Fact from Michael Pollan's book The Omnivore's Dilemma: A Natural History of 4 Meals: a single strawberry contains about five calories. To get that strawberry from a field in California requires 435 calories of energy.
The book is full of such interesting bits of information--also some disgusting ones (don't ask about fecal dust in cattle feedlots--or rather, do, if you are interested in what goes into your food). I'm reading it along with eating locally this month, and it's eye-opening.
He follows the food chain back to its sources--a corn field in Iowa, a feedlot in Kansas, farms that practice Big Organic (Earthbound Farms, for instance, from whom you have probably bought lettuce) and Small Organic (family-type, sustainable farms). In the last section, he forages and hunts his dinner--I haven't gotten that far. I'm just past the feedlots (fecal dust!), starting on his dinner from Whole Foods.
Here's an excerpt. If you're interested in this kind of thing, it's required reading. His earlier book, The Botany of Desire, which focuses on 4 particular plants (potatoes, apples, tulips, and marijuana) is also very good. (Check out that organic smile, right.)
For more diverse and interesting opinions on the local eating challenge, check out the ELC website.