There is a lot of disagreement out there.
If you are going to eat meat, then I would suggest that you eat free range organic meat and wild-caught fish. Don't worry about "lean".
You might want to check out this website: http://www.westonaprice.org
Now if you do want to become vegan, that's great. But I would say the important thing is for everyone (including vegans) to do a parasite cleanse. And if you do eat meat, get a clean source. It is not about "lean" being better, it is how the animals were raised.
By the way, tiny fish like wild-caught sardines are an excellent source of animal protein that is lower in mercury and pollutants.