Vegetarian bad?

you hear a hundred and one reasons why its get to be a vegetarian, what i want to know, is why its bad to be a vegetarian? like what are the bad things it does to your body and your health, an any other reason you can think of
Update: what about for sports?
is it easier, do you feel you can run faster, longer, ect? do you feel you can play a more variety of sports?
and stuff like that?
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