Meat-heavy diets are deleterious for human health and the environment—a fact that’s now been established by many studies. But new research reveals that there can also be large differences in the health and planetary effects of plant-based diets.  Several studies have paired up the health and environmental impacts of national diets to home in on foods that score well on both fronts. But the new Lancet Planetary Health research provides uniquely concrete insights on the issue, because it draws on real dietary records from a massive national survey of over 90,000 American nurses, whose diets and health were then tracked for almost 30 years.  Starting in 1991, the survey data…