The study, published in The Lancet Planetary Health, is the first to comprehensively assess the relationship between the health and nutritional impacts of different dietary-change strategies and their environmental impacts across all major world regions.

"The food people eat impacts their health and the health of the environment. Unhealthy diets, overconsumption, and hunger are leading to nutritional deficiencies and diet-related chronic diseases around the world. The food system is also a major cause for climate change, freshwater depletion, deforestation, and pollution of ecosystems, for example through overapplication of fertilizers," says Dr. Marco Springmann of the Oxford Martin Programme on the Future of Food and the Centre on Population Approaches for Non-Communicable Disease Prevention at the Nuffield Department of Population Health, University of Oxford, who led the study.

"We investigated the aspects of our diets that we should change to improve health and nutrition around the world whilst at the same time reducing environmental impacts whenever possible."

The study considered three different dietary-change strategies, including replacing animal products with plant-based ones, improving weight levels by addressing both over- and underconsumption of calories, and adoption of well-balanced and predominantly plant-based diets that reflect the current scientific evidence on healthy eating, including flexitarian (semi-vegetarian), pescatarian, vegetarian, and vegan diets. The researchers assessed the impacts of these strategies on nutritional deficiencies, chronic disease mortality, and environmental impacts, including greenhouse gas emissions, freshwater use, cropland use, and nitrogen and phosphorus application from fertilizers for more than 150 countries.

The results showed various trade-offs. Replacing animal products with plant-based ones was particularly effective in high-income countries for improving nutrient levels, lowering chronic disease mortality, and reducing some environmental impacts, in particular greenhouse gas emissions. However, it also led to increased freshwater use, and had little effectiveness in countries with low or moderate consumption of animal products.

Improving body weight by addressing the over- and underconsumption of calories led to similar reductions in chronic disease mortality, due to reductions in levels of overweight in high- and middle-income countries, and reductions in levels of underweight in low-income countries. However, it only moderately improved nutrient deficiencies and led to small reductions in environmental impacts at the global level, with reduced impacts in high- and middle-income countries balanced by increased resource use in low-income countries.

Adoption of healthy, energy-balanced, and predominantly plant-based dietary patterns addressed these problems and led to an adequate supply of most nutrients in most regions, and large reductions in chronic disease mortality that ranged from 19% for flexitarian diets to 22% for vegan diets.

The energy-balanced and predominantly plant-based diets also led to reduced environmental impacts globally and in most regions. Globally, the changes included large reductions in greenhouse gas emissions (54%–87%), with greatest reductions for vegan diets; medium-level reductions in nitrogen application (23%-25%) and phosphorus application (18%–21%), with greatest reductions for vegan diets; and small to moderate reductions in cropland use (8%-11%) and freshwater use (2%–11%), with greatest reductions for pescatarian and flexitarian diets, respectively.

"Our research suggests that we need a dietary-change strategy that encourages predominantly plant-based diets and improvements in weight levels and total calorie intake, in line with the latest scientific evidence on healthy eating, if we are to address nutrient deficiencies, lower the number of deaths from chronic diseases, and reduce environmental impacts," says Dr. Springmann.

"However, many national dietary guidelines do not reflect the latest scientific evidence on healthy eating and include no or very lax limits for animal products, particularly meat and dairy. Updating national dietary guidelines to reflect the latest evidence on healthy eating is important not only for improving human health, but also for preserving the health of the environment by reducing the environmental impacts of our food system."