The study examines the health and environmental impacts of diets across 195 countries from 1990 to 2011, using a health–environment efficiency indicator. This indicator measures the ratio of health benefits (reduced disability-adjusted life years, DALYs) to environmental impacts (greenhouse gas emissions, water withdrawal, acidifying and eutrophying emissions). The efficiency follows a nonlinear path, initially increasing due to reduced child and maternal malnutrition, then decreasing as animal product consumption increases, and finally showing slow growth in some developed countries as they shift towards healthier diets. The relationship is influenced by the Socio-Demographic Index (SDI), which measures socio-economic development. The study finds that the efficiency has an N-shaped trend, with positive correlation in the first stage, negative in the second, and positive again in the third stage. This pattern aligns with the global dietary transition framework. The findings highlight the need for policy interventions to improve health and environmental sustainability, particularly in low- and middle-income countries, where dietary transitions are rapid and often lead to increased environmental impacts.The study examines the health and environmental impacts of diets across 195 countries from 1990 to 2011, using a health–environment efficiency indicator. This indicator measures the ratio of health benefits (reduced disability-adjusted life years, DALYs) to environmental impacts (greenhouse gas emissions, water withdrawal, acidifying and eutrophying emissions). The efficiency follows a nonlinear path, initially increasing due to reduced child and maternal malnutrition, then decreasing as animal product consumption increases, and finally showing slow growth in some developed countries as they shift towards healthier diets. The relationship is influenced by the Socio-Demographic Index (SDI), which measures socio-economic development. The study finds that the efficiency has an N-shaped trend, with positive correlation in the first stage, negative in the second, and positive again in the third stage. This pattern aligns with the global dietary transition framework. The findings highlight the need for policy interventions to improve health and environmental sustainability, particularly in low- and middle-income countries, where dietary transitions are rapid and often lead to increased environmental impacts.