Vegetarianism

I have been trying to find out if it is practical for all the humans to be vegetarians. I read many qualitative statements mentioning how animals we consume eat more food than we do, if we chose to eat that food instead. If it is true for majority of the animals we consume, it is a powerful argument. There must be some research done in this regard and I am still looking for that data.

There are other angles to this issue. On one hand, we have people disregarding the wellbeing of animals in 'animal farming' and on the other hand, we have the concept of 'edible pets' (Yes, there are people that raise animals like pets and at the end of the day, cook them).

Philosophers Paul & Linda McCartney said, if slaughterhouses had glass walls, everyone would be vegetarian. But, I don't think treating animals with love and dignity before killing them is the solution.

Anyway, I am asking why we should eat animals instead of asking why we should only eat vegetarian food. I am looking for a convincing answer for why we should be non-vegetarians.