Extensive literature suggests that current levels of animal production are unsustainable in the global food system, resulting in irresponsible natural resource use and substantial environmental harms (Alsaffar, 2016; Fresán and Sabaté, 2019; Jarmul et al., 2020). The EAT-Lancet Commission estimated that animal-derived food production, especially red meat, is responsible for a large proportion of GHGEs in the food system, including carbon dioxide, methane, and nitrous oxide (Willett et al., 2019). An estimated 88% of emissions in the life cycle of animal products are generated on-farm, with only 12% derived from food system activities after agricultural production (e.g., processing, refrigeration, transport, retail, waste). Feed crop production activities, including synthetic fertilizer production, irresponsible land and soil management, and non-renewable energy use, are major sources of these emissions (Woods et al., 2010; Vibart et al., 2021). On-farm emissions produced by activities during livestock rearing include enteric fermentation and manure management. Together, environmental livestock production alone accounts for 14–51% of total anthropogenic GHGEs (Soret et al., 2014). Differences in the impact of certain animal products, however, do exist. Lamb production, for example, emits an estimated 39 kg of carbon dioxide equivalents per kilogram of food (kgCO₂eq/kg) compared to lower levels for beef (27 kgCO₂eq/kg), cheese (14 kgCO₂eq/kg), and eggs (4.8 kgCO₂eq/kg). Plant-based foods generally produce less GHGEs across their life cycles, including rice (4.1 kgCO₂eq/kg), dried fruit (3.6 kgCO₂eq/kg), almonds (2.1 kgCO₂eq/kg), tofu (2.0 kgCO₂eq/kg), avocados (0.9 kgCO₂eq/kg), tomatoes (1.1 kgCO₂eq/kg), and peanuts (0.6 kgCO₂eq/kg) (Environmental Working Group, 2011; Mejia et al., 2018; Willett et al., 2019; Seltenrich, 2020).

Of note, relative differences in the total GHGEs of plant-based products are largely influenced by production, processing, and other post-farm-gate activities. For example, air-freighted fruits and vegetables significantly contribute to total energy use and CO₂ emissions in food supply chains (Saunders and Hayes, 2007). Certain rice cultivation strategies (e.g., continuous flooding, nitrogen fertilizer use) and low-yielding varieties also emit extensive quantities of methane (Jiang et al., 2017; Wang et al., 2021). The farming and processing of nuts, seeds, and dried fruit sometimes requires greater energy inputs (e.g., fuel for irrigation, dehulling, and drying), material (e.g., fertilizer) and water resources than equivalent volumes of animal-derived foods (Eshel et al., 2019; Seltenrich, 2020). Thus, a reduction in meat consumption by itself may not necessarily lead to improvements in sustainability. Replacement of animal foods in the diet must be thoroughly evaluated according to the nature of the substituted product. This approach encompasses public and private initiatives to shift food consumption toward a sustainable diet with foods consistently associated with low environmental impacts (Hyland et al., 2017).

Land used as cropland or pastureland reaches about half of the ice-free land area of Earth, contributing to habitat clearance and species extinction (Tilman and Clark, 2014). Animal foods have particularly high ecological footprints, including milk and dairy products, and animal fat (Rosi et al., 2017). Estimations suggest that 0.85 ha/person (8,500 m²/person) are required annually to support an average global diet, with animal products accounting for as high as 87% of land use. Meat and fish alone account for almost half (44%) of land deterioration, largely due to terrestrial feed crop production for large-scale livestock and aquaculture operations (Davis et al., 2016; Rosi et al., 2017; Froehlich et al., 2018).

The US, a major global agricultural producer, contains approximately 391.5 M acres (1.58 M kilometers) of cropland, about a fifth of its total landmass and only 20% of which is used to grow food intended for direct human consumption; up to 42%, in contrast, is used for livestock feed or exports (Merrill and Leatherby, 2018). The high-intensity use of land for monoculture crop production results in a rapid rate of soil erosion and degradation, with about 90% of cropland losing soil at a rate 13 times above sustainable levels. US pastureland is similarly losing soil, with approximately 60% being overgrazed and subject to accelerated erosion (Baden et al., 2019). Using land for staple crops and livestock feed also implies carbon stock loss, with forest conversion (a proxy of deforestation) accounting for an estimated 2.9 Gt CO₂ emissions (metric system) globally every year (Food Agriculture Organization of the United Nations, 2021).

Freshwater use refers to the consumption of surface water (e.g., lakes, rivers, streams, reservoirs) and groundwater (e.g., aquifers) (Springmann et al., 2018). US food production uses about 80% of the country's available freshwater supply, more than any other activity (Clark et al., 2019). The water required to produce various foods and forage crops ranges from 500 to 2,000 L of water per kilogram of food produced, with animal protein production requiring about 100 times more water than that of grain. Although livestock watering directly uses only 1.3% of total agricultural water use, the water required for forage and grain production dramatically increase water requirements (Pimentel and Pimentel, 2003).

In the US, critical water issues exacerbated by agricultural practices include the pollution of surface and groundwater sources, over-drafting of aquifers, waterlogging and salinization of soils, wetlands loss and runoff, evaporation, and leakage from irrigation systems (Marlow et al., 2009). Studies suggest that progressively replacing animal products with plant-based foods could increase freshwater use up to 16% due to water-demanding crops, such as legumes, vegetables, and fruits (Aleksandrowicz et al., 2016; Springmann et al., 2018). Specific foods, such as nuts, also result in higher scarcity-weighted water use compared to dairy, eggs, processed meat, and fish (Clark et al., 2019). Transitions to plant-based diets in the US will thus require careful consideration of freshwater availability as well as innovations to improve irrigation efficiencies and reduce water pollution in plant-based food production.

Policies to accelerate environmentally sustainable production of plant-based foods have emerged in recent years at the federal, state, and municipal level. Strategies including incentivizing production and consumption of sustainable plant foods, prohibiting excessive use of natural resources in production, and funding conservation agriculture technology are needed to address the environmental impacts of the current food system. No-till farming techniques, for example, promote carbon sequestration in the soil (Islam and Reeder, 2014), enriching the nutrient quality of harvests (Montgomery and Biklé, 2021) while preventing eutrophication through runoff (Kleinman et al., 2015). Investments in low-resource crops such as legumes promote water conservation and preserve farmland, as they occupy minimal arable land; as such, production of these crops, consumed directly as food for humans instead of animal feed, can be combined with decreases in livestock grazing land to limit land use changes and preserve natural environments. Many species' root systems also efficiently facilitate water retention in the soil, diminishing excess water use and preserving soil health (Rubiales and Mikic, 2015; Stagnari et al., 2017; Ferreira et al., 2021).

Much evidence indicates that solutions to reduce the environmental impact of food production and augment sustainability require multi-sector collaboration, and context-specific policy solutions are needed across food systems sectors to adequately address diet-related climate concerns. While this review contextualizes intervention strategies in the US, there is great need to align domestic food systems initiatives with global frameworks. The United Nations Sustainable Development Goals are a collection of 17 goals that address interrelated global challenges including poverty, inequality, climate change, and environmental degradation (United Nations Department of Economic Social Affairs, 2015). This urgent appeal has spurred nations into action around climate-smart agriculture and advancing plant-based, sustainable diets (Food Agriculture Organization of the United Nations, 2019; European Commission, 2020). This review discusses existing national, state, and municipal policies in the US around food systems and sustainability using the social-ecological framework (Figure 1). Public health professionals must consider the range of interdependent policies that impact our food system and work together to improve human and planetary health.

Several US federal programs, such as the Supplemental Nutrition Assistance Program (SNAP), SNAP-Ed (an accompanying educational program with cooking, grocery shopping, and nutritional literacy curricula), and Eat Smart, Live Strong (a nutrition education program for older adults) generally promote fruit and vegetable consumption to their more than 40 million participants (Committee on Examination of the Adequacy of Food Resources SNAP Allotments, 2013). However, these federal nutrition programs do not explicitly discuss benefits of sustainable plant foods, nor the environmental concerns that arise with current reliance on animal products. Further, despite mounting evidence of the link between diet and environmental sustainability (Willett et al., 2019), the Dietary Guidelines for Americans Advisory Committee excluded sustainability as a topic of interest in the 2020 report (U.S. Department of Agriculture, 2017). A 2021 executive order establishing a National Climate Task Force to “increase resilience to the impacts of climate change, protect public health, conserve our lands, waters, and biodiversity” failed to mention the negative impact animal agriculture has on the planet and the positive impact that diets rich in plants can have on the environment (The White House, 2021). Omitted discussion on the sustainability of foods are a missed opportunity to empower consumers with education about environmentally responsible food choices.

Given the nutrition community's recommendations of a greater emphasis on plant foods in our diets (Intergovernmental Panel on Climate Change, 2018), the intersection of nutrition and sustainability is exceedingly relevant information to include in population-level dietary guidelines and education. However, we cannot rely solely on reframing federal nutrition education to fix our food system, as these policy reforms alone may not inspire change that reflects the urgency of environmental and human health crises (National Academies of Sciences, 2019; Bowman, 2020). It has been suggested to the United States Department of Agriculture Farm Bill to increase investment in resource-conserving agriculture and to limit the expansion of large-scale livestock operations (Dewey et al., 2017; Miller, 2017). Bold messaging is needed to explicitly promote sustainable, plant-based diets and must be brought to the forefront in these federal food programs with a multifaceted agricultural and consumer educational approach.

While food systems reform on the federal level has moved at a slower pace (Muller et al., 2009), state and municipal governance typically have more agility to enact climate- and food-related policies.

The US state of Massachusetts, for example, offers the Healthy Incentives Program (HIP) which reimburses SNAP participants when they purchase fruits and vegetables from local retailers including farmers' markets, community-supported agriculture programs, farm stands, and mobile markets (Commonwealth of Massachusetts, 2021). Program evaluation for the Massachusetts HIP show that more than 165,000 individuals increased their fruit and vegetable intake by one serving per day in between 2017 and 2021 (Commonwealth of Massachusetts, 2021). Of note, this increase does not necessarily imply a direct reduction in animal food consumption; however, this policy encourages behavior change that decreases demand for resource-intensive agriculture practices while increasing demand of healthy foods from local producers, representing a strong investment in health and nutrition for low-income consumers and local producers. Furthermore, local retail operations that provide food for programs like HIP are better positioned than conventional agriculture to mitigate the effects of climate change as they motivate positive behavioral changes and, thus, reduce demand for intensive agricultural practices (Goh, 2011).

The US state of California has also demonstrated innovation in promoting environmentally responsible plant food production in rural agricultural settings. Assembly Bill 1289, entitled “Smart Climate Agriculture Program: Plant-Based Agriculture,” aims to support small and mid-sized farmers who wish to transition their lands from livestock farming to low-resource plant-based agriculture, excluding crop production for livestock feed. If passed, the bill will provide technical assistance and best practices for transitioning farmland to participating farmers (California Legislature, 2010). While this initiative does not explicitly promote the consumption of plant foods, it represents a substantial commitment to sustainable diets by incentivizing the production of low-resource plant foods instead of high-resource animal food farming.

The C40 Good Food Cities Declaration is a coalition of cities worldwide to increase planetary health diets in the next 20 years (C40 CITIES, 2019). Los Angeles, California, the only US city to adopt this climate leadership initiative, has promised to leverage public property for plant-based, urban agriculture by increasing the number of produce gardens in parks and public libraries by 50% by 2021 (C40 CITIES, 2019). While urban farming is limited in scale and is not the sole solution to ensuring availability of nutritious and environmentally-sound food, this demonstrates an innovative and promising commitment to sustainable plant food production.

In efforts to boost the number of nutritious, sustainable lunch options in public schools, Sacramento passed the California School Plant-Based Food and Beverage Program in 2019 that incentivizes plant-based food in schools and provides additional funding to schools serving a plant-based entrée and non-dairy milk (California Legislautre, 2019). Though schools across the country are interested in increasing plant-based options, districts face cost barriers as animal-based foods and cow milk are more heavily federally subsidized than plant-based options (Sewell, 2020). Funding these initiatives will entail large-scale government buy-in for schools to be able to afford the high costs.

Behavior intervention strategies promise to be effective tools to nudge consumers toward plant-based food choices. A study performed in three university cafeterias, for example, showed that doubling the proportion of vegetarian meals offered increased sales of between 41% and 79%, indicating that food availability and substitution may be an effective strategy for increasing consumption of plant-based meals (Garnett et al., 2019). Another study measuring food choices in a university cafeteria demonstrated that taste-focused labeling of vegetables in dining halls increased vegetable consumption by 39%, compared to health-centered labeling of the same dish (Turnwald et al., 2017), adding to a growing body of evidence indicating that menu design and labeling can influence consumer acceptance (Vadiveloo et al., 2017). This compelling research suggests that enhancing the variety and availability of vegetarian dishes and using messaging that emphasizes indulgent flavor and culinary technique may be effective behavioral nudging strategies to increase consumption of plant-based foods in school or food establishment settings.

While these outcomes encourage choice architecture innovation to nudge consumers toward healthier choices (Ensaff et al., 2015; Mikic, 2020), behavioral interventions have limitations. For instance, campaigns promoting environmentally responsible consumption, labeling systems addressing ecological footprint, and other interventions targeting customers' food conduct are generally less effective than fiscal tools, advertising bans, and other restrictive market regulations (National Academies of Sciences, 2019). Focusing on individual-centered interventions rather than population-scale measurements has questioned the social-ecological validity of public strategies based on behavioral theory. The remaining gaps in the chain of evidence from small-scale controlled efficacy trials to population frameworks should be addressed to interpret behavioral interventions' effectiveness and generalizability (Hagger and Weed, 2019).