While this research posits that abstaining from meat can be detrimental to your mental health, there is mention of factors that can muddy that a bit. Keep in mind this publication deals in research before published peer review.
Perhaps it is the ever present mental and physical stress of our ‘profit over people’ system. The mental issues they blame on lack of meat may have been there already and were the source of the change in diet. Might just be the result of a reality based outlook. The anxiety of being overtly aware of the plight of most people on the planet. Those with severe enough cases have a strong impulse to actually do something about it. Some just live with, either not knowing what to do or figure it is the price of being a normal caring person in our current self destructive world.
Many people choose to eat substantially less animal products because of negative factors associated with meat as part of the ‘normal American diet’. Might be the massive animal harvesting industry and environmental factors associated with it. Might be the death and cruelty. Maybe it’s the reality that most meat, food in general for that matter, available today is not produced in a sustainable and healthy way and does not contribute to good health. Much of this death and destruction ends up in the dumpster behind your favorite junk food eatery. Or absorbed into a market that produces far more food than is needed.
Whatever it is, some people are just naturally aware everyday of the depressing, joy killing, and otherwise soul crushing reality of the way our society operates. In a big way indicated by its continuing tacit approval of the continuing destruction of the planet we need to survive.
Much the same, I think, as the idea that those that don’t believe in god(s) are not as happy as those that do. Maybe non-believers just see a natural world where we have the ability to create a well adjusted highly functional society and it has nothing to do with the supernatural. It is up to us. There is no reason to think we will be rescued by some benevolent species that takes pity on our stupidity. Likely just the opposite, I afraid. It is a reality that can certainly effect your mental state everyday, like it or not, know it or not.
There is a mental and emotional price to pay for keeping honest reality in the forefront, in order to fight for a better world.
Just some thoughts.