There are 20 amino acids that we need, of which we are incapable of creating 9 of them.
histidine
isoleucine
leucine
lysine
methionine
phenylalanine
threonine
tryptophan
valine
In addition, children need arginine.
As long as we get these nine in adequate amounts, our body is capable of breaking them down and reforming them to meet its needs. There are amino acids in almost all foods, including many fruits, not that I’m advocating a fruitarian diet. :)
When you consume meat, the body breaks it down to amino acids and reforms them into the particular combination it needs. If your source of amino acids is from plant material, it does the same thing.
It depends on how well educated a person is willing to become and the time they are willing to spend on their diet. Probably the main disadvatage a well-founded vegetarian diet is the time it takes to prepare meals. But the main advantage is that you can save a fair bit of cash.
Some dieticians have told me that if you have O blood, you should incorporate some animal protein for optimal health, even if it’s only eggs.