Changes in water use due to dietary transitions
Our study investigated the impact of different dietary scenarios on global water use from 2020 to 2100, including the baseline scenario (BaU), the EAT-Lancet diet (EAT), dietary guidelines for Americans (HUS), the Mediterranean (MED) and the vegetarian (VEG). In the BaU scenario, water use was projected to increase from 4448.13 km3 year− 1 in 2020 to a peak of 5203.03 km3 year− 1 in 2070 before declining to 4741.11 km3 year− 1 by 2100 (Fig. 1a). Water use in 127 countries by 2100 exceeded its 2020 level (Fig. 1b), with notable increases in Argentina, Brazil, Chad, Mexico, and Uruguay (Figure S3). The region with the largest projected increase in water use was Argentina (137.33 km3 year− 1). Conversely, with fewer changes in irrigated areas and constant water use efficiency, 51 countries were projected to use less water because of declining populations later in the century (Figure S4). India was expected to experience the largest reduction, with a decrease of 520.34 km3 year− 1. Under four dietary transition scenarios, reducing animal-based food demand led to a projected decrease in global water use to a range of 3893.06-4505.38 km3 year− 1 by 2100, in which VEG and EAT biased towards plant-based diets can significantly reduce water use, while MED and HUS diets were relatively less effective in reducing water use. Moreover, we observed a period of upward trends in global water use (except for the VEG scenario) due to higher food consumption. Among them, the HUS and MED diets were the most significant, and the water use peaks would be reached in 2060 (5072.13 km3 year− 1) and 2065 (4941.87 km3 year− 1), respectively (Fig. 1a).
Some African and Asian countries experienced water use challenges due to higher food consumption during the early stages of the transition (Fig. 1c-n). For example, in the EAT scenario, reducing water-intensive foods lowered water use for sugar, animal products, and feed. However, many African and Asian countries initially faced water use challenges due to higher food consumption. By 2030, 52 countries exceeded the BaU scenario in water use. Vietnam was projected to have the largest increase (39.40 km3 year− 1) due to increased cereal and fruit consumption. While water use gradually decreased with the dietary transition, it remained higher than the BaU scenario. Côte d'Ivoire and Laos also faced similar challenges, increasing by more than 5 km3 year− 1 each.
Under the HUS and MED scenarios, more countries (80 and 66, respectively) experienced increased water use compared to the EAT scenarios in 2030. The higher consumption of water-intensive products, particularly meat and sugar, contributed to this trend. Consequently, approximately 34% of countries faced more severe water use challenges. Despite reaching peak water use by 2080 in 50% of these countries, water use continued to increase in 23 countries due to higher food consumption. In contrast, the VEG scenarios substantially reduced water use by reducing meat consumption. This transition to a plant-based diet greatly conserved water resource. However, increased consumption of foods like legumes/nuts14 in certain poorer regions may still pose water use challenges.
Dietary quality of different dietary scenarios
With sustained economic growth, the consumption of meat and sugar products was expected to rise in future diets, leading to a continued decline in diet quality globally. In the BaU scenario, global dietary quality was projected to experience a decline from 52.34 in 2020 to 51.36 in 2100 (Fig. 2a). Notably, the decline in dietary quality could be observed in 102 countries, where increased meat and sugar products (-1.42) and decreased consumption of cereals (-2.39) offset the health benefits of certain food categories such as vegetables (+ 0.26), fruits (+ 0.42), seafood (+ 1.40), and that of nutrition indicator like PUFAs (+ 0.97). Consequently, the total Alternative Healthy Eating Index (AHEI) score of these countries was predicted to decrease by 2.69 ± 2.34 points (Fig. 2b,c). Additionally, this phenomenon was more pronounced in economically underdeveloped regions, specifically Africa (-2.64 ± 1.95), Latin America (-3.14 ± 2.25), and Asia (-3.78 ± 3.27), and these regions were expected to have a greater number of countries experiencing a decline in dietary quality, with 26, 22, and 20 countries, respectively.
In all four healthy and sustainable diet scenarios, varying degrees of improvement in dietary quality were observed and remained constant after the completion of the dietary transitions in 2050. The HUS diets exhibited the least improvement, with a score that increased from 52.34 in 2020 to 67.17 in 2050 (Fig. 2a). The MED scenario, with lower sugar and oil consumption, led to an additional 1.08-point improvement compared to the HUS scenario. The EAT scenario further reduced the consumption of dairy products, sugars, and meat while increasing the consumption of legumes/nuts and grains, bringing the total score to 78.12 in 2100. Moreover, the VEG scenario replaced all meat with legumes/nuts and showed further improvements in red meat and slightly higher scores for PUFAs and trans fats. Nevertheless, the lack of seafood/fish consumption results in a lower score of 70.97 in 2100.
Although dietary transitions had the potential to positively impact dietary quality in all countries, the impact was more pronounced in developed countries. Nonetheless, the gap in benefits between developing and developed countries was smaller when meat and sugar consumption is lower. For example, the VEG scenario had the smallest gap of 2.50, while the MED scenario had the largest gap of 4.08. Overall, countries such as Hungary (with scores ranging from 29.76 to 40.84), Slovakia (29.33–39.28), Bolivia (30.22–41.65), Argentina (30.35–40.46), and Switzerland (30.85–41.09) were expected to see the greatest improvement in dietary quality among the four scenarios. This improvement was largely due to the effect of food subcategories on PUFAs and trans fat, indicating that reducing certain food categories and including healthier alternatives could significantly improve the nutritional profile of diets in these countries.
Share of food expenditure
The share of food expenditure is determined by supply and demand balance, food prices and income. In the BaU scenario, food demand and prices were projected to consistently rise by 15.02% and 24.32%, respectively, with a notable rise in animal-based food expenditure by 68.91% in 2100 (Figure S5). In the HUS and MED scenarios, despite lower overall demand, food expenditure would increase due to higher demand for animal-based products and fruits/vegetables. Food prices were projected to increase by 5.76% and 12.77%, respectively. In contrast, in the EAT and VEG scenarios, the opposite effect was projected, as dietary changes would lead to a much larger reduction in demand for animal-based products. As a result, food prices were expected to fall by -25.96% and − 54.36%, respectively.
The share of food expenditure in relation to total income decreased across all scenarios at the global level as the economy developed. This indicates an improvement in global food affordability, with the share decreasing from 1.07–0.35% for each scenario (Fig. 3a), promoting positive prospects for food security worldwide. Nevertheless, there was significant variability in food affordability among countries due to disparities in economic development (Fig. 3c). In 2020, Africa and Asia faced substantial challenges in food affordability, with the top 20 countries in these regions having an average food expenditure share of 21.69%. The Central African Republic had the highest share at 52.43% in this region. In contrast, developed regions like the US and Europe had negligible affordability issues, with food expenditure shares at 1.22% and 1.49%, respectively. In the BaU scenario, food affordability ranged from 0.34–11.80% for each country, influenced by their economic development. Africa and Asia were projected to experience significant improvements in food affordability by 2100, with a decrease of 13.87% in Africa and 5.20% in Asia (Fig. 3d).
During the dietary transition certain countries, particularly those already facing poverty and economic challenges, encountered increased pressure on food affordability. For example, in 2030, during the transition to the EAT, HUS, MED, and VEG scenarios, 67, 94, 100, and 28 countries, respectively, were projected to face increased food affordability pressure (Fig. 3e), mainly in Africa and Asia. However, this pressure gradually diminished as dietary transitions completed. By 2100, in the MED scenario, only 19 countries experienced a higher share of food expenditure due to increased dietary demand, ranging from 6.32–48.31%, with Iraq being the highest.
Environmental-health-affordability trade-offs and synergies
Finally, we analyzed the synergies and trade-offs associated with different dietary transitions (see method). The results indicated that these transitions can significantly improve global dietary quality over time. However, they also came with trade-offs in food affordability and water use, especially in developing countries (Fig. 4). Among the different diets studied, the HUS and MED diets faced more prominent challenges. In the early stages of dietary transition, both the HUS and MED diets fell into quadrant III, indicating trade-offs between water use and food affordability. Despite the initial challenges, there were positive prospects for dietary transitions. As the transition progressed and was completed (post-2050), the advantages became more evident, with reduced calorie requirements and changing trade-offs shifting towards synergies between food affordability and water use, especially in the HUS and MED diets. Both diets gradually moved from food affordability and water use trade-offs quadrant to triple win quadrant.
It is important to note that the synergies and trade-offs of dietary transitions varied significantly between countries. We highlighted the challenges and potential benefits for different countries before and after the transitions. Due to space limitations, we focused on the dynamic changes of the EAT diet (Fig. 5), and the rest of the diet scenarios are provided in the appendix (Figure S6). In 2030, the adoption of the EAT scenario could have improved dietary quality, reduced water use, and lowered food expenditure in 82 countries by 2030 (quadrant I), but these countries were mainly in developed countries such as Europe or North America (Fig. 5a-b). During the early stages of the transition, many countries in Africa and Asia faced trade-offs in water use (quadrant IV), affordability (quadrant II), or both (quadrant III), despite the improved dietary quality. Specifically, 42 countries encountered food affordability trade-offs, primarily in Africa (-39.55 ± 30.33%), Latin America (-20.47 ± 18.56%), and Asia (-7.33 ± 2.99%). Notably, Burundi, the Democratic Republic of the Congo, and Malawi experienced the most significant declines by -132.28%, -105.17%, and − 66.69%, respectively. Additionally, 27 countries, predominantly in Asia and Africa, faced water use trade-offs (-21.99 ± 21.11%). However, as the dietary transition progressed and reached completion, the trade-offs were expected to gradually ameliorate, with the number of countries facing trade-offs decreasing from 92 in 2030 to 65 in 2050, and to 34 in 2100 (Fig. 5c-e). This was mainly because economic development and population growth would further drive the total food demand in different countries over time without dietary transitions intervention, thereby gradually widening the gap with the total food demand in the diet scenarios (Figure S7). Encouragingly, a significant majority of countries in the EAT scenario are poised to transition to quadrant I before 2060. However, 34 countries were still projected to face water use trade-offs, and 17 of these countries, mainly in Africa and Asia, encountered water trade-offs after the completion of the dietary transition.