There are some nutrients that we get primarily from the consumption of meat products. I'm not an expert so I won't try to explain which nutrients they are. Some of these nutrients can be gotten through eating specific vegetables, though generally in much larger quantities than would be required with meat. Some of them cannot, though, and to maintain a Vegan diet you have to take daily supplements. Usually these supplements are provided by the same companies that the PETA nuts boycott because of animal experimentation, though.