Look at ''The Adventist Study 2", which to my knowledge is the largest study ever conducted on the subject which includes not only the vegetarian diet but also the fully plant based diet. https://adventisthealthstudy.org/studies/AHS-2

"From 2001 and 2007, we enrolled as participants 96,000 Adventists, ages 30 to 112, from all 50 U.S. states and Canada."

"Vegetarian diets in AHS-2 are associated with lower BMI values, lower prevalence of hypertension, lower prevalence of the metabolic syndrome, lower prevalence and incidence of diabetes mellitus, and lower all-cause mortality."

"Cox proportional hazards regression analysis (adjusting for age, race, sex, smoking, exercise, education, marital status, alcohol, geographic region, menopause, and hormone therapy) showed reduced all-cause mortality for all vegetarians compared with nonvegetarians (HR: 0.88; 95% CI: 0.80, 0.97). For specific dietary patterns, the HRs were 0.85 (95% CI: 0.73, 1.01) for vegans, 0.91 (95% CI: 0.82, 1.00) for lactoovovegetarians, 0.81 (95% CI: 0.69, 0.94) for pescovegetarians, and 0.92 (95% CI: 0.75, 1.13) for semivegetarians."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4144107/#:~:tex....