Food miles are irrelevant, co2 ouput isnt, and most (not all) fruit and veg coming from a supermarket will be highly efficient with low co2 output. Even in places where local food could be exclusively consumed the effect is negligible. There is a good study about an area of California import 80% of their food but export nearly all food. Study showed the co2 saved would be less than 1% by eating exclusively locally.

A banana from bolivia you buy from a supermarket will quite possibly have a lower co2 emmision than almost thing you buy at a farmers market. Transport and so on is often so inefficient that even having a half full van means supermarkets will always outperform on co2 efficiencies. Pick your own (unless you walk/cycle to it) has got to be one of the most co2 intensive methods of shopping as you are driving an entire car to move a few punnets around.

Also you then get to support the development of the economies and improvement of living standards in those countries.

My favorite comment I read about this was ‘ I suspect the really eco-friendly way to shop is “go to the biggest supermarket you can in the smallest vehicle you can as late in the day as you can”; but cycling to the 24-hr Tesco at midnight isn’t quite as glamorous as striking up a conversation with the surprisingly handsome and well-read farmer’s son in the “local” farm shop’

And just lick some salt if you want potassium.