The canine gastro-intestinal tract is a battle ground, with the body having to fight off potential invaders, whilst still absorbing the nutrition essential to thrive and survive. With an often-unscrupulous canine diet to take into account, this fight can often be hard fought! The battle is conducted on two fronts, with both immune and non-immune factors playing an active role.Non-immune factors are the most important, providing the primary defence for canine guts against potentially harmful pathogens such as E. coli, salmonella and campylobacter. These critical non-immune factors include:
- A high cell turnover. The cells lining the gut, called enterocytes, are shed at a rate of 100000 every 30 seconds. That means the entire gut lining is replaced every three days! This keeps the cells fresh and in top condition.
- Tight junctions. The enterocytes hold on to each other via links known as tight junctions. These keep the cells close together, preventing water, pathogens, toxins or anything else unwanted from slipping into the body through intercellular gaps.
- Mucin. Specialised cells known as goblet cells produce a protective layer of mucous, which covers the enterocytes.
- Enzymes. Digestive enzymes for food also break down the pathogens in the dog digestive tract, reducing numbers.
- Good bacteria. Each dog has its own individual population of good bacteria, known as microflora. These are as individual as fingerprints! Good bacteria compete with pathogens, and while doing so produce a very mild acid called lactic acid. This lactic acid improves the environment for healthy microflora to replicate, whilst creating a more hostile environment for harmful pathogens.
The immune system plays a back-up role, making sure that bacteria not thwarted by the non-immune defence are dealt with. This involves both dogs innate and adaptive immunity and the utilisation of many types of white blood cells.
Both the immune and non-immune factors are supported by the diet. By optimising a dogs diet to provide the best nutrition for the gut, the guts own defence can be improved.
Amino acids provide a lot of support for enterocytes, immune cells and general gut health. Providing a healthy balance of the key amino acids for gastrointestinal support is an important part of protecting a very vulnerable part of a dogs body system.
“Dietary amino acids are major fuels for the small intestinal mucosa” – Wang 2009
Glutamine is a conditionally essential amino acid for gut health. It can be given directly, but also can be synthesised from the essential amino acid glutamic acid. The reason glutamine is so important is that it is the preferred energy source for cells that proliferate rapidly – the perfect description for canine enterocytes, as well as the white blood cells that guard them. Without energy, the barrier cannot replenish itself, and the barrier function of the gastrointestinal tract falls. Crucially, whilst glutamine can be made in the body, the ability to do this falls during acute illness or stress. Providing glutamine, or the ingredients necessary to create it, improves the energy balance within the gut.
L-Arginine is essential for making lots of critical molecules, including nitric oxide (NO), polyamines, and creatine. Nitric oxide mediated mechanisms, stimulated by arginine, initiate the secretion of intestinal fluid. This keeps the lining of the gut moist and healthy, of critical importance in the maintenance of the intestinal mucosal barrier. In addition, there is emerging evidence that arginine plays an important role in modulating mucosal inflammation, which may be helpful in dogs with inflammatory conditions such as Inflammatory Bowel Disease (IBD).
“Arginine supplementation is effective in improving intestinal barrier function and vascular development”. – Wang 2009
Sulphur containing amino acids – Methionine and Cysteine
The major end products of methionine and cysteine metabolism are glutathione, homocysteine and taurine, which play important roles in the intestinal immune response. Of these, glutathione is the most interesting. Glutathione is produced in the liver and is one of the primary endogenous antioxidants in the body. It has a short half-life (2-6 hours) so has to be constantly in production to keep up with the demands of the body. In times of oxidative stress, due to damage or inflammation, glutathione is one of the body’s main strategies against the rise in cell-damaging free radicals. Glutathione in the gut lumen and enterocytes is therefore of critical importance in maintaining normal intestinal function.
“Administration of specific dietary substrates and precursors for glutathione synthesis is an effective strategy to improve gut mucosal functions and may prevent or treat intestinal diseases” – Wu 2004
Intracellular glutathione concentrations are higher in actively proliferating cells, again like our canine enterocytes. It is thought that this is because glutathione, as well as protecting the enterocytes, also appears to be involved in the regulation of cell growth. A deficiency of glutathione leads to severe mucosal damage, marked by epithelial cell damage, mitochondrial degeneration and villus atrophy in mice, a clear indication of the importance of glutathione in gastrointestinal health.
Dogs commonly get acute diarrhoea, often from dietary indiscretion, and providing the appropriate amino acids for primary enterocyte nutrition is critical for getting them back on their feet as soon as possible.
Glycine and Lysine
Dietary glycine and lysine are directly utilized by the intestine for protein synthesis and other metabolic processes, so are important for the healthy cell turnover needed in the gut lining.
The secondary role of glycine as a powerful cell-protectant has only recently been recognized. The mechanisms of action include osmoprotection, extracellular signalling, scavenging of oxygen free radicals, and modification of biologically active molecules. One study suggested that local perfusion with glycine diminishes ischemia-reperfusion injury in the small intestinal mucosa, opening the door for more therapeutic use of glycine to be considered.
“Threonine is particularly important for mucin synthesis and maintenance of gut barrier integrity” – Bertolo 1998
In the intestinal mucosa, threonine is heavily used as part of mucins, which are the major glycoproteins protecting the epithelium from injury. A study with rats demonstrated that dietary threonine restriction dramatically and impaired the synthesis of mucins in all segments of the small intestine. In the duodenum (the first section of the small intestine) threonine reduction reduced mucin synthesis by forty percent! Helpfully, an increase in dietary provision of threonine and other amino acids can promote mucin synthesis and re-equilibrate the gut microbiota to favour intestinal protection and mucosal healing.
It is clear that giving the right balance of amino acids is critical for dog gut health, and even more so when the health of the gastrointestinal tract is already compromised. With nearly 18% of canine veterinary consults regarding a gastro-enteric issue, supporting our dogs in this area should be a part of everyday nutrition, and something we all need to consider when feeding our dogs.
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