You're not going to have "withdrawal". If you eat allot of tuna you had better do a bit of reading about how much mercury you are consuming. Even if you are not a female or pregnant, do you really think mercury is a great idea to eat? Here is a nice little list of symptoms and effects of excess mercury consumption.
http://www.mercury-poison.com/symptoms.htmAre you concerned about feeling weak because you fear you will not get enough protein? That is a TOTAL MYTH. Human breast milk gets 5% of it's calories from protein. Stay with me here. Your needs for protein and calcium are never as high as they are in the first year of life when you triple your weight in just those twelve months. It stands then to reason that any food you eat which derives 5% of more of its calories from protein will be a perfectly adequate protein source. If you ate nothing else but the common potato (11% calories from protein) you would be just fine. The fact is, anyone who is eating enough calories is virtually guaranteed to get enough protein.
Here's a great article on WebMD that discusses the disease prevention benefits of a plant-based diet.
http://my.webmd.com/content/article/64/72282.htm