You can't compare caloric content of lettuce to that of beef. Vegetarians don't stock up on lettuce to get calories. That being said, if every single person was

You can't compare caloric content of lettuce to that of beef. Vegetarians don't stock up on lettuce to get calories. That being said, if every single person was vegetarian, I would bet we could get the efficiency of vegetable farming to beat the current efficiency of cattle through indoor farming.