I think there should be an equation for how much food a city can send by trade route - if we consider the ancient world, grain & olive oil were really the only two food commodities that could be transported easily. Rome was fed on grain from Carthage & Egypt.
So perhaps wheat, rice or 'grain' resource tiles should be an important part of the calculation for how much food a city can send by trade-route.
It would certainly make wheat tiles much more valuable then just a simple local food source.
In fact for Civ 5 I really think an improved wheat or fish source should be necessary before a food trade route can be sent (along with the granary)
