A number of scientific studies have shown that moving towards a plant-based vegan diet can improve mood, lower depression risk, and improve quality of life. Here is a sampling of these studies. 
 
In a cross-sectional study of vegetarians and omnivores, vegetarians reported significantly less negative emotion than omnivores. 
 
In a randomized controlled trial, restricting meat, fish, and poultry in omnivores improved mood after two weeks.
 
In a daily diary study, on days when young adults experienced better moods, they reported eating more servings of fruits and vegetables. 
 
In a prospective cohort study of men and women in Taiwan >65 years old, frequent consumption of vegetables lowered the risk of depression.
 
In a multicenter randomized controlled trial, weekly instruction on following a vegan diet for 18 weeks reduced depression and anxiety and improved quality of life in a multiethnic adult population in the corporate setting.