Pet owners are no longer satisfied with the pet food only to provide nutritional functions, more hope that the pet food at the same time can solve or avoid some of the disease problems, such as the pet's digestive problems and obesity problems, etc., can be achieved to take care of the stomach and intestines to improve the hair, enhance immunity and other functions. Therefore, functional pet food came into being. At present, the commonly used functional pet foods include probiotics, oligosaccharides, plant extracts, functional amino acids, proteoglycans and so on.

Probiotics

Probiotics are a class of active microorganisms that can colonize in the host and change the composition of the flora in a certain part of the host. They can regulate the balance of flora in the intestinal tract and maintain intestinal health (Xu Wenjiao et al., 2020; Zou Peng et al., 2021). Probiotics added in pet food can colonize the pet's body, improve the pet's intestinal immunity, prevent intestinal diseases, and improve the pet's health (Longtan et al., 2017). Kun Liu (2009) found that Bacillus cereus could reduce diarrhea in puppies. In addition, some studies have shown that the addition of probiotics can increase the average daily weight gain of puppies by 18.76% (Huo Jun et al., 2018), and the addition of probiotics to pet diets can also improve immune function (Du Li et al., 2009). At present, the addition of probiotics such as Bifidobacterium bifidum, Streptococcus thermophilus, and Lactobacillus plantarum in canine and feline pet food has a large market.

Functional oligosaccharides

Functional oligosaccharide, also known as non-digestible oligosaccharide, is a low degree polymerized sugar with branched or straight chains formed by dehydration of 2-10 monosaccharide molecules connected by glycosidic bonds. Functional oligosaccharides have the function of reducing diarrhea and are purely natural, low-residue, and non-resistant, so they can be used as feed additives in place of certain antibiotics, and used in pet food to enhance the immune performance of pets, improve the intestinal micro-ecological environment, improve digestibility, and reduce the fecal odor (Liu Hualan et al., 2015; Hao LINH et al., 2005). Li Hong (2012) found that adding 0.5%~1.0% oligofructose to the diet of Rottweiler littermates could promote their growth and improve feed conversion ratio (SWANSON et al., 2012); oligofructose and oligomannose were found to improve the immune system function and fecal odor.

Functional amino acids

Functional amino acids have a regulatory effect on the life activities, growth and metabolism of organisms compared with traditional amino acids, such as glutamic acid, cysteine, leucine, tryptophan and arginine (Gao Chunqi et al., 2016). Arginine and taurine are commonly used in pet food.

Arginine can participate in the decomposition, metabolism and synthesis of nutrients in the body, promote the secretion of insulin, growth hormone and other hormones, and participate in the regulation of immune function, which can improve the immune function of pets and promote their growth (NEWSHOLME et al., 2005).

Taurine is widely found in various tissues and vital organs of animals, and is one of the most abundant free amino acids (HAYSEand STURMAN, 1981). Taurine has more multifunctional functions, which can improve the immune function of animals, enhance antioxidant capacity, improve immune function, and maintain intestinal microecological balance, but most mammals have limited ability to synthesize taurine by themselves, and exogenous supplementation is required (LIU et al., 2014). Wu Zhenming (2017) showed that the addition of taurine to broiler diets can enhance heat resistance and alleviate stress response. In addition, the addition of appropriate amounts of taurine to diets can improve growth performance and feed intake (LEE et al., 2004).