I think it was vitamin b11 that is only available from animal sources and is necessary for proper functioning of the human nervous system. Scott Adams, author of Dilbert, is a famous vegan, and he's also had a couple of severe nerve problems. An interesting coincidence.
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If ye love wealth better than liberty, the tranquility of servitude than the animated contest of freedom, go from us in peace. May your chains sit lightly upon you, and may posterity forget that you were our countrymen! - Samuel Adams