We have to do it. Some of us have to do it in different ways. Doing small things where you can is all well and good but literally none of us are making a single impact in capitalism by making different choices about how we're forced to participate in it. Focus on tearing it down together rather than picking apart individual choices because those individual choices still result in participating in capitalism.
I've never seen studies or anything about it, but I'm fairly sure that people who lose weight going vegan were overweight to begin with. Rice and beans don't have less calories than a steak.
There’s also A lot of people who lose weight when going vegan because they were going vegan for such a purpose or subconsciously have orthorexia. So it is an issue but their circumstances aren’t applicable to everyone if you pay attention to eating enough calories.