Assessing the protein content of plant-based dog foods: an analysis of the effects of plant-based diets on dogs of different ages
According to both the Plant Based Foods Association and The Good Food Institute, the COVID-19 pandemic caused a 27% growth in the plant-based market, with both plant-based meat and milk products nearly doubling their original purchase rates from past years [1]. The rising popularity of plant-based diets is associated with the novel desire for a healthier and more sustainable lifestyle compared to traditional omnivorous or animal-based diets. While many nutritional and clinical studies confirm the sufficiency of these diets in humans, there is a great lack of evidence supporting the effectiveness of these diets in dogs. Many dog owners assume that their dogs will thrive on a plant-based diet that they themselves were able to successfully convert to, not taking into account whether or not the nutritional needs of their pets are actually met.
Pet owners in general tend to attribute certain emotions to their animals that can end up being more of a detriment to their well-being, otherwise known as anthropomorphism. Anthropomorphism can lead to misinformation as pet owners let their own personal beliefs guide their perspectives of their pets. For example, many believe that dogs are strict carnivores, possibly due to how canines are portrayed in the media, but partly due to the owner’s belief that dogs prefer meat over vegetables, when in reality, dogs are omnivores and vegetation is a key part of their diet [16]. It is very easy to assume that one or two consistent trends between human and canine nutrition leads to similar projections into their futures. This theme of projecting owner feelings onto pets, however, is dangerous and certainly one of the main driving forces behind the increasing popularity of plant-based diets in our canine companions [16].
Understanding the changing dietary needs of dogs as they age should be of importance when owners consider acclimating their dogs to a plant-based diet. One nutrient of particular concern is protein, a primary source of nitrogen and amino acids critical to many metabolic processes. Research on the protein content of particular diets is, in general, a popular topic due to wide consumer awareness and knowledge of protein as a nutritional component. The trends of protein requirements across a dog’s lifespan have been adequately recorded, however this is only true for dogs consuming traditional carnivorous or omnivorous diets [2, 3]. This literature review will concern whether or not the protein content of plant-based dog foods meets the nutritional needs of dogs of different ages. The individual results of each study are difficult to generalize due to the great variety in both dog breeds and brands of plant-based dog foods. Overall, these studies hint at trends that can only truly be proven by a long-term study on the effects of plant-based diets on a dog’s health.
Ambiguity of age in dogs
Before we can discuss the connection between a dog’s age and the efficacy of a plant-based diet, it is important to recognize the inconsistencies that come with calling a dog “old.” There are many methods used to age dogs in clinical settings, two examples being dentition and ossification [4]. Dentition is a much less invasive method to age dogs as researchers and/or veterinarians can use eruption times for particular teeth as a gauge for canine age. This method, however, quickly becomes obsolete once canines reach an “adult” age where all their teeth are fully grown. Sutton et al. (2018) proposed the use of ossification, or the process of bone formation, across many different bones in the canine body as a measurement for age since each bone/class of bones has a particular range of times at which their sections fuse to form an epiphyseal plate [4]. While this method did accurately age dogs, it is also not very practical due to the nature of the sample. Obtaining bone samples is incredibly invasive and painful and will not be ethically acceptable in a clinical setting. While there are tools available to pinpoint a dog's age, many are not very accessible or practical in a clinical setting.
Besides complications in approximating canine age, dogs also age differentially relative to their body size. A study by Kraus et al. put this idea to the test while attempting to correlate body size to senescence, or declining rates of reproduction with increased age [5]. Kraus et al. used breed, body mass, and age of death data for 55,000 dogs (both purebred and mixed breed) in combination with a variety of other parameters and algorithms in an effort to draw a connection between the dog’s body size and senescence. Despite the large sample size of this experiment, Kraus et al. note the confounding nature of many body mass records obtained for their study particularly from the young and elderly categories. The ambiguity of “young” in canines indicates that weight is not a reliable indicator of growth relative to size; similarly, elderly dogs are more susceptible to diseases that can cause weight loss, ultimately rendering the weight of those individuals to not be representative of a “normal” individual at that age [5]. Kraus et al. found no significant relationship between body size and senescence, but instead, a strong proportional relationship between canine size and aging rate [5]. This means that larger dogs tend to die younger (death between 5-8 years) than smaller dogs (death between 10-14 years) since they have faster aging rates and tend to show signs of aging before they are fully grown [5]. Such an intricate relationship between body size, growth rate, and longevity highlights the drastic cost of expending lots of energy during the early stages of life to grow quickly in order to keep up with large body size demands, ultimately resulting in reduced survival rates later in life.
The relationship between breed and dog size is clear: chihuahuas are small and mastiffs are large with very little variation within each breed. Thus, when aiming to age a dog, it may be beneficial to take both size and breed into account to get the most accurate prediction for age. Using breed to age dogs has some gray area when it comes to mixed-breeds since it is difficult to predict what size they will grow into given their diverse heritage. That being said, one solution may be to compare the “adult” size of a mixed-breed dog to the “adult” sizes of other purebred dogs to approximate the aging rate of the mixed-breed dog.
Of the studies in this review concerning age, none use breed or dog size as a method of aging, instead favoring the use of metabolic rate, growth rate, or owner-reported age to categorize their subjects into separate age groups [3, 6, 7, 8, 11]. Although age may be somewhat ambiguous in dogs, the influence of age on dietary protein requirement is clear and will be discussed further in the sections below.
Nutritional Requirements of Dog Foods
Many research studies on plant-based dog foods discuss whether or not these foods provide all necessary nutrients according to standards defined by either the Association of American Feed Control Officials (AAFCO) [6, 7, 9, 10] or the European Pet Food Industry Federation (FEDIAF) [13]. Inconsistencies in experimental methodology exist due to differences in established standards between AAFCO and FEDIAF. Although AAFCO and FEDIAF distribute nutritional guidelines to various parts of the world, these organizations have no federal regulatory authority, ultimately reducing their “guidelines” to mere suggestions that pet food companies do not have to legally abide by. An investigation of AAFCO and FEDIAF standards reveals that both corporations outline different protein requirements based on different stages of living (ex. weanling, growing, adult, etc.):
Recommended grams of protein for growth and reproduction | Recommended grams of protein for adult maintenance | ||
AAFCO | 53.6g | 45.0g | |
Early Growth (< 14 weeks) | Later Growth ( ≥ 14 weeks) | ||
FEDIAF | 62.5g | 50.0g | 45.0g |
This table (Table 1) summarizes the discrepancies between AAFCO and FEDIAF protein guidelines while also highlighting a secondary issue: there is no category for elderly dogs. Stipulations are only outlined for other life stages such as puppies (“early growth” to “later growth”) and adults, indicating a need for regulated nutritional standards for older dog populations [11, 12]. The growth stage for dogs in itself is highly variable depending on the breed as it represents the amount of time needed to reach adult size. As dog breeds vary greatly in their size, it is easy to see how this time period for growth can vary as well, generally anywhere between six and 24 months [12]. FEDIAF does acknowledge in their 2017 Scientific Advisory Board Statement that elderly dogs experience bodily changes that alter their nutritional requirements, however, this notice lacks an advised protein content value for elderly dogs [12]. It is necessary to first establish if plant-based diets contain enough protein to satisfy the nutritional requirements of a dog throughout its life.
A recent study by Wehrmaker et al. used a computer program and an ingredient database to formulate various plant-based recipes and predict the nutritional efficacy of each recipe according to FEDIAF standards, ultimately concluding that all recipes either met or drastically exceeded canine protein requirements [13]. At its core, Wehrmaker et al.’s study focused on producing a “sustainability value” (a number representative of the environmental impact this recipe would have upon production) for plant-based dog foods based on the ingredients of each recipe. The most unreliable part of this study concerns the ingredient database: not all nutritional values (ex. calcium, zinc, methionine, cystine, etc.) were present for each ingredient listed in the database. The absence of critical nutritional data led the computer program to create recipes that would exclude necessary ingredients [13]. It is also important to note that none of the recipes in Wehrmaker et al.’s study are real, they only pose suggestions for the components of plant-based dog foods and have yet to be tested in a real population of dogs. However, the researchers note the high likelihood of plant-based diets meeting protein requirements. Although evidence supports the sufficient protein content of plant-based dog foods according to international standards, whether such a diet is able to fulfill the nutritional needs of dogs at all life stages remains unknown.
Two studies on varied animal-based diets determined that diets containing a protein content of 12.8% are sufficient for growing dogs and a protein content of about 16% sustains both adult and elderly dogs [2, 3]. Protein content above these percentages produced no significant differences within the dogs in terms of growth or whole-body protein turnover [2, 3]. Typically used as a proxy for metabolism, whole-body protein turnover compares amounts of synthesized protein to amounts of consumed protein in the body, overall creating a picture of growth quantified by protein usage. In addition to protein, there are a host of other nutrients for which there exists a significant correlation (p < 0.05) between sufficient quantity and a dog’s age: cholesterol, nitrogen-free extracts (starches and sugars), and nitrogen; nitrogen in this context as associated with a dog’s maximum protein reserve, otherwise known as stored protein within the body [6, 7, 14]. Although protein can be stored in the body, it is not one of the body’s preferred energy sources and instead primarily exists as metabolites (ex. enzymes and amino acids) or as muscle tissue. Overall, there appears to be a proportional (but not exclusive) relationship between a diet’s protein content and age in dogs, suggesting that increasing the age of a dog causes similar increases in necessary dietary protein.
As dogs age, cholesterol levels in particular also tend to increase, which can be explained by the increase of adipose (fat) tissue with age [3]. Cholesterol is a type of fat that can be stored in adipose tissue and tends to accumulate in the body with age due to a slower metabolism, less active lifestyle, and long-term consumption of foods that are high in saturated fats. Williams et al.’s experiment comparing cholesterol levels between plant-based and animal-based diets produces consistent results: senior (which Williams et al. defined as dogs over eleven years old) and weanling dogs (zero to eight weeks old) consuming plant-based diets had lower cholesterol levels compared to those consuming animal-based foods, most likely due to the high fiber and low saturated fat content of plant-based foods [6]. However healthier results from plant-based foods are not always guaranteed and should not be readily assumed.
Methodology Used to Estimate the Protein Content of Plant-based Dog Foods
We can reliably compare many of the studies mentioned in this review due to their similarities in subjects of study (primarily beagles, who are considered model organisms for canines) and methodology [2, 3, 6, 7, 8, 14]. Many dietary nutrients can be studied through hematology [3, 6, 8, 9] or by comparing digestibility [3, 6, 7, 9]. Hematology is the study of blood and how its components change over time. Protein is not the only component in blood, so many studies using hematology tend to be broad assessments monitoring change over multiple body systems. In that sense, hematology is a good method to use when studying more complex and multifaceted topics, such as aging, disease, and metabolism.
Besides hematology, many of the studies that have been and will be discussed utilize the concept of digestibility [6, 7]. Digestibility is a measure of how well nutrients are absorbed by the gut and therefore “digested.” When the digestibility of a certain food item is low, it indicates that most of the contents of the food passed completely through the body to be excreted as waste rather than being absorbed by the body to be either stored or broken down to produce energy. At its core, digestibility measures the nutritional value of food. The digestibility of food can be affected by both the contents of the food as well as whether the food is highly processed or not [6, 15]. Generally, plant-based foods tend to be more processed than animal-based foods as an attempt to maximize protein content while reducing fiber content [13]. Greater fiber consumption associated with plant-based diets, for example, implies a difference in the digestibility of plant-based foods compared to animal-based foods; since plant-based diets contain high levels of fiber, dogs must intake larger quantities of food to meet the same nutrient levels as smaller amounts of animal-based foods [6]. The processed nature of these foods, however, has led to mixed results on its effect on protein digestibility [15]. A study by Lappi et al. used protein digestibility as a proxy for the nutritional quality of plant-based foods compared to animal-based foods undergoing various processing techniques. Processing does affect the digestibility of certain ingredients, but at varying degrees; foods that contain the same protein content can still have different digestibility levels depending on the type of processing that was used to make the food [15]. More research, however, is necessary to truly prove this correlation. It is important to note that Lappi et al.’s study was conducted with respect to human diets, so their results may vary according to differing necessary dietary protein intake values between canines and humans.
Another form of processing includes the final state of the food: dry or wet. Various results from Wehrmaker et al. validate the relationship between protein digestibility and food state, denoting the high protein content and high average crude fiber rates of both wet and dry plant-based dog foods [13]. As previously mentioned, the recipes constructed by Wehrmaker et al. were mere simulations, implying that all of their results have yet to be validated by reality. While combining the ingredients in each recipe would not be too difficult, the real issue would be making these recipes palatable to canines without additional supplementation or flavor enhancement that could alter the nutritional content of the food. Additionally, the nutritional content of each recipe presented by Wehrmaker et al. are most likely diluted versions of the real nutritional values of each recipe. Missing nutritional values in the ingredient database led to inflated supplement amounts in each recipe, including additions of calcium, iodine, zinc, vitamin D, vitamin B12, and many others [13].
However, there are many other factors which can affect digestibility besides processing and ingredient type. Trends of protein digestibility decreasing with age, as previously discussed, have been demonstrated by Williams et al., who found overall lower protein turnover rates in older dogs compared to younger dogs [7]. Kim et al., on the other hand, observe no differences in protein digestibility between wet and dry food, which may be considered a form of processing and therefore consistent with the work done by Lappi et al. [7, 15]. Kim et al. conclude their research with a broad statement, speculating that food moisture content may impact the digestibility of certain nutrients in aging dogs [7]. The lower digestibility of protein in older dogs may explain why elderly dogs need about 3.2% more protein in their diet compared to the average adult dog to maintain their health [13]. Consuming greater amounts of protein, however, may not be easily resolved by giving dogs bigger portions of food as results from Lawler et al. suggest that dogs consuming larger quantities of animal-based food experience shorter lifespans and greater risk of illness [8].
As mentioned throughout this review, the main drawback of all research conducted on nutritional studies is the lack of generalizability due to the specificity of each experiment. Results that become inconsistencies in reality are one of the main drawbacks of experiments utilizing simulation, ultimately causing such studies to lose their applicability and generalizability. Similarly, the great variation within both canine breeds and dog food brands presents an overwhelming number of dietary combinations that could not be feasibly accounted for in scientific research. This is, of course, why studies choose to operate on either large populations or model organisms as results produced in this fashion can be assumed to match reality. Despite individual flaws, many of these research studies work together to fill in each other’s gaps [3, 7, 8, 9, 11, 14]. Certain protein thresholds are clearly indicated for each life stage where the dog has no more nutritional benefits to gain upon consuming more food (Table 2). In fact, consuming excessive amounts of protein may be dangerous for dogs.
Recommended percentage of crude protein in diet | |
Puppies (< 14 weeks) | 12.8% - 17.2% |
Young Adult Dogs (2-8 years) | Up to 16% |
Geriatric Dogs (8+ years) | Up to 16% |
Health-related impacts of plant-based diets on aging dogs
The discrepancies between AAFCO and FEDIAF reveal a lack of standardization in the protein content for dogs past the adult age range. Given the rising popularity of vegan and vegetarian dog foods, it would be beneficial to investigate any temporal-related effects of committing one’s dog to a plant-based diet to determine if this lifestyle is manageable and healthy in the long-term [1, 16].
Cavanaugh et al. and Lawler et al. attempted to assess any correlations between particular diet restrictions and disease as dogs age [8, 9]. Cavanaugh et al.’s study honed in on the correlation between amino acids, specifically taurine, and heart disease; taurine is one of ten essential amino acids to canines that must be ingested to perform vital functions throughout the body, and unfortunately, deficiencies in this amino acid have been linked to various heart issues [9]. Lawler et al. and Williams et al. postulated that differing concentrations of amino acids may explain both the slower immune system decline in diet-restricted dogs and the increase in necessary dietary protein between adult and elderly dogs [3, 8, 9].
One major concern in comparing these studies is that Lawler et al. and Williams et al’s conclusions stem from research on animal-based diets while Cavanaugh et al’s study was based on plant-based diets. Thus, while Cavanaugh et al. determined that there was no significant difference (p < 0.05) between the taurine levels of plant-based and animal-based diets, this result cannot be directly applied to Lawler et al.’s and Williams et al.’s studies due to inconsistencies in methodology [9].
Lawler et al.’s study over the course of 20 years took various samples from 48 labradors to determine how different quantities of the same animal-based dog food affected a dog’s long-term health [8]. The study thoroughly compared two groups of dogs by taking annual blood, fecal, urine, and serum samples as well as maintaining physical records for each dog in the study. Dogs were paired up based on their physical similarities (ex. size and gender) so that each pairing consisted of one dog from the control group and another from the experimental, or diet-restricted, group. The control group (consisting of 24 dogs) ate “normal” amounts of animal-based food relative to their size while the diet-restricted group (also consisting of 24 dogs) consumed about 25% less food than their control group partner. The study found overall that dogs consuming about 25% less food compared to the control group over the course of their entire lives had an increased lifespan by about 2 years [8]. This experiment also reported that diet-restricted dogs had significantly slower declines in their immune systems as they aged [8].
Unfortunately, the main reason why many studies evaluate associations between diet and disease is because many commercial diets formulated according to either AAFCO or FEDIAF standards still result in nutrient deficiencies. This sort of deficiency may be related to the fact that dogs of different ages require different nutrient concentrations in their diets, causing many commercial dog food brands to become obsolete the older a dog becomes. While the studies described above complement each other in garnering both the short- and long-term effects of certain dietary restrictions, there is a sense of ambiguity caused by inconsistencies in the methodology of each experiment. Compared to all other studies described in this paper, Lawler et al.’s study is the only one performed on labrador retrievers instead of beagles, which may lead to some results that are specific to labradors and cannot be generalized to other dog breeds. On the other hand, Cavanaugh et al.’s study had many limitations pertaining to their experiment, including the lack of a randomized control group, high variability of plant-based foods in the experimental group, and a relatively short study period. Many dietary and nutritional studies admit that their results are difficult to generalize to overall dog populations and, therefore, their results should be applied with caution to other breeds. Similarly, Cavanaugh et al. concluded that their results could not be generalized to all plant-based diets due to high variation in the nutrient composition between brands.
Results from various studies agree that dogs consuming plant-based diets must consume more food to meet the same nutritional requirements as animal-based diets due to the lower digestibility of plant-based foods [6, 8]. As demonstrated by Lawler et al., dogs consuming smaller quantities of animal-based food experience longer lifespans and lower risk of illness [8]. Therefore, if elderly dogs require higher protein content to maintain their health, they will need to consume greater amounts of plant-based foods to meet those protein needs [2, 3, 6, 13, 14]. However, excessive amounts of protein are associated with certain renal and heart diseases in dogs [9, 10]. This last conclusion in particular indicates the possibility of a connection between greater food consumption and increased risk of certain diseases as discussed by Lawler et al.
Ephraim et al.’s study was tested with these conclusions in mind, using the ambiguity of optimal protein intake for adult dogs to test if diets of varied protein concentration (18.99%, 25.34%, and 45.77%) are linked to the development of particular renal diseases in canines [10]. All three diets were in dry form and used either chicken or soybean protein isolate as the main protein source. 30 beagles were evenly split into six groups and randomly assigned to consume each diet type for 90 days (270 days total) and urine, fecal, blood, and serum samples were taken annually from each dog. Thorough analysis of each dog’s metabolites and gut microbiota led to the conclusion that increasing the protein content of a dog’s diet is significantly correlated (p < 0.0001) with an increase in metabolites associated with kidney disease and inflammation in otherwise healthy dogs [10]. This result becomes especially concerning when put in the context of high-protein plant-based diets and the low digestibility of protein in older dogs, a combination that suggests that plant-based diets may not be entirely suitable for elderly dogs. While there may be a correlation between protein consumption and kidney disease in older dogs, it is important to recognize that kidney disease is already widely associated with aging, and thus the role plant-based diets play in expediting this disease is ambiguous.
Conclusion
Results from studies on plant-based dog foods are difficult to generalize due to the vast diversity of plant-based dog food brands and dog breeds to be accounted for. It becomes difficult for consumers to trust the nutrient content advertised by many plant-based food brands, as there appears to be a lack of evidence concerning the specific long-term effects of plant-based diets on dog health. To ensure complete transparency, dog food brands should communicate the associated risks with increasing protein content rather than generalizing the protein content of their food to dogs of all ages.
Despite the abundance of research on alternative diets, consumers fail to apply their knowledge to commercially available dog food. Such claims of proper nutrient content are relative to the standard each brand chooses to adhere to (AAFCO or FEDIAC) and may not be truly healthy for one’s dog due to hidden deficiencies [9]. While this review confirms the possibility of healthy plant-based diets for dogs of all ages, dog owners should refrain from letting their own personal choices influence their pet’s lifestyle. Proper research should be conducted before committing one’s dog to this diet.
The best outcome to ensure the health of canines as they age would be for brands to curate diets for dogs of all ages, or rather, age categories. It may be beneficial, therefore, for future studies to amass a large sample of dogs of various sizes and/or breeds, producing more applicable results for dog owners by increasing ecological relativity. Although the underlying mechanisms of these trends will likely be inconclusive in such broad studies, the value of relativity to the general population may outweigh the need for scientific specificity.
About the Author: Alyssen Wong
Alyssen (Aly) Wong plans to graduate from UC Davis in 2024 as an Animal Biology major (and yes, you would be correct in your assumptions) to pursue a career in veterinary medicine. Currently, Aly works for two different branches of UC Davis’ School of Veterinary Medicine: the Veterinary Genetics Lab (VGL) and the Mammalian and Ecology Conservation Unit (MECU). At both the VGL and MECU, she is largely responsible for extracting DNA from the hairs, scats, and tissues of various animals such as horses, bears, coyotes, wolves, foxes, deer, kangaroo rats, and many others to produce data for research and clientele.
Author's Note
According to the American Society for the Prevention of Cruelty to Animals, more than 23 million American households (my family included) adopted a pet during the COVID-19 pandemic, many of which were affectionately termed “pandemic puppies.” Unfortunately, many of these so-called “pandemic pets” were adopted on a whim, solely to keep their lonely owners company. Consequently, many families neglected to perform proper research before getting their new pet. One area that can be particularly difficult to approach is diet: the wide array of commercially-available dog foods spanning various ages, sizes, breeds, and diet types.
There’s no question about the safety of plant-based foods for canine consumption – their presence in our local pet shops assures us of this. Yet, many of these plant-based foods also claim to provide the same health benefits and nutritional value as animal-based foods with the added bonus of sustainability, but to what extent are these claims true? Among humans, plant-based diets have garnered a lot of attention in recent years because of the healthier lifestyles they boast, so it was only a matter of time before these same thoughts became integrated into the pet food industry. In fact, research from various sources proves that these alternative diets can be suitable for dogs while meeting nutritional standards outlined by either the Association of American Feed Control Officials (AAFCO) or the European Pet Food Industry Federation (FEDIAF). In spite of research and the advertised nutritional quality of plant-based dog foods, the long-term effects of these diets have yet to be truly studied due to the recent emergence and popularity of plant-based diets.
My initial investigation into plant-based foods led me to a secondary topic: the changing dietary needs of dogs as they age. As an analogy, over the first couple years of life, humans eventually transition from milk formulas and baby foods to solid foods. These early life-stage foods contain supplemental nutrients which boost growth and/or healthiness in babies. Puppy foods exist for this same purpose: to provide extra nutrients during a critical period of growth in a dog's life. What my research introduced to me was the fact that elderly dogs (an age range that is generally breed-dependent) have different dietary needs compared to adult dogs, highlighting the need for “geriatric dog formulas” to keep elderly pups healthy. Unfortunately, there is a clear lack of commercial pet foods curated towards elderly dogs compared to puppies, and this is especially so for plant-based foods. With these two ideas combined, I strove to investigate the long-term sufficiency of plant-based diets in dogs by using dietary protein content as a proxy for healthiness in dogs of various ages.
References
- Robbins M & Voss S. 2020. Plant-based food retail sales grow 27 percent to reach $7 billion in 2020. Good Food Institute [Internet]. Washington (DC):Good Food Institute. [cited 2023 May 11]. Available from: https://gfi.org/press/plant-based-food-retail-sales-grow-27-percent-to-reach-7-billion-in-2020/
- Gessert CF, Phillips PH. 1956. Protein in the nutrition of the growing dog. The Journal of Nutrition [Internet]. 58(3):415-421. doi: 10.1093/jn/58.3.415
- Williams CC, Cummins KA, Hayek MG, Davenport GM. 2001. Effects of dietary protein on whole-body protein turnover and endocrine function in young-adult and aging dogs. Journal of Animal Science [Internet]. 79(12):3128-3136. doi: 10.2527/2001.79123128x
- Sutton, LK, Byrd, JH, Brooks, JW. 2018. Age Determination in Dogs and Cats. Veterinary Forensic Pathology [Internet]. 2:151-163. doi: 10.1007/978-3-319-67175-8_11
- Kraus C, Pavard S, Promislow D. 2013. The Size-Life Span Trade-Off Decomposed: Why Large Dogs Die Young. The American Naturalist [Internet]. 181(4):492-505. doi: 10.1086/669665
- Swanson, KS., Kuzmuk, KN., Schook, LB., Fahey, GC. 2004. Diet affects nutrient digestibility, hematology, and serum chemistry of senior and weanling dogs. Journal of Animal Science [Internet]. 82(6):1713-1724. doi: 10.2527/2004.8261713x
- Kim K, Seo K, Cho H-W, Jeon J-H, Kim C, Jung J, Chun J. 2021. Age-related digestibility of nutrients depending on the moisture content in aged dogs. Journal of Animal Science and Technology [Internet]. 63(6):1355–1361. doi: 10.5187/jast.2021.e116
- Lawler DF, Larson BT, Ballam JM, Smith GK, Biery DN, Evans RH, Greeley EH, Segre M, Stowe HD, Kealy RD. 2008. Diet restriction and aging in the dog: major observations over two decades. British Journal of Nutrition [Internet]. 99(4):793-805. doi: 10.1017/S0007114507871686
- Cavanaugh RP, Cavanaugh SM, Gilbert GE, Ketzis JK, Leavitt EL, Vieira AB. 2021. Short-term amino acid, clinicopathologic, and echocardiographic findings in healthy dogs fed a commercial plant-based diet. PLoS One [Internet]. 16(10):e0258044. doi: 10.1371/journal.pone.0258044
- Ephraim E, Cochrane C-Y, Jewell DE. 2020. Varying protein levels influence metabolomics and the gut microbiome in healthy adult dogs. Toxins (Basel) [Internet]. 12(8):517. doi: 10.3390/toxins12080517
- AAFCO Dog and Cat Food Nutrient Profiles [Internet]. 2012. Champaign (IL):Association of American Feed Control Officials. [cited 2023 May 7]. Available from: https://www.aafco.org/wp-content/uploads/2023/01/Model_Bills_and_Regulations_Agenda_Midyear_2015_Final_Attachment_A.__Proposed_revisions_to_AAFCO_Nutrient_Profiles_PFC_Final_070214.pdf
- Nutritional Guidelines For Complete and Complementary Pet Food for Cats and Dogs. 2021. Brussels (BE):FEDIAF EuropeanPetFood. [cited 2023 May 7]. Available from: https://europeanpetfood.org/wp-content/uploads/2022/03/Updated-Nutritional-Guidelines.pdf
- Wehrmaker AM, Draijer N, Bosch G, Van der Goot AJ. 2022. Evaluation of plant-based recipes meeting nutritional requirements for dog food: the effect of fractionation and ingredient constraints. Animal Feed Science and Technology [Internet]. 290(0377-8401):115345. doi: 10.1016/j.anifeedsci.2022.115345
- McCoy JR, Wannemacher Jr RW. 1966. Determination of optimal dietary protein requirements of young and old dogs. The Journal of Nutrition [Internet]. 88(1):66-74. doi: 10.1093/jn/88.1.66
- Lappi J, Silventoinen-Veijalainen P, Vanhatalo S, Rosa-Sibakov N, Sozer N. 2022. The nutritional quality of animal-alternative processed foods based on plant or microbial proteins and the role of the food matrix. Trends in Food Science & Technology [Internet]. 129(0924-2244):144-154. doi: 10.1016/j.tifs.2022.09.020
- Wells S. 2023. Vegan Dog Food Is On The Rise but Veterinarians Give One Reason To Avoid It. Inverse [Internet]. New York (NY):Inverse. [cited 2023 Dec. 7]. Available from: https://www.inverse.com/science/vegan-dog-food-popular-veterinarians-give-reason-to-avoid