I'd say the biggest concern is B12, since that only comes from animal products (which can be milk or eggs, doesn't have to be meat). Iron may be an issue too but it can be obtained from plant foods if you know which ones have more and eat them with something that has lots of vitamin C. My take on it is that the natural human diet is omnivorous but should contain a lot more plant than animal products; most of us here in America eat too much meat and that's not healthy especially given modern farming practices. I seldom eat red meat anymore, mostly poultry and fish, and those not even every day. Ditto to those who said to stay away from excessive amounts of processed foods--those are unhealthy whether vegetarian or not!