This academic and food writer says the best evidence is anecdotal–look at healthy vegans, look at what they eat, and put two and two together. Conversely, look at unhealthy and failed vegans and what they eat to get a better sense of how some vegan diets are as unhealthy as diets get:
Someone can live on potato chips, pot, and cherry soda and call himself a vegan. Many recidivists have evidently tried to do just that.
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