My take on this (an I’m vegan so there’s a possibility of bias) is that most of the mainstream claims such as there being no health downside and a plantbased diet is significantly less harmful for the environment are simply true.
I try to avoid talking BOTH environment and health in the same discussion because I find the risk of it turning into a gishgallop back-and-forth between the triforce of plant (environment, health, ethics). And obviously ethics can become a nuclear bomb dead-end of different fundamental positions. But I do feel of the two sides, the vegan side is definitely the one benefitting most from misinformation. I don't think you'll do the gishgallop thing (you seem to be pretty good-faith), so I'll give my thoughts on both. But if you did decide you wanted to discuss one in detail, I'd ask that we keep it to one of your choosing :)
I'm not sure if I've provided these links (lemmy context issues). This guy is not biased, a lot health focused, and did a LOT of research on the topic of nutrition and veganism.. There's a lot to pack/unpack, but most of it is based around the fact that a supermajority of vegans are suffering from malnutrition in one way or another, compared to a significantly lower number of non-vegans. As for B12, it is 92% of the population. He also did research into the environmental impact of the meat industry (in another video), and it's equally eye-opening. I'm not blindly believing him, and I don't expect you to. My experience has been growing up in non-mega farm areas, so I have seen the things he's saying firsthand. The river in my hometown died from plant farming; it was a fairly big deal, and a friend of mine (from a totally different area) did her Environmental Engineering thesis on it. It wasn't about the plants as much as the overfarming altogether. Which is similarly true with meat, imo.
One thing as a meat-eater I find is that vegans often do one inadvertant disservice (the same types of vegans who throw false shit out). They change the focus from how to improve a healthy balance (less red meat, more white meat and healthy seafood, regulating away processed meats that are confirmed carcinogens, etc) to "veganism is the cure". And instead of focusing on some very real issues with global warming, they focus on the meat industry of countries like the US that are simply an insignificant part of the environmental threat. If every American and European quit meat cold turkey tomorrow, and the most optimistic non-bs numbers were true, it would slow global warming by a tiny fraction of a percent. Look at this map for a second before reading my next line ( full context article here ) . Instead of focusing on meat emissions in ways that probably will never happen and won't do much if it does, we could be focusing on regulating and presssuring India and Africa (and maybe China) to clean up their act.
When we look at the supposed environmental impact of meat, it's pretty important to know that there are African countries that produce more agriculture-related GHGs than all of the US and Europe combined. It's important to note that the second biggest meat producer in the entire world (US) isn't even a blip on the radar. There is a right way to do meat, at scale, that is environmentally friendly. And as bad as the US supposedly is, they're pretty good in aggregate. If there's room for improvement in the US meat industry... well, I don't expect you to believe the next sentence, but I have come to believe that if meat is grown correctly, the symbiotic Meat+veggie+grain farm is simply better for the environment than plant alone. Lots of reasons (fertilizer, the manure->field efficiency, waste products not usable for any other purpose, etc). But suffice to say even if I'm wrong, it approaches zero impact to do meat as long as you do it right.
