It may come as a surprise to some, but even in a densely populated area like the close-in suburbs of Washington, D.C., farmers markets are numerous and popular. Within five miles of where we live, there are at least five or six large, weekly venues where the quality of goods puts the grocery stores to shame.

I wouldn’t go so far as to say shopping at the markets is “buying local.” The fact is, nearly all of the farmland that existed within an hour’s radius 20 or 30 years ago has given way to planned communities, shopping malls and office parks.

Most of the farmers at markets we frequent come from Virginia, Maryland’s eastern shore and Pennsylvania — a 90 minute to 2 hour drive or more. Obviously, the demand for fresh foods in this highly populated area is great enough to make the trip profitable for the farmers.

The point is, we shop the markets because we find it enjoyable, the foods are more nutritious and better tasting, and by supporting the farmers, we hope they’ll continue supplying our communities. Until fairly recently, we never thought about whether it took more or less energy to bring those “local” items to our table than the supermarket foods that typically travel much greater distances.

One of the justifications often used to support claims that buying local is indeed less energy consuming is the notion of food-miles. The basic idea is that the more miles food travels to get to your table, the more energy was required to get it there. Shorter distance, less energy. Makes sense until you start to think about it.

The problem is, transportation costs are only part of the total energy required to produce and deliver food. I have no idea whether the methods used by our “local” farmers are more or less energy efficient than those used by more distant growers. I do know that agricultural runoff from Pennsylvania farms is a major contributing factor to the ill-health of our Chesapeake Bay. Shouldn’t I consider that environmental cost against the benefit of any energy that might be saved in transport? Rather quickly, it becomes evident that food-related energy and environmental impacts aren’t as easy to assess as they first appeared.

As for food-miles, is it really more energy efficient to transport many small payloads in small trucks a short distance than it is to ship a much larger load a significantly greater distance in an 18-wheeler? How do we factor in the probability that the farmer’s trip home is an empty run, whereas the 18-wheeler is more likely to haul another load on his return? And how do these compare to the energy efficiency of shipping by air? I honestly, don’t know, but have an inkling the answer probably includes a qualifying, ‘that depends.’

Ronald Bailey, in a November 4 article at reason.com, “The Food Miles Mistake: Saving the planet by eating New Zealand apples,” attempts to make sense of food-related energy consumption. You may not like his tone. You may not like what he has to say. But he definitely deserves a read.

For a different take on the subject, listen to David Steinman’s interview — right here at Ecology Radio — with Jonathan Harrington, author of “The Climate Diet.”