From Grist.org:
What’s the most worrisome aspect of the current U.S. food system?
That’s a tough one. But the thing that really struck me is just how much energy goes into the process. The most recent study I’ve seen, from the University of Michigan, says that 20 percent of our fossil-fuel consumption is going to feeding ourselves. This happens at three different stages. One is on the farm, because we use synthetic nitrogen fertilizer, which is made from natural gas and a great deal of electricity. Then we take commodity crops, such as corn and soybeans and wheat, and we process them intensively, adding another seven calories of fossil-fuel energy for every one calorie of food. It’s a very intensive process to take the corn and turn it into the high-fructose corn syrup, or take the corn and turn it into the chicken, and the chicken into the Chicken McNugget. As we move further away from eating food to eating highly processed, complicated food products — as we move from yogurt to Go-GURT — it takes more energy, and more energy in the packaging. We’re putting a lot of time into redesigning our whole food supply so we can eat in the car. Nineteen percent of meals [and snacks in the U.S.] are eaten in the car right now. And then we drive [the food] around the country, if not fly it around the world. You can get your organic asparagus from Argentina, you can get your grass-fed beef from New Zealand.
So given that our most serious environmental problem is global warming, I’d have to say the most serious problem with the food system is its contribution to global warming.