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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