If it could be delivered by boat or camel it could be sustainably delivered today.
As carbon foot prints help define sustainability, Ancient trade routes can help us determine what can be delivered efficiently.
Through out the world ancient trade routes sold food that was dry, preserved, or could last for a long journey. This still make sense for long distance trade today.
Fresh foods should always be as local as possible, for the sake of sustainability and quality, however preserved food from afar could have a significantly positive political influence as well as having a good carbon foot print.
The Mayans trade was geared to feed huge cities, with out having beasts of burden. The carbon foot print was solely based on feeding people (deforestation being the primary issue).
The carbon foot print of an item that is shipped by freight from Michigan to California is much higher than on a slow boat from China, though present diesel engines in ships presently have a high sulfur oxide output Products that need refrigeration increase energy use hourly, so air freight might end up being better than ground for chilled items, due to shorter traveling time. There are many variables in predicting carbon foot prints. 
One exception to the camel rule is local products grown in hot houses, may have a larger carbon foot print than those shipped by air freight .
Developing sustainable markets in a global scale can be just as important as developing local sustainability. We are all in one large ecosystem, dust from the Gobi desert blows into North America, so bad agricultural practices spread through out the world, so if our decisions can limit pesticides in china it helps our local environment as well as globally.
Local food is normally the most sustainable but outside of a 100 mile range the the advantages become cloudy. The question then becomes how does the product get here, and how much energy does it take to produce it at its origin. Political issues must also be considered. If a region is developing it is better to ensure that it starts sustainably than trying to do catch up later. After pesticides and herbicides have already been used, it takes years to convert back to organic production.
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