Plant-based diets (PBDs) are gaining popularity due to their perceived health and environmental benefits. However, definitions of PBDs vary, influenced by social, cultural, and agricultural factors. While PBDs generally offer sustainability advantages and lower risks of diet-related noncommunicable chronic diseases (NCDs), the transition needs careful investigation to address potential challenges. Meat production contributes significantly to greenhouse gas emissions (approximately 57% of global food production emissions), while plant-based food production contributes about 29%. Shifting toward PBDs is crucial for climate action, and is associated with reduced risks of NCDs, particularly cardiovascular diseases. The adoption of PBDs has increased, evidenced by growing vegetarianism rates and the expanding plant-based food market. However, previous studies have focused on individual food items rather than composite diets. This study uses the Food4HealthyLife calculator to model composite diet scenarios, partially replacing animal-based meats with legumes, to assess nutritional quality, environmental impacts, and trade-offs, aiming to identify feasible and sustainable dietary patterns.

The introduction provides a comprehensive literature review on the definitions and benefits of plant-based diets, highlighting the conflicting definitions and the significant contribution of meat production to greenhouse gas emissions. It also discusses the established health benefits of plant-based diets in reducing the risk of diet-related noncommunicable diseases. The review also touches upon the increasing trends of plant-based food consumption and the market growth in the United States and globally. Existing research gaps concerning the environmental impacts of composite diets are highlighted, setting the stage for the current study.

The study employed a four-step methodology. First, three consumption models (M1, M2, M3) were developed, each targeting a partial replacement of meat with legumes. Second, 24 diet scenarios (S) were created within these models, including current, feasible, and optimal diet scenarios from the Food4HealthyLife calculator. Third, the nutritional quality of the diet scenarios was assessed using the Food Compass scoring system and the Health Nutritional Index (HENI). Environmental impacts were estimated using midpoint impact values reported by Stylianou et al. (2021) for foods in the What We Eat in America (WWEIA) database. Fourth, a trade-off analysis using dual-scale data charts was conducted to explore the relationship between nutritional quality and environmental impacts, using Food Compass scores and HENI scores against global warming (short term), ionizing radiation, and freshwater eutrophication. Statistical analyses including Pearson correlation and Kruskal-Wallis tests were performed.

Nutritional quality, as measured by Food Compass scores (FCS) and HENI scores, increased as the diet scenarios transitioned from the current diet to the optimal diet. The FCS ranged from 65.46 (current diet) to 81.73 (optimal diet), with most scenarios exceeding the recommended score of 70. HENI scores showed a strong positive correlation with FCS, ranging from 60.21 min per 100 kcal (current diet) to 320.85 min per 100 kcal (optimal diet). Domain contribution analysis showed vitamins contributing most to FCS, while specific lipids contributed the least. For HENI scores, vegetables and fruits were the main contributors. Environmental impact analysis revealed that several scenarios, particularly S2M1, S2M2, and S2M3, initially showed higher impacts than the current diet due to increased consumption of sustainable foods. However, as animal-based meats were partially replaced with legumes, the environmental impacts decreased significantly. S9M3 (10% legumes, 0.11% red meat, 0.28% processed meat, 2.81% white meat) showed the lowest environmental impact, reducing global warming by 54.72% while maintaining a high FCS of 74.13. Trade-off analysis confirmed that S9M3 was the most sustainable scenario, achieving high nutritional quality and substantial environmental gains. The optimal diet scenario (S10M1/M2/M3), while having highest nutritional quality scores, also had higher environmental impacts due to increased consumption of fruits, vegetables, cereals and legumes.

The findings highlight the complex interplay between nutritional quality and environmental impact in dietary transitions. The study confirms that simply increasing plant-based food intake does not guarantee superior environmental performance; the type and quantity of foods are crucial. The success of the S9M3 diet scenario underscores the importance of strategic substitution of animal products with legumes to achieve both nutritional and environmental benefits. The lack of perfect correlation between nutritional quality and environmental impact necessitates a holistic approach to dietary assessment, considering both aspects simultaneously. The study also notes the limitations of relying on secondary data for environmental impact assessment and the exclusion of certain nutritional factors.

This study presents a novel approach for assessing the combined nutritional and environmental benefits of diets, emphasizing the need for sustainable dietary modeling that considers both health and environmental impacts. The optimal scenario, S9M3, which included a significant reduction in animal-based meats and an increase in legumes, showcased the potential for achieving substantial environmental gains (55% reduction in global warming) while maintaining high nutritional quality. Future research should expand this model to different regions, incorporate more detailed data, and consider factors like cooking methods and bioavailability for a more accurate assessment.

The study relied on secondary midpoint environmental impact data from the WWEIA database, limiting the generalizability of findings to other regions. Data on phytochemicals and carotenoids were unavailable, impacting the Food Compass scoring. Cooking losses and bioavailability were not considered, potentially influencing the nutritional quality assessment. Future studies should address these limitations.