It’s only one study, and any single study such as this should be taken with a grain of salt. But an Austrian matched sample study of people with different dietary habits contradicts the conventional wisdom that vegetarians are healthier:
… Subjects were matched according to their age, sex, and socioeconomic status leaving 1320 people – 330 vegetarians, 330 that ate meat but still a lot of fruits and vegetables, 300 normal eaters but that ate less meat, and 330 on a more carnivorous diet.
After controlling for variables, they found that vegetarians did have lower BMI and alcohol consumption but had poorer overall health. Vegetarians had higher incidences of cancer, allergies, and mental health disorders, a higher need for health care, and poorer quality of life.
As a result, vegetarians take more medications than non-vegetarians.
I’m guessing that the vegetarians’ problem is insufficient alcohol consumption.