Its a belief that you can't try to get it from other sources though meat contains vital nutrients most stuff we eat does that, the argument is that with globalisation there is relatviely easy access to a large variety of plants that provides similar nutrients.
Keyword there is omnivore, you know quite a few of those can live on a purely plant based diet right? Or a mostly meat based diet if it should come down to that. Being an omnivore doesn't mean MUST EAT MEAT but that we are able to eat and process all kinds material both plant based, meat based and fungi.
Or you know studies by people who where actually interested in the asnwer? They are of course affected by other factors too, like the fact that we use a salt to preserve meat thats on the list of cancer inducing chemicals, or the fact that people who eat meat generally eat too much of it, (with the exeption of old people where the extra meat consumption is helpfull for their health)
There are thousands of factors and a million of datat points and and people still haven't found a good way to measure health impact.
The point is that there simply is not enough data, some vegans claim that you can live a healthy life on a plant and fungi based diet, others claim that you cannot survive without meat. Neither is backed by significant amounts of reliable data because surprise the human body is complex and so is our bodies which makes studies much more about statistics than pure cause and effect.
The idea that the nutrients need to come from meat is weird though what is important is that you get them not whether they where on a cow or on a piece of corn, theoretically I should be able to make it all from ground up in a cehmistry lab and live from that.
Pretty sure that the expensive tissue hypothesis, still classifies as exactly that though a hypothesis, meaning that it an idea backed by data, but the amount of data is not big enough to actually be able to with reasonably security call it a fact.
Personal unpopular opinion: Both sides of the veganism health debate are idiots and should spend more time on research instead of debating dietary choices.
Somebody should make a meta study of all the meta studies to see if they actually conflict with each other or people are jsut talking about different data. (Or something if it would just help people understand that most of our scientific results within the health sector could use a healthy dose of relativism)
