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I have seen a lot of non-veg turning to vegan, began posting veganism on fb and in just few months when their bodies start to give up, they again turn to non-veg.

Turning vegan just for few months is not on.

No idea how many such people do you know but I am vegan from more than 3 years and stronger than before. I do not know any such people who turned vegan and then turned non-vegan again because their body become weak.