Eating local is not a new concept in health consciousness. The sooner the piece of food goes from the earth to your mouth, the better. As it sits, whether being transported from, say, Brazil to your grocery store, or from your grocery store to your fridge, it loses its nutrional value. I learned this during my nutrion class at UMass back in 1998. But it makes sense.
If you think of an apple: when you bite into it, then put it down, then remember it about a half hour later, it's a little brown & wilted where it's been exposed and sitting around. The same things happens with food, even if it's been protected during shipping. That's why food rots.
Now, while it may not be easy or even economical for one to buy fresh food from the local co-op -- let's say because it's just you and your roommate at home -- there's little things you can do to mimic the local-vore diet. At my grocery store, they say where the food comes from "Strawberries from New Zealand" or "Apples from Washington". What I try my best to do is buy the "local hothouse tomatoes" as opposed to the ones from far far away.
Debates abound on whether organic wins out over local. Is a locally grown tomato better for the earth and for you than an organically grown tomato from across the country? You figure in the gas mileage and deterioration of nutrients during the transport of the organic veg vs the pesticides used in the local farm and the conclusion is...well, people still argue.
Here's the thing that I think about: whatever I can do to avoid artificial flavors and preservatives and "high fructose corn syrup" is the thing I should focus on the most. Next is locality. If that's not an option or if that is an option in both organic and non-organic form, go for the organic, if you can afford it. It's awfully hard to make a good go of the organic stuff these days with the poop economy, believe me, I know.
But if your choice is between fresh spinach not organically grown from Maryland, real cheese from Wisconsin cows and whole wheat pasta processed in Italy versus a Lean Cuisine Spinach Alfredo - go with the fresh stuff, always. Even if it's not local or organic, it's better than the over-processed, over-preserved, energy consumptive microwave meal. Not only has that stuff come from far away to get to the plant where it's made, but then it's added with preservatives. Then it's frozen. Then it's shipped. Then it stays frozen in the grocery store. Then in your freezer. Then gets zapped. Then you eat it and it's mushy and funky and almost tastless, and you've lost twice as much nutrients and used up twice as much energy than if you went with the fresh stuff.