To be sure, I wasn't saying it was impossible to survive well on just plants (though it's certainly a bit more challenging and risky). It's just that naive comp

To be sure, I wasn't saying it was impossible to survive well on just plants (though it's certainly a bit more challenging and risky). It's just that naive comparisons of food efficiency don't tell the whole story. You need a lot of trace vitamins which you can only obtain by 1) taking a lot of a low-density source and concentrating it (like cooking an entire bag of spinach into a souffle to get iron), or 2) eating something which has done the concentrating for you, usually over a period of time. Either way, living off plants is considerably less efficient than "meat vs wheat" would suggest. "Plenty of fish" is a problematic addition to the diet you describe - that's meat, plain and simple, with all of the bioefficiency loss and ecosystem disruption that entails. It's just less visible because it's underwater.