each section of the gut has unique characteristics and different functions, such as nutrient digestion in the small intestine or water absorption in the colon. Different types and different quantities of bacteria populate the various sections of the gut.
In the stomach, bacteria levels are typically quite low. Bacteria in a healthy small intestine are primarily anaerobic (they thrive without oxygen), and their populations increase along the GI tract as they get closer to the colon. The colon contains the greatest number and variety of bacteria, as intestinal transit time is reduced here, and bacteria have more time to flourish. The colon is inhabited by bacteria like Bifidobacterium, Lactobacillus, and Enterococcus.
Every cat and dog has a unique microbial profile, and healthy cats have higher bacterial counts per gram of intestinal contents than dogs. Even though differences exist between individual animals at the bacterial species and strain levels, studies show that Bacteroides, Clostridium, Lactobacillus, Bifidobacterium, and Enterobacteriaceae are the predominant bacterial groups cultured from canine and feline intestines. Many of these microorganisms are beneficial, but some are potentially pathogenic.
A balanced microbiome
Maintaining the balance of "good" and "bad" bacteria in the gut improves fecal quality, but it also has immune benefits that extend far beyond the gut itself. The most important immune-related function of the good bacteria is to protect the pet from infection by potentially harmful bacteria. These good bacteria prevent potentially pathogenic bacteria from flourishing by competing for space, secreting antibacterial substances, and creating an environment that is unfavorable for pathogens.
Many factors can negatively affect the balance of intestinal bacteria. For example, kittens commonly experience GI upset associated with microbiota imbalance during weaning. The microbiome balance in older animals tends to shift toward more pathogenic organisms like Clostridium perfringens. The use of broad-spectrum antibiotics commonly used to treat infections causes a disturbance in intestinal microbiota, as does stress, which is often brought on by changes in environment or diet, or travel.
A microbiota imbalance can often lead to diarrhea as well as general GI upset in the pet. It could also result in increased quantities of harmful bacteria in the pet's feces, which could affect other animals or even humans in the environment. A balanced microbiome can help pets to maintain normal fecal quality during times of stress.