I'm very familiar with Dr Greger. I have that book and I have read it. Dr Greger's diet advice is pretty good and his work at nutritionfacts.org is a good reference. But Dr Greger over-states the case for a pure plant-based diet. He seems to have gotten excited by feedback from his followers, driving him to overstate a number of things. Gil Carvalho (who thinks a lot like I do so I respect his view) has a fact-check on him: https://www.youtube.com/watch?v=3XjmvBFt63k I think Gil is being overly harsh here, but it is still useful. The diet-health-vegetarianism connection comes out of Seventh-Day Adventism which got it from Ellen G White (a women hit in the head with a rock so hard as a kid that she started having "visions"), who thought she got it from God, but probably got it from Reverend Sylvester Graham. All of this happened before 1900 without the benefit of science. But once people have a belief, it is hard to shake, and since SDAs were some of the first doing medical research into diet, their religious beliefs (including vegetarianism) became (unsuprisingly) the healthiest diet. Not only is it very clear that a pure plant-based diet is deficient in a lot of things (B12, iron) but a lot of research shows that unprocessed low-saturated-fat meats are at least as healthy as grains, and the cohort of SDAs who add fish to a plant-based diet live longer than those who don't.