Transitioning to healthy and sustainable diets has higher environmental and affordability trade-offs for emerging and developing economies.

Switching to alternative global diets offers established benefits, but the challenges and opportunities for individual countries during and after the transition remain unassessed. In this study, we project changes in water use, dietary quality, and food affordability under four dietary scenarios (including Mediterranean diet, the EAT-Lancet diet, the Healthy US-Style diet, and Vegetarian diet), assessing the potential implications at the country level from 2020 to 2070. Here, we show that by 2070, transitioning to healthy and sustainable diets can improve dietary quality by 30.29 - 45.43%, with all countries reducing water use (1.21 - 14.73%) and increasing food affordability (9.29 - 63.23%). However, in the initial phases, increased food demand escalated water use and worsened food affordability, especially in emerging and developing economies, with the maximum average deterioration being 2.62% and 13.06%, respectively. These highlight the need for long-term planning and financial support to ensure successful global transitions.