The proportion of the dietary energy in the US consumed by dogs and cats was calculated as the sum of the energy consumed by dogs and cats (203 ± 15 PJ y -1 ) divided by human energy intake (1051 ± 9 PJ y -1 ), with the result that dogs and cats consume about 19.4 ± 1.6% of the energy that humans in America do ( Table 1 ). 
 
 Cats require ~544 kJ (kg BW) -0.67 d -1 energy [ 29 ]. The body weight of cats varies less than that of dogs, so the average and standard deviation of cat weight in Bermingham et al. [ 33 ] (4.2 ± 0.2 kg) were used to represent average cat weight, resulting in a total cat energy requirement of 1,426 ± 79 kJ d -1 (341 ± 19 kcal d -1 ). Multiplied by the estimated number of owned cats in the US ( Eq 1 ), this results in an estimate of 45 ± 2.5 PJ y -1 consumed. 
 
 The American Pet Products Association (APPA) estimates that there were 77.8 million dog and 85.6 million cats owned as pets in the United States in 2015 ( Table 1 ) [ 8 ]. Dogs’ energy requirements are taken as ~544 kJ (kg BW) -0.75 d -1 [ 29 ]. Dogs’ body weight (BW) varies greatly by breed. To estimate the average BW of dogs, the average weight of the American Kennel Club (AKC)’s list of the 10 most popular dog breeds in the US was used [ 30 ]. Average breed weights were taken either from the AKC or other sources [ 31 ]. This resulted in an average US dog BW of 22 kg. The standard deviation of the average breed weights represents the variability among breeds, rather than uncertainty in the average dog weight and is therefore inappropriate for the uncertainty analysis done here. To estimate the uncertainty in the average dog weight, data from Meyer et al. [ 32 ] were taken for 10 breeds of different sizes. For each breed, Meyer et al. [ 32 ] reports the mass and standard deviation of the samples (n = 4 to 9). The standard deviation was regressed against the mass (r 2 = 0.87) and standard deviation at 22 kg was estimated as 1.2 kg. Therefore, the estimated average US dog BW that will be used hereafter is 22 ± 1.2 kg giving an average energy requirement of 5,594 ± 443 kJ d -1 (1337 ± 106 kcal d -1 ). Multiplied by the estimated number of owned dogs in the US ( Eq 1 ), this results in an estimate of 159 ± 13 PJ y -1 consumed by dogs [ 29 ]. 
 
 The Census Bureau estimates that the total population of the US was 321 million in 2015, with roughly equal proportions of men and women [ 27 ] ( Table 1 ). The USDA Agricultural Research Service estimates that on average, US males (age 2+) consume 10,330 ± 91 kJ d -1 (2,469 ± 81 kcal d -1 ) and US females (age 2+) consume 7,607 ± 64 kJ d -1 (1,817 ± 15 kcal d -1 ). Therefore, the average daily energy consumption for both males and females is 8,966 ± 155 kJ d -1 (2,143 ± 37 kcal d -1 ) [ 28 ]. Using Eq 1 , these estimates result in a total human energy intake of 1,051 ± 9 PJ y -1 . 
 
 Energy from animal sources 
 
 For humans, the fraction of energy that is derived from animal sources, F A , can be calculated as: (2) where E A,C is the energy consumed by humans from animal sources (subscript A). E A,C can be calculated from data available from the U.S. Department of Agriculture (USDA) (Table 2): the total amount of red meat (including beef, veal, pork, and lamb), poultry (including chicken and turkey) and fish (including fish and shellfish) eaten by each Americans is 59.6 kg yr-1. Given the energy density of each food used by the USDA (Table 2), and with the conservative assumption that this meat provides the only animal-derived energy consumed by Americans, it is calculated that Americans consume 206 PJ yr-1 from animal sources, which constitutes 20% of their total energy intake. 
 
 For dogs and cats, direct data on consumption is not available and therefore F A cannot be calculated directly using Eq 2. Instead, new calculations must be made based on available data: ingredient lists for dog and cat foods and the composition of these ingredients in terms of substrates which have well-known energy densities (i.e., Atwater factors for protein, carbohydrate, and fat). 
 
 To do this, the ingredient lists for individual pet foods were used. Individual ingredients were considered in terms of the content of energy-providing substrates, protein, fat, and carbohydrate and non-energy providing components like water, ash, and fiber. Compositional data analysis is required for these calculations because the substrate components must sum to unity [34]. For a particular pet food, m, the center (analogous to the arithmetic mean) dry mass fraction of substrate k (protein, fat, carbohydrate, other), expressed as average grams of k per gram of m, was calculated as the closed geometric mean: (3) where is the mass fraction of substrate k in one of the first five ingredients, i, in a particular food (i.e., grams of k per gram of i). For these calculations, the category ‘other’ was included to provide closure [35], that is, so that the fractions of all categories would sum to unity. was estimated for each ingredient by equating it with a general ingredient category for which substrate content is available (Table 3) [29, 36]. 
 
 Similarly, the average dry mass fraction of animal-derived substrate k for a particular food (i.e., average grams of animal-derived k per gram of m) was calculated as the closed geometric mean: (4) where is the mass fraction of animal-derived substrate k in one of the first five ingredients, i, in a particular food (Table 3 asterisks indicate animal-derived). For these calculations, the same approach was used in calculation of , except non-animal derived protein, fat, and carbohydrates were added to the ‘other’ category to maintain closure. and are (geometric) average mass fractions and therefore explicitly assume that the first five ingredients in a food are present in equal proportions and that they constitute nearly all of the mass of pet food m. This assumption is wrong, but conservative, as explained below. Uncertainty in was calculated as the variance across all m for each substrate k [35]. 
 
 The fraction of energy derived from animal products in a food m (animal-derived joules per total joules) was calculated as: (5) where E k is the energy density of the substrates (i.e., Atwater factors: E protein = E carbohydrate = 4J/g, E Fat = 9 J/g [29]). E other was set to zero in for both total and animal-derived calculations. In the former case, water, ash, and fiber, which provide no dietary energy, comprised the ‘other’ category. In the latter, ‘other’ contained water, ash, and fiber as well as non-animal derived protein, fat, and carbohydrates, on the logic that these do not provide animal-derived dietary energy. The total animal-derived energy was calculated as (6) which is the weighted average fraction of animal-derived energy in four categories: premium dog food (n = 102), market-leading dog food (n = 9), premium cat food (n = 163), and market-leading cat food (n = 9). is the annual total energy consumed by dogs and is the annual total energy consumed by cats (Table 1) P x,y is the proportion of dog or cat owners and (x = Dog and Cat, respectively) who prefer premium or market-leading foods (y = P and N, respectively). Likewise, M x,y is the number of foods considered here in each category. More premium foods were used in these calculations because there is more diversity in this market sector. For dry dog food, nine foods from just five manufacturers constitute 48% of the market [37]. For dry cat food, nine foods from just four manufacturers constitute 49% of the market share [38]. 
 
 Dry foods were used for these calculations. For both dogs and cats, dry food sales dominate wet food sales (billions of US dollars in sales for various foods in 2012: 8.7 (dry dog food) vs 2.3 (wet dog food) [39], and 3.6 (dry cat food) vs. 2.4 (wet cat food) [40], and thus are more representative of the foods fed to cats, and especially, dogs. The dominance of dry food as the preferred form is especially true when the price per serving is taken into account. One market-leading wet cat food costs approximately $0.83 per serving while a dry food by the same manufacturer costs approximately $0.23 per serving. Using the this per-serving price ratio, dry cat food outsells wet cat food on a per-serving basis by a factor of about 3 to 1. Furthermore, dry food typically has lower animal content (as determined by the list of ingredients in descending order of mass contribution) than wet food. Thus, use of dry food for these calculations provides a conservative estimate of the greatest proportion of dog and cat food sales in the U.S. 
 
 USDA labeling rules require that pet food ingredients be labeled in descending order of weight contribution, as they do with foods intended for humans. Calculations were made on the assumptions that 1) each of the first five ingredients contributes, by mass, equally to the mass of the pet food and 2) collectively, these first five ingredients make up nearly all of the mass of the pet food (that is, there are no other ingredients that contribute substantially to the mass of the food). With regard to the former, for marketing purposes, animal-derived ingredients typically appear in in the top couple of places in the ingredient list. This is particularly true of premium foods, where 100% of both dog and cat foods examined here had animal-derived products as the first ingredient (Table 4). For all types of dry food examined here (market-leading v. premium dog and cat foods), animal-derived ingredients appear among the first two ingredients more commonly than among the third and fourth ingredients (Table 4). Thus, the calculations made here over-weight the later ingredients, which are less likely to be animal-derived, compared to the earlier ingredients, which are more likely to be animal derived. Although there is no way to know, in proprietary recipes, the exact proportions of ingredient, by weighting the first five ingredients equally, a minimum overall estimate of animal-derived energy in dog and cat food is produced. 
 
 PPT PowerPoint slide 
 
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 larger image TIFF original image Download: Table 4. Frequency of an animal derived ingredient in one of the first two positions or one of the two following positions in the ingredient list of dry foods considered here. https://doi.org/10.1371/journal.pone.0181301.t004 
 
 With regard to the second assumption, that the first five ingredients make up nearly all of the mass of the pet food, ingredients appearing past the first five in the ingredient list are often nutrients (e.g., tocopherol) added in trace quantities. If ingredients past the fifth are not trace, then given the requirement that ingredients be listed in decreasing mass contribution, the sixth ingredient must contribute less than 16% of the mass of the food. In the case of seven substantive ingredients, the maximum fraction of the mass in the 6th and 7th places is 29%. Among the premium brands that were examined, the proportion of animal-derived product decreased as they occurred later in ingredient lists with only 21% of the sixth ingredients in dry dog food being animal-derived. Thus, even in the extreme case, a maximum of 3–6% (21% of 16% = 3.5%; 21% of 29% = 6%) of the animal-derived content may be missing in the foods examined here. Although the methodology use here cannot give exact amounts of animal-derived content from foods, the potential maximum exclusion of 3–6% of animal-derived products is sufficient to draw important conclusions about the amount of animal-derived energy consumed by dogs and cats. 
 
 The APPA’s annual pet-owners survey [8] provides data that can be used understand consumer preferences, thus providing information about ratio of premium vs. non-premium (market leading) foods consumed. Non-premium brands tend to have lower animal-derived content whereas premium brands tend to have higher animal-derived content. The premium brand category used here includes the 'premium' and 'gourmet' survey categories. For dogs of all sizes, the average percent of owners who usually feed these meat-rich dog foods is 38%. For cats, this number is 30% (Table 5). 
 
 The final market-wide estimates of the fraction of energy in dog and cat foods that is animal-derived are 34% ± 4% and 31% ± 4%, respectively (Table 5). In total, Eq 6 yields an estimate that animal-derived energy constitutes 33% ± 6% of the diets of dogs and cats in the US. This is significantly higher than the fraction of humans’ dietary energy that is animal-derived (19%). Because dogs and cats consume, together, 203 ± 15 PJ/year, it is estimated that dogs and cats consume a minimum of 67 ± 17 PJ/year in animal-derived energy, which is 33% ± 9% of the animal-derived energy consumed by humans in the US or 25% ± 6% of the total. 
 
 An important caveat for the calculations of the relative consumption of pets and humans is that the sources of the data, and mode of calculation, are dramatically different. As a result, their ratios may be systematically biased. Nonetheless, the calculations of absolute amounts (e.g., PJ/yr) are informative, and the relative amounts still provide important insight into the magnitude of pets’ consumption. ||||| Turns out that chow you’re feeding Fido and Felix produces a pretty big carbon pawprint. 
 
 In a study released Wednesday, a geography professor at UCLA calculated that the meat-based food Americans’ dogs and cats eat – and the waste those pets produce – generate the equivalent of about 64 million tons of carbon dioxide a year. 
 
 That’s as much as about 13.6 million cars driving for a year, says professor Gregory Okin in a paper published in the journal PLOS One. 
 
 Put another way: Dogs and cats are responsible for 25 to 30 percent of the environmental impact of meat consumption in the United States. 
 
 Livestock emit methane, which is the source of about 10 percent of all greenhouse gases. Previous studies have found that Americans’ meat-heavy diet results in the release of 260 million tons of carbon dioxide – the leading greenhouse gas – through livestock production. Okin used similar estimates to calculate how much meat the nation’s 163 million cats and dogs eat and its carbon equivalent. 
 
 Okin said he isn’t suggesting Americans put their pets on a vegetarian diet, “which would be unhealthy.” However, Okin – who is allergic to dogs and cats but keeps two fish as pets – said he wants pet owners to consider the climate impacts of pet ownership, particularly as many Americans switch their dogs and cats to high-protein premium foods. 
 
 “This was not a study to make anybody feel guilty,” Okin said in an interview Wednesday. “I’m not saying people should go out and kill their animals or feed them something that isn’t appropriate....It was a study just to figure out how big these numbers are, and the numbers are surprisingly large.” 
 
 Frank Mitloehner, an animal science professor at UC Davis who specializes in livestock emissions, said the study shows that pets “clearly have an environmental footprint, and it is not a small one.” 
 
 But he said almost all of the meat pets consume in their food comes from rendered livestock byproducts. Those are the nasty bits that meat processors would otherwise toss because people won’t eat them. 
 
 “If this material wouldn’t go into pet food than what would be the alternative use?” he said. “It would most likely be wasted.” 
 
 Jennifer Fearing, a Sacramento lobbyist for the Humane Society of the United States, said the study shows that “everybody who’s eating meat is contributing to global warming.” 
 
 Fearing said her own dog, a shepherd mix, is doing well on a vegan diet, as did her previous dog, who died last year at age 15. 
 
 “He thrived on it,” she said. “He was a remarkably healthy dog.” She cited studies, including a 2009 report in the British Journal of Nutrition, showing dogs can do well on a meat-free diet. 
 
 However, many veterinarians warn against feeding dogs and cats a purely plant-based diet. Cats are especially prone to heart and eye problems if they don’t get enough of a particular amino acid called taurine found in meat. 
 
 A 2015 study by the Journal of the American Veterinary Medical Association tested 24 types of dry and canned vegetarian pet foods and found that most weren’t compliant with minimum labeling standards set by the pet-food industry. At least six of the foods tested were lacking in certain amino acids critical to pet health. ||||| Credit: CC0 Public Domain With many Americans choosing to eat less meat in recent years, often to help reduce the environmental effect of meat production, UCLA geography professor Gregory Okin began to wonder how much feeding pets contributes to issues like climate change. 
 
 All that meat has important consequences. Okin calculated that meat-eating by dogs and cats creates the equivalent of about 64 million tons of carbon dioxide a year, which has about the same climate impact as a year's worth of driving from 13.6 million cars. 
 
 "I like dogs and cats, and I'm definitely not recommending that people get rid of their pets or put them on a vegetarian diet, which would be unhealthy," Okin said. "But I do think we should consider all the impacts that pets have so we can have an honest conversation about them. Pets have many benefits, but also a huge environmental impact." 
 
 In a paper publishing Aug. 2 in the journal PLOS One, Okin says he found that cats and dogs are responsible for 25 to 30 percent of the environmental impact of meat consumption in the United States. If Americans' 163 million Fidos and Felixes comprised a separate country, their fluffy nation would rank fifth in global meat consumption, Okin calculated, behind only Russia, Brazil, the United States and China. And it all has to go somewhere—America's pets produce about 5.1 million tons of feces in a year, as much as 90 million Americans. If all that were thrown in the trash, it would rival the total trash production of Massachusetts—from the humans, at least. 
 
 Compared to a plant-based diet, meat requires more energy, land and water to produce, and has greater environmental consequences in terms of erosion, pesticides and waste, Okin noted. Previous studies have found that the American diet produces the equivalent of 260 million tons of carbon dioxide from livestock production. By calculating and comparing how much meat 163 million cats and dogs eat compared to 321 million Americans, Okin determined how many tons of greenhouse gases are tied to pet food. 
 
 His calculations start with publicly available information, like the number of dogs and cats in the country and the ingredients in market-leading pet foods, producing estimates that create a starting point for conversation. 
 
 He found that the nation's dogs and cats eat about 19 percent as many calories as the nation's people, on par with all the calories consumed by the population of France in a year. Because dog and cat food tends to have more meat than the average human diet, this means that dogs and cats consume about 25 percent of the total calories derived from animals in the United States. 
 
 Okin, a member of UCLA's Institute of the Environment and Sustainability, usually researches dust bowls, desert landscape dynamics and wind erosion, and how those things can impact individual ecosystems and the global climate. Pinning down the environmental impact of canine companions and feline friends was more of an—ahem—pet project that occurred to him while he was thinking about the growing trend of raising backyard chickens. 
 
 "I was thinking about how cool it is that chickens are vegetarian and make protein for us to eat, whereas many other pets eat a lot of protein from meat," he said. "And that got me thinking—how much meat do our pets eat?" 
 
 Okin recognizes that some of the products in pet food aren't something people should or would eat. But some of it is. In his research, he confirmed his hunch that premium pet foods usually contain more animal products than other brands, and that premium pet food purchases are increasing. As growing numbers of people consider pets less as animals and more as family members, Okin said, pampering has increased and the options for pet food with high-quality meat has kept pace. This means pets are increasingly eating cuts of meat suitable for humans. 
 
 "A dog doesn't need to eat steak," Okin said. "A dog can eat things a human sincerely can't. So what if we could turn some of that pet food into people chow?" 
 
 A commitment to snout-to-tail consumption, where as much rendered product as possible is produced for human use, could significantly reduce national meat consumption. Okin estimates that if even a quarter of the meat in pet food could be consumed by humans, it would equal the amount of meat consumed by 26 million Americans, nearly the population of Texas. Okin noted that ideas about what is edible vary dramatically by culture. He also pointed to a controversy in 2012 about "pink slime," also called lean finely textured beef. 
 
 "It's perfectly edible and completely safe, but it's unappetizing, so people don't want it in their food," Okin said. "But frankly, it's a good, inexpensive protein source." 
 
 As eating less meat expands from vegetarian to environmental circles as a way to reduce one's carbon footprint, considering what to feed pets is a natural next step, Okin said. It's not just an issue in the United States, he noted. In places like China, Brazil and other emerging countries, as the population becomes more affluent, they're eating more meat and they're getting more pets. 
 
 "I'm not a vegetarian, but eating meat does come at a cost," he continued. "Those of us in favor of eating or serving meat need to be able to have an informed conversation about our choices, and that includes the choices we make for our pets." 
 
 He doesn't see a simple solution. Pets provide friendship and other social, health and emotional benefits that can't be discounted, Okin said. People concerned about meat intake could consider vegetarian pets, like birds or hamsters, he suggested. The pet food industry, he noted, is also beginning to take steps toward sustainability, and could work to reduce overfeeding and consider alternative sources of protein. But it's a complicated issue, and where pets are concerned, Okin knows it's important to have a sense of humor about it. 
 
 "Maybe we could all have little ponies," he said, half-jokingly. "We'd all get more exercise taking them for walks, and they would also mow the lawn." 
 
 Explore further: Is a grain-free diet healthier for my dogs and cats? ||||| We all know that owning a dog or cat can cause problems such as chewed shoes, “accidents” on the carpet and a proliferation of shedding hair. But have you considered the impact pet ownership can have on the environment? 
 
 A UCLA study released Wednesday found that a key requirement of pet ownership — feeding them — creates about 64 million tons of carbon dioxide a year in the United States, or the equivalent of driving 13.6 million cars for a year. 
 
 The problem comes from the environmental impacts of meat production, and the fact that dogs and cats are responsible for 25 to 30 percent of those impacts, according to the study by UCLA geography professor Gregory Okin. Compared to a plant-based diet, meat production “requires more energy, land and water and has greater environmental consequences in terms of erosion, pesticides and waste,” the study found. 
 
 In terms of waste, Okin noted, feeding pets also leads to about 5.1 million tons of feces every year, roughly equivalent to the total trash production of Massachusetts. 
 
 “Given the significant environmental impact of meat production, the contributions of our omnivorous and carnivorous pets deserve special attention,” according to Okin’s study, published in the journal PLOS ONE. “The U.S. has the largest population of pet dogs and cats globally, with an estimated 77.8 million dogs and 85.6 million cats in 2015.” 
 
 While previous studies have examined the impact of pet ownership on carbon use, water quality, disease and wildlife, Okin’s study delved into its impact on total U.S. energy and meat consumption, and the environmental impact of that consumption. 
 
 “This analysis does not mean to imply that dog and cat ownership should be curtailed for environmental reasons, but neither should we view it as an unalloyed good,” Okin wrote in the study. “It is clear that a transition to pets that eat less meat, and therefore have less environmental impact, would reduce the overall U.S. consumption of meat.” 
 
 Okin’s report notes the social and emotional benefits of owning dogs and cats, insisting the study is intended to increase the awareness of the impact such pets have on the nation’s meat-production industry and its environmental effects. 
 
 “Additional research is needed to evaluate the animal content and human- edibility of ingredients in dog and cat food after processing, but the calculations presented here indicate that these pets comprise a significant proportion of U.S. energy and animal-derived product consumption, with the consequent environmental impacts, including greenhouse gas emission and feces production,” he wrote. 
 
 Okin noted that the pet-food industry has made advancements in manufacturing, product design and alternative protein sources, but more can be done. 
 
 `Simple measures like feeding domestic dogs and cats nutritionally appropriate amounts will certainly reduce their environmental and energetic impact, Okin wrote. “However, without large-scale reduction in their number and changes to the food system that drastically reduces the per-capita animal product consumption, the environmental and energetic impact of these animals will remain significant.”
– A UCLA professor isn't saying anyone who cares about the environment should get rid of their beloved pet dog and/or cat. But he's not not saying that either. It's widely accepted that eating meat is bad for the environment for a number of reasons, and it turns out our pets eat a whole lot of meat—in fact a country comprised only of the US' cats and dogs would place fifth in global meat consumption, according to a press release. Professor Gregory Okin was interested in the environmental impacts of our pets and—in a paper published Wednesday in PLOS One—calculates that American cats and dogs create about 64 million tons of carbon dioxide per year due to eating meat. That's equivalent to the yearly output of 13.6 million cars. "The numbers are surprisingly large," he tells the Sacramento Bee. All told, cats and dogs account for 25% to 30% of the environmental impact of meat consumption in the US. Okin says that's "huge." And yet, he's "not saying people should go out and kill their animals or feed them something that isn’t appropriate"—like an all-veggie diet. Okin does identify a number of possible ways to lessen the problem: people could transition to vegetarian pets, such as hamsters, birds, and chickens (which have the added benefit of providing protein for their owners); the pet food industry could get more sustainable and find alternate sources of protein; and, according to the Santa Monica Mirror, pet owners can make sure they're not over-feeding their dogs and cats. (These states have the fattest cats and dogs.)