 First up, Mike's quite clear on one point. Eating meat, any meat, is always going to be carbon heavy. There's just no getting away from that fact. The most efficient thing to do by far is to grow plants directly for human consumption. If instead you grow plants and then feed them to animals, they do all sorts of inefficient things that take energy. So they walk around and they keep warm and they metabolise. And all of that builds a massive inefficiency into the food system.