Why diet is number one in gastrointestinal disease?

1. The intestinal tract functions best with food

The GI tract is built to digest food—mechanically and chemically breaking down ingredients to extract nutrients. When food is withheld for more than 24 hours, nutrient absorption in the small intestine can decrease by up to 50%. This means fewer nutrients are absorbed, and more undigested material reaches the large intestine, potentially disrupting the gut environment³.

2. The gut microbiome: a delicate ecosystem

The large intestine houses a diverse microbial community—bacteria, fungi, protozoa, viruses, and their genetic material—collectively known as the gut microbiome. In healthy animals, this ecosystem performs essential functions, including fermenting non-absorbed nutrients. Fermentation in this anaerobic environment produces various metabolites, depending on the nutrient and microbial species involved.

For instance, fiber-fermenting bacteria produce short-chain fatty acids (SCFAs), which nourish epithelial cells, reduce inflammation, and even cross the blood-brain barrier⁵. These metabolites also act as neuromodulators, neurotransmitters, and energy sources for both host and microbial cells.

However, antimicrobial use can disrupt this balance. Metronidazole, for example, has been shown to reduce beneficial SCFA-producing bacteria, increase E. coli levels, and decrease P. hiranonis, a bacterium responsible for converting primary bile acids into secondary bile acids¹(Figure1). Without this conversion, primary bile acids accumulate in the large intestine. Remember that in the small intestine the role of primary bile acids is to emulsify and absorb fat, fatty acids, and fat-soluble vitamins. A characteristic of primary bile acids is that they are cytotoxic and hence play a role in regulating growth of bacteria in the microbiome. Bile acid diarrhea can occur when there is an excess of primary bile acids which can lead to an increase in water secretion and hypermotility in the intestinal tract. This microbial imbalance may explain why some patients experience recurrent diarrhea after antimicrobial therapy.

While antimicrobials have their place, indiscriminate use—especially broad-spectrum anaerobic agents—can lead to long-term dysbiosis and inflammation. Antimicrobial stewardship is essential, particularly when systemic infection is not evident⁴.

Figure 1

Result of qPCR on 18 dogs with acute diarrhea who were either prescribed a 7 day course of oral metronidazole or administered a fecal microbiota transplant (FMT) enema compared to a healthy set of dogs. Both groups experienced similar fecal score improvement, those treated with metronidazole had altered microbial and metabolic profiles at day 28 compared to those treated with FMT or control groups.

3. Therapeutic diets target key nutritional factors

Complete and balanced diets are composed of ingredients that provide essential nutrients for energy and metabolic function. Therapeutic diets are formulated based on scientific evidence to address specific health conditions. These diets undergo rigorous testing to ensure nutrient levels are appropriate both in raw ingredients and post-manufacturing.

Key nutrients are often supplemented to support GI health. One key component is fiber, which plays a crucial role in modulating the microbiome. The type of fiber and the nature of the GI illness determine which fibers are most beneficial. Insoluble fibers bulk up stool and stimulate mucus production, protecting epithelial cells. Soluble fibers absorb water, forming a gel that improves stool quality and regulates intestinal contractions. Some soluble fibers, such as oligosaccharides, are highly fermentable and promote SCFA production⁷.

Let’s take a moment to compare a common recommendation of chicken and rice versus a complete and balanced gastrointestinal diet. If we compare a recipe from a complete and balanced diet, to a diet that contains only chicken and rice (Balance.it) we can see that chicken and rice is deficient in at least 7 recommended vitamins as per the Association of American Feed Control Official (AAFCO). (Figure 2). Another interesting fact is that chicken and rice is less calorically dense, meaning a larger, much larger, volume of chicken and rice is needed to meet daily energy requirements compared to an average gastrointestinal formulated complete and balanced diet. (Figure 3)

Figure 2

Each circle represents a nutrient identified as being part of a complete and balanced diet for a dog. The red indicates the portion of that need that is being met in the diet. While the need amount is not listed, red indicates the percentage of need being met with a fully red circle indicating 100% of the need is met in the formulation. On the left, the figure shows a complete and balanced diet. The picture on the right shows 7 vitamin deficiencies in a diet created with chicken and rice only (Balance.it 2024).

Figure 3

Comparison of energy needs and feeding volume of a commercial complete and diet and a chicken and rice diet. Chicken and rice energy content obtained from Fascetti, 2012. The commercial diet has a general energy content based on gastrointestinal diets formulated for adult maintenance on the market⁶.