Environmental sustainability is one of the most pressing new areas of research on food today. Food production and consumption face unprecedented scrutiny as their impact on the natural environment and human health becomes more evident. How foods are grown (including water use), processed, sold, prepared, cooked, consumed, and disposed of is crucial. In studying nuts and environmental sustainability, all these stages of production and consumption (the ‘food system’) must be considered.

The global food system produces enough calories for a growing world population. However, about half of the population is malnourished with almost 1 billion people not consuming enough food and experiencing hunger, and almost 2 billion overconsuming foods low in nutritional quality associated with micronutrient and phytonutrient deficiencies, obesity, and increased incidence of chronic disease. Globally, the non-optimal intake of foods in the diet is estimated to account for approximately 22% of all deaths among adults and 15% of disability-adjusted life-years ^[1]. At the same time, agriculture uses ~70% of the global freshwater ^[2]. The food supply chain is responsible for ~26% of global greenhouse gas emissions (GHGe), occupies ~43% of habitable land, causes ~78% of the ocean and freshwater eutrophication, and ~32% of terrestrial acidification worldwide ^[3].

Several strategies have been proposed to decrease the environmental pressures exerted by the food systems. These include (1) improving agricultural technologies to enhance productivity and reduce harmful emissions; (2) reducing food loss and waste to decrease food production requirements and waste emissions in landfills; and (3) shifting to the production and consumption of foods that support human and planetary health ^[4][5][6][7]. Thus, the type and amount of food produced and consumed are major determining factors in promoting human health within planetary boundaries ^[8][9]. Research can identify where nuts fit within these parameters. Addressing the environmental impacts of the lifecycle of nuts (LCA, Lifecycle Analysis), from production to consumption, is one way to approach this.

According to the Food and Agriculture Organization (FAO), environmentally sustainable diets are “those diets with low environmental impacts which contribute to food and nutrition security and to healthy life for present and future generations (…) while optimizing natural and human resources ^[10].” Scholars have previously identified four determinants for, or dimensions of, a sustainable diet from the consumer’s perspective. These dimensions are based on the ratios of dietary characteristics. They are (1) the proportion of foods in the diet of animal versus plant origin, (2) the proportion of processed versus whole foods, (3) the proportion of seasonal/locally sourced foods versus out-of-season/context, and (4) the proportion of foods consumed versus wasted ^[9] (Figure 1).

Most diets (and most meals) have a mix of foods, each having different characteristics. The inclusion of nuts in the diet is variable. The larger the amount and proportion of foods in a meal or diet whose constituent foods are animal-sourced, processed, out of season or context (requiring transportation or refrigeration for storage), and wasted, the less sustainable the diet is. Reciprocally, the higher the proportion of foods of plant origin consumed, minimally processed, in season, and locally sourced, the more sustainable the diet is. Thus, nuts would appear to have a place in sustainable diets.

2. Nuts and Environmental Sustainability

2.1. Nuts as Sustainable Foods

At the consumption level, nuts are plant foods consumed whole or very minimally processed; they have a “long season” (since they do not require refrigeration for storage, can be transported and stored with minimal energy use), and have little waste. Thus, nuts appear to be sustainable foods. At the production level, however, concerns have been raised regarding the high water usage and chemical inputs in the production of nuts under intensive agricultural practices. Importantly, this is not the case when nuts are grown in extensive or traditional agricultural practices.

2.1.1. Life Cycle Analysis

There is a relative paucity of published data from Lifecycle Analysis (LCA) on nuts. Environmental impacts of foods can be measured using various units, including, per weight (of edible amount), per serving, or, depending on the nutritional contribution of each food, per grams of protein or energy (kcal) ^[11][12][13]. This can be problematic for some foods such as nuts. For example, many food comparisons regarding environmental impacts have been based on edible amounts (by weight), giving nuts mixed results. A commonly recommended nut serving size is small (approximately 30 g) compared to other foods (often 50–120 g or even 240 g for most beverages). However, nuts are also energy-dense foods (less than 5% water) whose main nutrients are fat and protein. As a result, the environmental impacts of certain nuts measured per grams of protein are very low compared to other animal-sourced protein-rich foods. ThWe scholars conducted a LCA of five common food sources of protein: legumes, nuts, eggs, poultry, and red meat—specifically, kidney beans (Phaseolus sp.), almonds (Prunus dulcis), eggs, chicken, and beef as produced in Californian agricultural practices ^[14]. When using beans as the reference (legumes are nature’s most efficient production of protein), almond protein is the second best ranked after beans for most environmental parameters except for pesticides (Figure 2). Clark and colleagues ^[15] analyzed LCA data from meta-analyses to determine the impact of fifteen foods on environmental depreciation, encompassing five components: plausible acidification, eutrophication, GHGe, land use, and scarcity-weighted water use. Of the fifteen analyzed foods, red meat (100 g), chicken (100 g), eggs (50 g), legumes (50 g dried weight; DW), and nuts (28 g) represent protein sources ^[16]. Each food was depicted in a radar plot, illustrating the rank-ordered impingement on designated environmental parameters per daily food serving. When comparing nuts, eggs, and red meat, nuts performed relatively well on all environmental parameters except water use. The environmental impact per serving of eggs per day serves as an intermediate. Red meat received the highest or most detrimental rank in all five environmental parameters, thus corresponding with previous research ^[14]. The foods were rank-ordered from least to most environmentally impactful per serving produced ^[15]. Nuts ranked lowest (least harmful) for GHGe among all fifteen foods. Among the five protein food sources, nuts ranked lowest in eutrophication potential and second lowest in acidification and land use. However, in conjunction with previous scholarly ^[17] and media ^[18][19] critiques on nut production and water use, nuts ranked second highest in scarcity-weighted water use, only exceeded by red meat. These findings imply that the environmental impact of a 28 g serving of nuts is less aggravating than that of a 100 g serving of red meat, although water use is of concern.

2.1.2. Water Footprint

The situation for nuts requires further examination of the term ‘water use’. This complex issue may be better addressed by considering distinct ‘water footprints’ referring to ‘blue water’ (surface plus groundwater, often used in irrigation ^[20]) and ‘green water’ (rainwater consumed with agricultural production ^[17]). Mekonnen and Hoekstra calculated the water footprints of nuts ^[17] and farm animals ^[21] from 1996 to 2005. Vanham and colleagues ^[22] applied the water footprint computations to demonstrate the blue and green water footprint of nuts and animal proteins (beef, eggs, chicken, pig meat, and sheep meat) in liters per kilogram and liters per gram of protein. In the context of liters per kilogram, shelled cashew (Anacardium occidentale) nuts have the most prominent combined blue and green water footprint among all nuts and triple that of beef. The water footprints of almonds and pistachios are also considerable, exceeding 10,000 L per kilogram. Among the selected animal proteins, eggs have the lowest water footprint. When comparing the water footprint by food weight, the sustainability of nuts appears unfavorable; however, as previously mentioned, protein is a sizable component of nuts’ nutrients, suggesting that water footprint expressed per gram of protein may be a more accurate representation for this environmental impact. Further clarification on the position of different types of nuts is seen through the water footprint articulated in liters per gram of protein. Accordingly, the water footprint of cashews remains higher than that of beef, whereas that of peanuts is lower than all five animal proteins ^[22]. Moreover, the average green and blue water footprint of almonds, hazelnuts (Corylus sp.), pistachios (Pistacia vera), and walnuts (Juglans sp.), combined, remain lower than beef’s water footprint when based on liters per gram of protein. Comparing the environmental impact of foods based on protein content rather than weight may be valuable from a nutritional perspective, as eating patterns focus on nutrient levels rather than weight. Researchers must also address the location of nut production, as agricultural production in water-scarce areas may have a greater impact on water footprint than locations with greater water availability. Indeed, water stress, defined as the ratio of water used to available water ^[23], varies according to region. For example, in California, the powerhouse of almond production ^[24], high water stress ^[22] and water demand ^[25] are evident even though the carbon footprint is relatively low ^[26]. Agricultural practices, including intensive versus extensive agriculture ^[27], require further consideration because these determine the magnitude of resource utilization.

2.2. Future Research on Nuts and Sustainability

First and foremost, future research should prioritize collecting data on nut production and sustainability using environmental parameters beyond water and GHGe. Although data regarding acidification and eutrophication potency are available ^[15], they are limited, and this confines the current comprehension of the environmental impact of producing nuts. Furthermore, collecting and comparing data on the environmental impact of nut production according to agricultural methods (intensive versus extensive), climate setting, and location may clarify whether previous critiques of nut production and sustainability are consistently reasonable across varied agricultural conditions. Certainly, an increase in the production and consumption of nuts, as recommended by the EAT–Lancet Commission ^[28], a global initiative on food and planetary health, beckons the question of whether eating more nuts is more sustainable than “healthier” diets. If “healthier” diets were to incorporate conventional meat analogs, it would be necessary to compare their sustainability with that of nuts. This would require quantifying the LCA of nuts versus conventional meat analogs in isocaloric and isoprotein conditions. Additional modeling or use of diet records can be useful in describing the environmental impact of nuts in previously defined dietary patterns. The future of nuts in the diet may even involve replacing well-known meat analogs, including texturized vegetable protein (TVP) ^[29], with nut-based meat analogs. Notably, this implies assessing and comparing the sustainability of nut-based meat analogs with current TVPs using the parameters discussed above. In addition, nuts are an excellent source of fat, almost entirely unsaturated. They contain mainly monounsaturated fatty acids (MUFAs) and some polyunsaturated fatty acids (PUFAs), mostly n-6 PUFAs, while walnuts are a good source of vegetable n-3 PUFAs ^[30]. Given the importance of these essential and healthy fats, relevant research could include the computation of LCA environmental inputs relative to the different food sources of unsaturated fats, including n-3 PUFAs. Unsustainable and unhealthy foods harm both planetary health and human well-being. Further research is required demonstrating and quantifying the environmental sustainability of nuts. Specifically, more needs to be known about the efficient use of natural resources and environmental protection in the production, preparation, and disposal of nuts. Consumption needs to be addressed in terms of nuts as a single food, as an alternative to other foods, and in the context of healthy dietary patterns.