People within the United States (U.S.) have been making dietary choices based on health claims and ethical considerations for generations. It is becoming increasingly apparent that the impact of one’s diet does not simply end at their health, as there are broader, planetary impacts of food choices. One of these planetary impacts is measured by carbon footprints. The carbon footprint of an item is generally defined as “the amount of greenhouse gas and, specifically, carbon dioxide emitted by something (such as a person’s activities or a product’s manufacture and transport) during a given period” [1]. However, a generally acceptable and consistent academic definition has yet to be agreed upon [2]. In terms of food, carbon footprints depend on the source and composition of each food item. Each stage in the supply chain contributes to differential emissions, ranging from land-use changes, farms, and animal feed, to transportation, retail, and packaging. These emissions add up quickly; in the U.S., food is responsible for 26% of total greenhouse gas emissions [3]. This statistic is not widely known by Americans; but, if it were, it may affect their decision-making on what they consume [4]. The EAT-Lancet Commission produced guidelines for following a flexitarian diet that benefits human health and the environment. Healthy diets were described to have “an appropriate caloric intake, a diversity of plant foods, low amounts of animal source foods, contain unsaturated rather than saturated fats, and limited amounts of refined grains, highly processed foods and added sugars.” With this dietary framework, food systems can potentially provide this flexitarian diet “for an anticipated world population of nearly 10 billion people by 2050 and still stay within a safe operating space on Earth. However, even small increases in red meat or dairy foods would make this difficult or impossible” [5].

Carbon-footprint information can not only assist consumers but also food brands that already have climate in mind during production. Although processed foods often have higher footprints than their whole-food counterparts, they do not always have to. Some food producers are adjusting their farming and production processes with climate in mind. Several food producers have focused their production on foods that reduce their carbon footprints, including Airly Foods ^[6][7], Moonshot Snacks ^[7][8], Quinn Snacks ^[8][9], Alter Eco ^[9][10], and Patagonia Provisions ^[10][11]. For example, a box of Airly’s Oat Clouds cheddar crackers removes 21 g of CO₂ from the air ^[6][7]. Simple food choices to replace traditional crackers with climate-friendly snacks, like this, can help reduce the carbon footprint. These foods that are developed with climate change in mind illustrate a growing trend in the market appeal of sustainably produced food and food commodities.

The Standard American diet, more technically described as the “Western Diet” in academic and scientific literature, is less of an intentional meal plan and is commonly focused on food for sustenance instead of the prioritization of health ^[11][26]. The Standard American diet is included in this paper as a control or basis from which to compare the other diets. The Western Diet rose to prominence as a meal plan (and has been continually expanding in reach) beginning in the 1990s. Exponential economic growth and the globalization of western food supply chains had led to the expansion of modern industrial manufacturing in the food industry, giving way to a new standard for access and distribution ^[12][27]. Many of these foods included synthetic sweeteners, engineered oils, and syrups that would come to dominate the newest foods in the market, becoming staples in what is now coined as the diet of the Standard American. Few would state that they adhere to the “Standard American diet,” so one must approximate the number of followers. The Center for Disease Control and Prevention (CDC) reported that “in 2015–2018, 17.1% of U.S. adults aged 20 and over were on a special diet on a given day” ^[13][28].

The eating habits of many Americans are often described as lacking in adequate and substantial portions of fruits, vegetables, and whole grains coupled with an excess of processed foods and a heavy emphasis on salt, sugar, and saturated fats ^[13][28]. Thus, the diet is inherently deficient in many of the valuable micronutrients, macronutrients, vitamins, and minerals necessary for optimal health. Furthermore, western food practices often seek “highly palatable foods that can trigger eating addictive-like behaviors” (which are often satisfied by highly processed or pre-packaged food items) and result in many preventable diseases such as “atherosclerosis, type II diabetes, hypertension, and, [most commonly], obesity” ^[14][15][30,31]. The well-established and increasingly prominent obesity epidemic within the U.S.—in 2018 the obesity prevalence was found to be 41.9% in the U.S. population of adults—is proof enough of the damage that can be caused by a calorically rich yet nutrient-poor dietary regimen ^[16][32]. The Western Diet is not typical in its intention as a deliberate and actual diet in the traditional meaning of the word. Thus, it is not surprising that there are no apparent health benefits for those that subscribe to this particular style of eating. In fact, the elevated intake of highly processed food items combined with low quantities of whole foods only seems to coincide with or result in a multitude of adverse health effects such as heart disease, metabolic and gastrointestinal disturbances, neurodegenerative diseases, diabetes, and infertility, to name but a few ^[17][33]. Furthermore, because the Western Diet is widely considered to be notoriously unhealthy, those that follow the American regimen are less likely to engage in daily physical activity and are more likely to engage in the consumption or recreational use of alcohol and drugs, which lead to diseases such as coronary heart disease, diabetes, obesity, and depression ^[18][19][34,35].

The meal-plan scenarios outlined for the purpose were chosen to optimize health, and thus will not include as many processed foods as may be considered typical for the average American. In order to combat the epidemic of diet-induced illness, the U.S. Department of Agriculture developed MyPlate^® to educate young children and adults on the five food groups that make up the ideal food pyramid ^[20][18]. Government-sponsored programs such as those that aim to demonstrate the importance of building balanced proportioned meals are filled with each of the five food categories (fruits, vegetables, grains, protein, and dairy) and oils that are included on a plate ^[21][36].

In regards to the environmental footprint of the Standard American diet, it is considered to be quite high as a result of its emphasis on the daily consumption of meat and dairy, two food items that have the highest greenhouse gas emissions; its land-use activity; and its acknowledged environmental impact(s) in each of the industries’ respective production processes ^[22][37]. In addition, the demand for highly processed foods is itself an energy-intensive manufacturing process, requiring an increased energy input (in the form of fossil fuels) to meet said demand. The intake of pre-packaged and processed foods also produces excessive plastic pollution. To serve as a comparison for the other considered diets, nutrient requirements for typical meals following MyPlate were used to represent the Standard American diet with an understanding that the lack of processed foods may represent an underestimation of the environmental impact from an ‘average’ American diet.

The Mediterranean diet first gained international popularity in the 1980s as the medical community began to note the long life expectancies and low risks of cardiovascular disease in people who resided in Mediterranean Basin regions, such as Crete and Southern Italy ^[23][38]. In the U.S., an estimated 5% of the population currently follows the Mediterranean diet [24].

Less of a diet and more of a lifestyle, the Mediterranean diet has no hard and fast rules or restrictions but rather emphasizes a focus on whole grains, fruits and vegetables, nuts, seeds, legumes, fish, and olive oil. It does include meat, dairy, egg products, and even red wine in moderation; however, consumption of these foods tends to be lower than in the Standard American diet ^[25][26][39,40]. The traditional Mediterranean diet tends to avoid heavily processed foods as well as desserts.

The main difference that sets the Mediterranean diet apart from others is its focus on monounsaturated “good fats,” namely nuts and olive oil, that lower low-density lipoprotein cholesterol—the “bad” cholesterol ^[27][41]. Studies have shown that consumption of monounsaturated fatty acids (MUFAs) is associated with many health benefits, including a reduction in the risk of cardiovascular disease, stroke, diabetes, and cancer and an improvement in mental health ^[28][29][19,42]. The results of randomized trials found that those in the Mediterranean diet intervention had lower risks of heart attack, stroke, and death from cardiovascular disease than the control group ^[22][30][37,43]. In addition, the Mediterranean dietary intervention group had a lower overall mortality rate than other comparison groups and a lower overall cancer mortality rate in a recent meta-analysis of the Mediterranean diet and cancer ^[31][44]. The lack of evidence for any negative health effects from this diet ^[32][45], combined with the extensive research on its health benefits, has promoted the general public to recognize the Mediterranean diet as one of the healthiest diets to follow.

The Mediterranean diet also has a comparatively favorable impact on the environment. The carbon footprint of the standardized Mediterranean diet was lower than that of the Standard American, keto, and paleo diets. These three diets include high intakes of red meat and dairy products known to elicit high greenhouse gas emissions and cause excessive land use by the beef and dairy industries. Low consumption of processed foods in the Mediterranean diet also decreases plastic consumption and emissions associated with the processing, packaging, and transportation of these products ^[33][46]. The guidelines of the Mediterranean diet allow it to be adjusted with sustainability in mind ^[34][47]. One can easily adopt a more climatarian approach by following the Mediterranean diet and focusing on consuming seasonal, locally sourced, and sustainably harvested and processed ingredients to further minimize emissions.

The term “veganism” was first coined in 1944 by Donald Watson and Dorothy Morgan. Veganism has evolved from a strict form of vegetarianism, a diet that was first observed in ancient India in conjunction with the Jain principle of ahimsa, or nonviolence ^[35][48]. Today, an estimated 5% of the U.S. population is considered to be plant-based, with around 16 million adults identifying themselves as vegan or plant-based consumers [24]. Veganism has become increasingly popular and has experienced immense growth since the 2010s, which can be observed in the surge in the number of plant-based food options to choose from at restaurants and grocery store chains and of product brands specifically marketed as “vegan” to the everyday consumer. Such advancement has been announced through a rapid market expansion, as “the retail market for plant-based foods is worth $7.4 billion, up from $6.9 billion in 2020”, according to the Good Food Institute ^[36][49].

Strict plant-based diets, such as vegetarianism and veganism, are often defined by which categories of food are excluded rather than included. Both diets share the elimination of meat, whereas vegetarianism still allows for the consumption of animal products such as eggs and dairy. A true vegan avoids the use of any animal products in their diet, including meat, dairy, eggs, and animal-derived materials or ingredients (i.e., honey, rennet, and gelatin). Because of broad restrictions in the major food categories, there is an emphasis on the consumption of fruits, vegetables, grains, legumes, nuts, and seeds. Thus, veganism is typically low in fat and protein but high in carbohydrates.

Nutritionally, “vegan diets are rich in fiber, magnesium, Fe3+, folic acid, vitamins C and E, polyunsaturated fatty acids (PUFA), carotenoids, flavonoids, other phytochemicals and antioxidants” that are often needed for the prevention of several chronic diseases ^[37][50]. Vegans that are able to eat balanced diets that fulfill their body’s essential nutritional requirements have “been observed [to have] had lower risks of obesity, hypertension, cardiovascular diseases, diabetes, arthritis, cancers (especially colon and prostate cancer) and fatal ischemic heart disease, thanks to protective substances found in fruits, vegetables, legumes, seaweed, seeds, whole grains, vegetable oils, and other plant-based foods” ^[38][51]. However, vegans are also prone to deficiencies in omega-3 fatty acids, iron, calcium, vitamin B₁₂, vitamin B₂ and B₃, zinc, vitamin D, and iodine, which can become detrimental toward the maintenance of strong immune-system support when combined with a low intake of protein ^[32][37][45,50]. It is also worth mentioning that veganism has been linked to low energy levels, weight fluctuations, hormone disruption, and an increased risk of depression if a well-rounded and balanced diet is not followed ^[39][52].

There is still much academic and scientific debate about whether veganism is actually considered to be healthier than a typical omnivorous diet. A lifestyle in which one consumes low amounts of meat has been linked to an increase in life expectancy due to a reduction in the risk of developing cardiovascular disease, but the associated low protein content as well as vitamin and mineral deficiencies in the vegan diet can weaken the immune system ^[32][39][45,52]. Conflicting studies have yet to agree upon the nutritional soundness of veganism, but many have suggested that a well-balanced and fortified vegan meal plan can overcome many of these perceived health barriers to make the diet sustainable in the long term.

As for the environmental considerations of maintaining a vegan diet and lifestyle, an increased intake of fresh fruits and vegetables allows for a considerably lower carbon footprint compared with that of many other dietary practices. A vegan diet with low amounts of the highly manufactured and processed foods that are typical of the Standard American diet and supplemented with those that follow the likes of keto and paleo will have a lower environmental impact. However, as veganism has grown in popularity in recent years, there has been an observable increase in the number of vegan or plant-based meat alternatives that may distract from the environmental and sustainability benefits of veganism ^[38][51]. Because the alternative-meat industry continues to rapidly expand as a result of higher demand, the carbon footprint of a vegan diet may begin to trend toward that of the Standard American diet as more vegans consume higher amounts of processed plant-based meat-substitution products.

The paleo diet (also known as the Paleolithic diet or the Old Stone Age diet) is a style of eating that was first suggested by gastroenterologist Walter Voegtlin in the early 1970s ^[40][53] but was catapulted to mainstream popularity with the publication of Loren Cordain’s The Paleo Diet Revised: Lose Weight and Get Healthy by Eating the Foods You Were Designed to Eat in 2001. Today, around three percent of Americans follow this dietary practice [24].

Paleo emerged as a nutritional regimen dedicated to the re-creation of the early human diet of the Paleolithic Age (approximately 2.5 million years ago) before the influence of modern agriculture and the widespread availability of food, and even longer before complex manufacturing of processed foods became common practice. The paleo diet places a heavy emphasis on the that the human body is slow to evolve and is, therefore, gastrointestinally incompatible with the majority of foods one can buy in a grocery store today. Thus, paleo aims to limit one’s food intake to only foods that could be obtained via hunting and gathering. This means that those on paleo rely on lean meats, seeds, eggs, fruits, and vegetables as their dietary staples. As a result, the paleo diet is generally low in carbohydrates but high in protein, cholesterol, and fats. One of the main tenets of the paleo diet is the emphasis on the avoidance of processed foods, refined/artificial sugars, salt, grains, legumes, and most dairy products ^[41][54].

While scientific and academic research concerning the nutritional and health benefits of the paleo diet is still ongoing, it appears that the adoption of a Paleolithic style of eating has positive results in reducing cardiovascular disease risk factors and inducing weight loss in overweight subjects ^[42][55]. The diet also has encouraging effects, such as decreasing levels of systolic blood pressure, diastolic blood pressure, total cholesterol, and low-density lipoprotein cholesterol while increasing concentrations of high-density lipoprotein cholesterol ^[43][56]. In addition to lessening the effects of cardiovascular disease risk factors, “previous studies reported positive effects of a [paleo diet] on energy intake, body composition, [and] insulin sensitivity” ^[44][22]. Beyond the perceived health benefits, paleo has been recognized to be high in fiber, carotenes, and vitamins C and E as a result of its emphasis on the consumption of fruits and vegetables. Furthermore, the low sodium content of paleo, due to its elimination of processed foods, has often led to a decrease in blood pressure, which is another stressor in the development of cardiovascular disease ^[43][56]. However, the limitation of salt may lead to a deficiency in iodine, and the “limitation of grains and dairy [may lead to a deficiency in] vitamin D, calcium, thiamin, riboflavin, and iron” ^[45][57]. It has also been suggested that those who adopt a diet similar to that of hunter-gatherer ancestors may experience discrete metabolic and physiological decomposition related to the improvement of lipid profiles and glucose sensitivity without inducing unhealthy weight loss ^[46][58]. Thus, it can be expected that the philosophy of the paleo diet may lead to several positive health and nutritional effects that may mask other deleterious ones.

The diet’s emphasis on lean meats and avoidance of highly processed food items may indicate a relatively moderate carbon footprint, as compared to that of the Standard American diet. Many substitutions can be made with regard to one’s selection of meats to make the paleo diet more environmentally friendly, such as lean beef, fish, or chicken as opposed to other beef and pork products that are associated with higher carbon footprints.

The ketogenic diet, also known as keto, was originally coined in the early 20th century as a way to prevent seizures in those with epilepsy ^[47][59]. It has since been used to treat various conditions, such as brain injuries and Alzheimer’s disease ^[48][60]. The keto diet is gaining in popularity in the U.S., with about 5% of the population reporting to be on the high-fat diet [24].

Keto is characterized by the consumption of foods that are rich in fat and protein accompanied by a significant limitation of carbohydrates, such as sugars and grains. The goal is for the body to reach a state of ketosis, in which it uses fat stores instead of carbohydrates for fuel ^[49][61]. As fruits and vegetables tend to be rich in carbohydrates, they are limited to select varieties such as berries, leafy greens, broccoli, mushrooms, garlic, squash, etc.

According to the literature, both benefits and risks to health are evident in those who adopt a keto diet. For the average person, who does not suffer from the conditions listed above, the keto diet may assist in weight loss, reducing sugar intake, improving cardiovascular-related lipid markers, microbiome healing, improving epigenetic markers, and reversing diabetes ^[48][50][60,62]. As the diet does not focus on caloric restriction, it has helped obese patients who had previously struggled with other diets to lose weight. However, there are also risks associated with the keto diet. There may be an excessive load on the liver and kidneys in order to process all the fat and protein from the diet, constipation due to the lack of fiber, fuzzy thinking, mood swings, an increase in LDL “bad” cholesterol, and nutritional deficiencies due to the limited range of fruits and vegetables ^[48][60]. The diet also does not restrict the consumption of the unhealthy saturated fats often found in foods such as bacon and butter. Although many individuals have achieved successful weight loss with this diet, the results vary on an individual basis and the sustainability of the diet is questionable along with its potential benefits ^[51][52][63,64].

The lack of restrictions on high-fat animal products and red meat indicates a high environmental impact of the keto diet. Although it is possible to alter the diet to be more climate-friendly by focusing more on white meat and plant-based foods that are high in fat, keto would inevitably make a climate-friendly diet, which is already restrictive, more difficult to maintain.

The climatarian diet is a relatively new development that has a focus on eating foods that minimize one’s impact on the environment. Formulated to be flexible, there are no forbidden foods; rather, the diet emphasizes making informed decisions about the food one eats and choosing low-impact options whenever possible ^[53][6]. There is no data on the number of followers of the climatarian diet within the U.S., because the new diet has yet to become as popular as the other diets discussed. However, a Food and Health Survey by the International Food Information Council [24] found that climate-friendly diets are followed by a combined 13–15% of the population (flexitarian, plant-based, vegetarian, vegan, and pescatarian). The diet also has the potential to expand as the population becomes more conscious of the environmental impacts associated with individual behaviors such as food consumption.

The climatarian diet encourages the consumption of local, seasonal, and fresh food that requires minimal transportation, refrigeration, and processing. It also emphasizes an overall reduction in meat consumption, with a significant avoidance of ruminant meats (lamb, goat, and beef) and a moderate intake of pork, poultry, and sustainable fish. The diet is relatively flexible otherwise. It is noted that pescatarian, vegetarian, and vegan diets are all climate friendly as well, which is mainly due to the lack of consumption of red meat ^[53][6].

There is no research on the health impacts of the climatarian diet, as it is new and has yet to become popular. However, much evidence exists for an association between the reduction in red and processed meat consumption and many health benefits including reduced risks of cancer, heart disease, and stroke ^[54][65]. The consumption of high-quality plant proteins such as tofu, lentils, nuts, seeds, and vegetables can also lower the risks of cancer, heart disease, and stroke ^[55][66]. Conversely, processed foods are associated with an increase in the risk of noncommunicable disease ^[56][67]. Because the climatarian diet relies on minimally processed, locally grown foods, it may indirectly provide extra health benefits. The potential health risks of the diet have not been investigated.

The climatarian diet provides guidelines on how to eat popular foods in a more environmentally conscious manner. With a lack of restrictions on the food itself and a higher focus on its source, production, and transportation, the climatarian diet encourages its followers to focus on plant-based, whole foods while still enjoying non-red meat and animal products that are local, more sustainably sourced, and require less environmental resources. This flexibility makes the diet more accessible to some, while the high costs and inconveniences of buying local and sustainable food may render it less accessible to others.

Table 1 contains the summarized carbon footprint values of the food items in each meal for the six comparison diets. For the diets that customarily include ruminant meat (Standard American, paleo, and keto), researchers created higher and lower carbon-footprint ranges. Ruminant meats represent the highest carbon-footprint values, and any diets that include ruminant meats will automatically have higher values even though the rest of the diet may be more carbon friendly. With higher and lower footprint ranges, researchers were able to describe diets with ruminant meats (higher footprint) and without ruminant meats (lower footprint). The climatarian diet was also split into vegetarian and meat-eating variations to investigate the difference between its lower- and higher-impact variations.

After curating the daily meal scenarios for each of the diet types identified above (Table 1), the carbon footprint of each food item was determined using a combination of the SEL and Song et al. databases ^[57][58][13,14]. Looking at the meal scenarios that have the lowest environmental impacts, it was calculated that the vegan diet scenario held a footprint of 1.63 kg CO₂-eq, the Mediterranean diet held one of 2.17 kg CO₂-eq, and the climatarian diet ranged from 1.88 to 2.54 kg CO₂-eq. Looking at the diets that are more dependent on ruminant meats and considered to be less environmentally sustainable, the Standard American’s footprint ranged from 2.63 to 8.14 kg CO₂-eq, paleo’s ranged from 3.11 to 5.91 kg CO₂-eq, and keto’s ranged from 4.85 to 9.72 kg CO₂-eq. All of the diets’ footprints were dependent on the inclusion or exclusion of meat, particularly ruminant meat.