Researchers have linked dietary data from over 55,000 individuals with data on the environmental impacts of the foods they eat. The team, from the Livestock, Environment and People (LEAP) project at the University of Oxford, found that the dietary impacts of vegans were around a third of those of high meat eaters. They also saw a 30% difference between high- and low-meat diets for most of the measures of environmental harm.
A vegan diet is far, far healthier than the average American diet even if the vegan doesn’t have a clue about nutrients and just eats a balanced diet with good healthy food. The only way you end up with significant B12 deficiency or other vitamin/mineral deficiencies is if you eat an even more restricted diet (for example, raw vegan), and even then a B12 supplement is much healthier than red meat.
Eating 2/3 of your meals made entirely of pureed dog shit is probably healthier than the average American diet. So, not exactly a ringing endorsement.