Introduction

The rising global adoption of pets has driven the pet food market which is projected to grow from USD $115.50 billion in 2022 to USD $163.70 billion by 2029. The five main players, Mars Inc., Nestle Purina Petcare, J.M. Smucker Company, Colgate Palmolive Company, and General Mills, Inc. hold more than 60% of the pet food market [1]. Although dry commercial pet food dominates the pet food market, other unconventional pet diet types and the use of innovative ingredients are growing [2]. Pet humanization and consumers increasing concerns for the role of nutrition in pet health are demanding pet foods that they believe provides better nutrition. The pet food industry is responding with more diet types such as raw, grain free, dehydrated, freeze dried, home-prepared, and fresh refrigerated. Today’s veterinarians will need to understand and address the growing diversity of pet food types as consumers want more information and education about the role of ingredients and whole diet on their pet’s health and longevity.

Applying Food Processing Categorization System to Pet Foods

There is a need to apply a food categorization system to the vast array of pet foods in the marketplace. Studies of dietary effects in the human nutrition literature often use one of three food processing categorizations systems: Nova, International Food Information Council (IFIC) or University of North Carolina at Chapel Hill (UNC). These food processing categorizations systems were developed in the 1980s with the arrival of ultra-processed (UP) human foods, i.e. fractionated recombined foods with added ingredients and/or additives. For example, ready-to-eat breakfast cereals are produced with mechanical treatments (flour refining), thermal treatment to gelatinize starch, and extrusion cooking with high pressure and heat. Finally, there is addition of salt, sugar, fat and/or numerous additives and preservatives to create these convenient, ready to eat, shelf stable products. Breakfast cereals produced with these multiple treatments are now categorized as “ultra-processed foods” in food processing categorization systems. Dry pet foods are produced similar to breakfast cereals with multiple processing steps using heat and pressure and retorted or canned pet foods also undergo multiple thermal processing steps [3–6].

Nutritionist and food scientists must balance food processing to deliver safe, palatable, shelf stable foods that are also healthy and sustainable. Currently numerous published epidemiological studies have determined that populations consuming higher amounts of UP foods exhibit a higher prevalence of obesity and chronic disease. These studies utilize and support the classification systems according to the degree of processing i.e. un/minimally processed (MP), processed, and ultra-processed (UP) of human foods and ingredients [6–14].

With the increase and diversity of pet food types and the consumer demand for identifying healthy pet diets, the application of a pet food categorization system based on the level of processing is relevant. A pet food categorization system has been previously published using the level of processing to produce the final product i.e. minimally processed (MP) and ultra-processed (UP) derived from human food/ingredient categorization systems. These pet food categories, “minimally processed” (MP) and “ultra-processed” (UP) can then be combined with the traditional pet food definitions of “commercial and home-prepared diets” where appropriate [15, 16].

The term commercial diet (CD) infers a pet diet made by a manufacturing pet food company to be sold in the pet food market. Typically, in the United States, commercial pet diets are complete and balanced commercial products, and are intended to be fed as the main source of nutrition for designated life stages, while unbalanced products are intended to be fed supplemental to kibble, canned, or other balanced diets. For example, ultra-processed commercial diets (UPCD) are the conventional dry and retort canned pet diets that dominate the pet food market [15, 16].

The popularity of feeding minimally processed (MP), or previously known as raw meat-based diets (RMBDs) to companion dogs and cats has been increasing in popularity in recent years. Other MP diet types would include the fresh-type pet foods that are cooked and potentially pet diets that are dehydrated or freeze dried. The practice of feeding minimally processed diets can be grouped into two major types: MP commercial diets (MPCDs) or MP home-prepared diets (MPHDs). MP diets of either type may be “complete and balanced” or unbalanced [15, 16].

Pet Food Research

Utilizing a pet food processing categorizations system with these definitions is a starting point that enables studies of UP and MP diets to determine if there is a similar role of these diet types of the health, chronic diseases, and obesity in companion animals. Currently, there is a paucity of quality studies i.e. peer-reviewed meta-analysis or randomized clinical control trials evaluating UPCD compared with MPCD over a significant period measuring whole-diet effects on the health, disease, and life span of a population of dogs or cats. In fact, published peer-reviewed studies evaluating whole-diet effects are scant and more often involve veterinary therapeutic diets [17, 18]. The most evidence-based statement that can be said about pet diet impact on health and longevity would be that dogs fed to maintain a lean body condition score result in statistically significant increased life span. This finding is based on a lifetime study of 48 Labrador retrievers fed a dry UPCD, which resulted in statistically significant longer life span in the diet-restricted group as compared with the control-fed dogs. Because all dogs ate the same UPCD, this study is not evidence that UPCDs are “best nutrition” but demonstrates the benefits of restricting caloric intake [19]. However, similar to studies of human consumption of UP foods, this study could also be interpreted as evidence that the dogs consuming more UPCD had more chronic disease and a shorter life span emphasizing the role of ultra-processed foods, disease states and longevity.

Translational Studies in Food Processing: Advance Glycation End Products

Because most dogs and cats obtain consume UPCD for their entire lives, it is important to understand the effects of food processing on health. Most commercial pet foods and treats are heated to improve safety, shelf life, nutritional characteristics, texture, and nutrient digestion. When heat is applied to food, the structure of sugars and proteins are rearranged. Some of the newly formed compounds are Maillard reaction products (MRP) some termed melanoidins that improve color, flavor, and aroma, but others termed advanced glycation end products (AGEs) can lead to the loss of essential amino acids and may negatively affect animal health [20, 21].

AGEs are also formed in the body by in vivo glycation of tissue proteins, and these are termed “endogenous AGEs.” Endogenous AGEs are produced and accumulate with the physiologic processes of aging and disease states. One of the first recognized endogenous AGE identified was hemoglobin A1c, a glucose-derived hemoglobin increased in diabetes mellitus. Both endogenous AGEs and dietary AGEs contribute to the body’s AGE pool [22, 23]. Studies implicate that dietary AGEs that add to the total body AGEs have a role in aging and the pathogenesis of chronic disease states. AGE-associated diseases develop via two mechanisms: (i) structural alteration of intermolecular and intramolecular cross-linking of tissue proteins that change molecular properties and function, or (ii) by activation of cellular signaling pathways through receptor binding or direct activation to produce reactive oxygen species and an inflammatory response [24–26]. Receptors for AGEs termed RAGE are widely expressed on cells playing roles in oxidative stress, vasoconstriction, excessive collagen deposition, and inflammatory responses. RAGE activation, via AGE binding, may result in development of a chronic inflammatory state, which is observed in many disease states in both humans and dogs. Furthermore, dietary AGEs can impact the intestinal microbiota, compromise epithelial barrier functions, and cause immune stimulation resulting in diseases [27]. This absorption of AGEs from the diet and their accumulation into the AGE pool in the body may be one of the ways diets can impact age-related diseases in humans and animals. AGEs have been associated with age-related diseases in humans, such as diabetes mellitus, atherosclerosis, nephropathy, retinopathy, osteoarthritis, neurodegenerative diseases, and neoplasia [22, 27–29].

Similar to what has been reported in human foods, pet foods are subjected to thermal processing which facilitates the Maillard reaction and formation of AGEs [30]. Studies have measured the levels of AGES in different types of processed pet foods [20, 30–36]. Elevated levels of AGEs in tissue proteins were observed in aging dogs with diabetes mellitus, cataracts, osteoarthritis, neurodegenerative diseases, vascular dysfunction, and atherosclerosis [30–36]. Serum and urine AGEs have been measured in dogs and cats with increasing levels reported with increases in dietary AGEs intake [30–36]. These studies align with human studies of dietary AGEs intake and chronic disease states. To promote human public health and prevent chronic disease, it is currently recommended to limit the intake of UP foods containing high dietary AGEs [37–40]. As pets are now family members, some owners are looking to apply these same recommendations. Avoiding UPCDs may be applicable to consumers looking to feed some or all MP diets. As more is learned about the role of AGEs in processed foods, whether for human or pet consumption, food manufacturers will face the challenge to produce foods with lower AGE content while still being palatable, shelf stable, and safe [41–44].

Pet Food Safety

Health risks for both UPCDs and MPCDs include bacterial pathogens, nutritional imbalances, aflatoxins, and other toxic contaminations which have been previously summarized as pet food recalls or “contamination incidents” that are reported by the Food and Drug Administration (FDA) Pet Food Recalls and/or publications [15]. Currently, the American Animal Hospital Association (AAHA), American Veterinary Medical Association (AVMA), and other veterinary organizations discourage the practice of feeding MP due to pathogenic risks to dogs, cats, and humans. It is important to note that historically bacterial-contaminated pet foods were recognized as a risk to humans from large outbreak in 2006–2008 with Mars Petcare’s dry UPCDs that was linked to human infections with Salmonella [45, 46] and again in 2012 with human Salmonella infections that occurred across the USA and Canada from Diamond Pet Foods dry UPCDs [41]. Following these outbreaks, the Center for Veterinary Medicine’s Veterinary Laboratory Investigation Response Network began testing UPCDs as well as MP diets, exotic animal feeds, and treats. As a result of these outbreaks and surveillances, in July 2013, the FDA set forth Compliance Policy Guide Sec.690.800 Salmonella in Food for Animals [46], establishing a policy of zero tolerance for Salmonella in pet food in the USA, which is more stringent than policies for human food. This means in the USA, all commercial pet diets whether UP or MP are held to the same standard (e.g., zero tolerance policy).

European organizations like European Pet Food Industry (FEDIAF) which is like AAFCO in the USA, has taken a different approach to pathogens in pet food by providing MP diet handling and food safety information and education to the public to help mitigate risk [16]. FEDIAF recognizes recognize that pet owners are likely to continue feeding MP diets despite discouragement of such practices from any regulatory or veterinary authority and therefore has been proactive with campaigns to educate pet owners on proper food safety and handling techniques to mitigate risk [16].

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