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 Fiber 

The addition of fiber at optimal levels in a diet is essential for the normal function and health of the gastrointestinal tract [5]. 

Dietary fibers are plant carbohydrates which cannot be digested by mammalian enzymes [5]. These structural plant carbohydrates include pectin, lignin, cellulose, hemicellulose, muclinage, and gums [5]. Different types of fibers have varying levels of solubility and fermentation; this ranges from pectin which is highly fermentable, to beet pulp which is moderately fermentable, to cellulose which is non-fermentable [5]. Non-fermentable fibers helps with satiety, maintenance of a normal intestinal transit time and gastrointestinal motility as well as increasing diet bulk [5]. Fermentable fibers, on the other hand, are fermented to short chain fatty acids by bacteria in the colon and have variable effects on gastric emptying [5]. Moderately soluble fibers have been linked to increased colon weight as well as an increased mucosal surface area for absorption of nutrients [5]. 

Fiber, though it is not an essential nutrient, is important for a healthy gastrointestinal tract [5]. The microbes found in the cat’s large intestine have the ability to ferment dietary fibers to short chain fatty acids [5]. Cells of the gastrointestinal are constantly dying and being replaced by new cells, which requires a lot of energy [5]. The fatty acids produced are used as energy sources for these epithelial cells which line the gastrointestinal tract [5]. As a result of the presence of energy from the fatty acids, colonic cell proliferation is increases [5]. 

Short chain fatty acids production from dietary fibers have many other advantageous effects on the gastrointestinal tract [5][16]. They increase motility by stimulating rhythmic contractions of the distal portion of the small intestine, which potentially decreases fermentation in the small intestine while increasing it in the large intestine for further fatty acid absorption [5]. Blood flow to the colon also increases with the presence of short chain fatty acids [5]. These fatty acids also increase sodium absorption which helps maintain normal electrolyte and fluid balance in the intestine, reducing the risk for diarrhea [5]. These homeostatic conditions of the intestinal tract promote the growth of beneficial bacteria while inhibiting the proliferation of pathogenic ones [5]. A healthy and balanced gut microbiome is important for maintaining a healthy digestive tract [5]. 

Fibers promote bacterial growth and activity in the large intestine [5]. It is essential for a healthy gastrointestinal tract to have a healthy and stable gut microbiota [14]. The microorganisms present in the colon are responsible for the fermentation of the fiber into short chain fatty acids and for the production of some vitamins [5][15]. 

Prebiotics 

Prebiotics are short-chain carbohydrates classified as fibers with an added aspect as they selectively promote the growth of beneficial bacteria [5][13][4]. By promoting the health and proliferation of beneficial bacteria, they suppress the growth of pathogenic ones by outcompeting them [4]. Insulin, galactooligosaccharides, lactulose, fructooligosaccharides (FOS) and mannanoligosaccharides (MOS) are all examples of prebiotics [5].  

Probiotics 

Probiotics are becoming increasingly popular in the diets of felines (1). They are included in the diet to increase the number of bacteria and microbes that are normally present in a healthy gut (1). Probiotics are considered a supplement rather than part of nutrition (1). Therefore, there are no strict regulations to the amount of probiotics that should be included in foods (1). The function of probiotics goes beyond basic nutrition and has many benefits to the health of the organism (1). The use of probiotics can help in the prevention and treatment of certain diseases or disorders of felines (2). Examples include prevention of allergies, diarrhea, symptoms relating to stress, etc. (2). The health of the cats is very much dependent on the fermentation that occurs through gut biota (2). The gut biota has an important role in the metabolism, absorption, and protective functions of the gastro intestinal tract (2). Felines have different gut bacteria than canines (2). However, the most common biota that are found in both felines and canines are Firmicutes, Bacteroidetes, Proteobacteria, and Fusobacteria (2). Although these are the most common types of gut biota found in felines, there are variances between independent cats (2). Each cat has their own unique and independent number and type of gut microbes (2).  

Antioxidants 

Nutraceuticals such as antioxidants are considered to be additives of gastrointestinal diets to prevent digestive upset [5]. Antioxidants have the ability to remove free radicals from the body which can cause damage to cell membranes, and are involved in chronic degenerative diseases [3]. Free radicals amplify inflammation by causing release of proinflammatory cytokines [6]. Free radicals can be caused by many factors such as stress, disease and age [3]. Some oxygen-derived free radicals can produce ischemia in the small bowel and stomach of cats [4]. Combinations of antioxidants have been reported to improve serum vitamin status, suppress lipid peroxidation and distributes the effects of exercise on the immune system [8]. The most common antioxidants found in cat gastrointestinal diets are vitamin E and vitamin C.   

Vitamin E 

The addition of Vitamin E as an antioxidant in gastrointestinal health cat food diets can have a positive effect of improving the animal’s immune function and prevent against infections [7]. Vitamin E is a free radical scavenger that functions as a chain-breaking antioxidant to prevent free radical damage of cell membranes [6][9]. Vitamin E aids in protecting cells from highly reactive oxygen species within the lungs, muscles, skin, brain, tissues and red blood cells [3]. Supplementation of vitamin E in the diet benefits the immune system and improves resistance to infections and diseases [3]. 

The National Research Council (NRC) suggested a ratio of vitamin E to polyunsaturated fatty acids be 0.6:1 to ensure enough vitamin E to combat any free radicals [8]. Polyunsaturated fatty acids (PUFA) are prone to oxidative destruction in cellular membranes and increases the requirement for antioxidants [3].   

Vitamin C 

Vitamin C (ascorbic acid) is a water-soluble antioxidant and a free radical scavenger where it will donate an electron to compounds with unpaired elections or reactive but not radical compounds [6][11]. Supplements of Vitamin C reduced oxidative DNA damage in cats prone to renal insufficiency, and can be beneficial to add into diets for cats suffering from renal diseases [10]. Vitamin C is not essential for cats as it is not required by the Association of American Feed Control Officials (AAFCO), however is commonly added into pet foods as an antioxidant [10]. Ascorbic acid is known to not only be an antioxidant, but also to function in gene expression, as a co-substrate, and have unique biosynthetic pathways in different organisms [12].  

1. Weese, S. J., & Arroyo, L. (2003, March). Bacteriological evaluation of dog and cat diets that claim to contain probiotics. The Canadian Veterinary Journal, 44(3), 212-215. 
2. Lukasz Grześkowiak, Akihito Endo, Shea Beasley, Seppo Salminen, Microbiota and probiotics in canine and feline welfare, In Anaerobe, Volume 34, 2015, Pages 14-23, ISSN 1075-9964, https://doi.org/10.1016/j.anaerobe.2015.04.002. 
3. Bauer, J. (June 2001). “Evaluation of nutraceuticals, dietary supplements, and functional food ingredients for companion animals”. Journal of the American Veterinary Medical Association. 218 (11): 1755-1760.  
4. Perry, M.; Wadhwa, S.; Parks, D.A.; Pickard, W.; Granger, D.N. (1986). “Role of oxygen radicals in ischemia-induced lesions in the cat stomach”. American Gastroenterological Association. 90: 362-367. 
5. Case, L. (2011). “Canine and feline nutrition: a resource for companion animal professionals”. Maryland Heights, Mo.: Mosby. 421-422. 
6. Conner, E.M.; Grisham, M.B. (February 1996). “Inflammation, free radicals and antioxidants”. Journal of Nutrition. 12 (4): 274-277. 
7. Puertollano, M.A.; Puertollano, E.; Alvarez de Cienfuegos, G.; de Pablo, M.A. (2011). “Dietary antioxidants: immunity and host defense”. Current Topics in Medical Chemistry. 11 (14): 175-176. 
8. Bontempo, V. (2005). “Nutrition and health of dogs and cats: evolution of pet food”. Veterinary Research Communications. 29: 45-50. 
9. Jewell, D.E.; Toll, P.W.; Wedekind, K.J.; Zicker, S.C. (2000). “Effect of increasing dietary antioxidants on concentrations of vitamin E and total alkenals in serum of dogs and cats”. Veterinary Therapeutics. 1 (4): 264-272. 
10. Yu, S.; Paetau-Robinson, I. (2006). “Dietary supplementation of vitamin E and C and beta-carotene reduce oxidative stress in cats with renal insufficiency”. Veterinary Research Communications. 30: 403-413. 
11. Padayatty, S.J.; Katz, A.; Wang, Y.; Eck, P.; Kwon, O.; Lee, J.H.; Chen, S.; Corpe, C.; Dutta, A.; Dutta, S.K.; Levine, M. (2003). “Vitamin C as an antioxidant: evaluation of its role in disease prevention”. Journal of the American College of Nutrition. 22(1): 18-35. 
12. Arrigoni, O.; De Tullio, M.C. (2002). “Ascorbic acid: much more than just an antioxidant”. Biochimica et Biophysica Acta. 1569: 1-9. 
13. Barry, K. A.;  Wojcicki, B. J.; Middelbos, I. S.; et. al. (May 2010). "Dietary cellulose, fructooligosaccharides, and pectin modify fecal protein catabolites and microbial population in adult cats".  Journal of Animal Science. 88: 2978-2987. 
14. Barry, K. A.; Middelbos, I. S.; Boler, B. M. V.; et al. (October 2012) "Effects of Dietary Fiber on the Feline Gastrointestinal Metagenome". Journal of Proteome Research. 11: 5924-5933. 
15. Zoran, D. L. (November 2008).  "Nutritional Management of Feline Gastrointestinal Disease". Topics in Companion Animal Medicine. 23(4): 200-206. 
16. Sunvold, G. D.; Hussein H. S.; Fahey G. C.; et al. (July 1995). "In Vitro Fermentation of Cellulose, Beet Pulp, Citrus Pulp, and Citrus Pectin Using Fecal Inoculum from Cats, Dogs, Horses, Humans, and Pigs and Ruminal Fluid from Cattle". Journal of Animal Science. 73: 3639-3648.