Why are Americans so fat? According to Michael Pollan, it's not just supersized portions and sedentary lifestyles that make obesity the second-highest cause of preventable death in the United States. It's corn.

When exploring the causes of the obesity epidemic, Pollan, a contributing editor to the New York Times Magazine and proponent of "food-chain journalism," focused on the subsidized overproduction of corn. One result is a surfeit of high-fructose corn syrup, which accounts for 20 percent of the daily calories of many children.

Our dependence on maize, he explains, is an environmental problem as much as a public-health one: "Modern corn hybrids are the greediest of plants, demanding more nitrogen fertilizers than any other crop. Runoff from these chemicals finds its way into groundwater and into the Mississippi River and the Gulf of Mexico, where it has already killed off marine life in a 12,000-square-mile area."

Pollan's best-selling book, The Botany of Desire, was published in 2001. He teaches writing at the University of California at Berkeley Graduate School of Journalism.

Sierra: How has your work influenced your eating habits?

Pollan: When you learn about the industrial food system, certain foods become unappetizing. Now that I know how supermarket meat is made, I regard eating it as a somewhat risky proposition. I know how those animals live and what's on their hides when they go to slaughter, so I don't buy industrial meat. I won't say I don't ever eat it because I don't reject things people serve to me; I respect the host-guest relationship, to the point that it can override my environmental ethic or sense of personal safety.

At home I serve the kind of food I know the story behind. My work has also motivated me to put a lot of time into seeking out good food and to spend more money on it. It's a worthwhile thing to do from a selfish point of view--it's invariably fresher, better food--as well as from an altruistic point of view.

Sierra: It doesn't seem to be making you fat.

Pollan: High-quality food is better for your health. When you go to the grocery store, you find that the cheapest calories are the ones that are going to make you the fattest--the added sugars and fats in processed foods. The correlation between poverty and obesity can be traced to agricultural policies and subsidies.

Corn is an efficient way to get energy calories off the land and soybeans are an efficient way of getting protein off the land, so we've designed a food system that produces a lot of cheap corn and soybeans resulting in a lot of cheap fast food. The added sugar in our diet is coming from corn, and the added fat is coming partly from corn but mostly from soybean oil. Everything at McDonald's is, in some shape or form, a product of corn and/or soybeans.

Sierra: Both of those crops are now widely grown in genetically modified versions. Do they provide any benefits?

Pollan: Genetically modified organisms are a tool, and tools help you do what you want to do. So what is it we want to do? We need to stop spraying so much pesticide. Are GMOs the only way to do that? No. There are other ways: We can plant a polyculture instead of a monoculture, for instance. But Monsanto doesn't like that strategy because it wants to sell as much of its product as possible. So far, GMOs have mainly been a way to sell more Roundup herbicide.

The first generation of GMO products offered the consumer nothing. The food was not cheaper, and it was still grown with pesticides--and in some cases required even more pesticides. In the late 1990s, the companies told me about this second generation of products that was going to provide superior nutrition. Where are they?

We still have the same crops that were rolled out in 1996. It suggests that either the capital to do research and development is drying up, or they've found it's harder than they thought to make these more complex products work. Either way, the industry is on the ropes. I don't think in ten years we'll be talking about GMOs. I can easily see the industry withering away.

Sierra: Can corporate agriculture be reformed?

Pollan: There already has been reform. Perhaps more than any other, the food industry is very sensitive to consumer demand. Every major food company now has an organic division. There's more capital going into organic agriculture than ever before. If consumers make good choices, the industry will respond. Will it be everything we hope? Probably not.

They didn't come up with organic, after all. That came from small farmers and consumers working together in relative obscurity. We need to sustain a noncorporate food chain to serve as the antennae for culture and agriculture. Whatever works will be picked up by the larger companies.

Sierra: You've expressed mixed feelings, though, about large food corporations jumping on the organic bandwagon.

Pollan: It's a very mixed bag. If you have organic Coca-Cola you're still feeding people junk and making them fat. Additionally, the high-fructose corn syrup used in it would still probably come from a monoculture of corn. When you go to monoculture you've got huge problems with pests, weeds, and pathogens, so you'd become very dependent on organic pesticides and fertilizers. On the other hand, if thousands of acres of corn in America will no longer be sprayed with the notorious herbicide atrazine, that is a good thing.

The answer to either/or questions is "both": We need corporate organic and we need true organic. When Wal-Mart and McDonald's start selling organic food, it will drive down the price to farmers and risk growing a new monoculture. On the other hand, the whole country will be educated about the virtues of eating organic food. So the center will move, which is how change always comes to this country.

When the choice comes down to industrial organic or local, I opt for the local, because it supports much more than good agricultural practice. It also tends to support polyculture, since local farms are usually diversified, and it helps to stop suburban sprawl by keeping small farms in business.

Sierra: That sounds like the "Slow Food" movement.

Pollan: People in Slow Food understand that food is an environmental issue. They're interested in the biodiversity of crop plants and food animals, and understand that the culture surrounding food is vitally important, just getting people to sit down together for meals and eat as families. Why don't we pay more attention to who our farmers are? We would never be as careless choosing an auto mechanic or a babysitter as we are about who grows our food. Slow Food is nurturing a culture that demands that information.

There's been progress toward seeing that nature and culture are not opposing terms, and that wilderness is not the only kind of landscape for environmentalists to concern themselves with. That's very encouraging for someone whose stock-in-trade is ideas. It's heartening to see that these conversations, this sort of writing, can have an effect on how people look at, and decide what to do with, a piece of land. I have had the good fortune to see how my articles have directly benefited some farmers and helped build markets for their products in a way that preserves land from development. That makes me a hopeless optimist.

Helen Wagenvoord is a writer in Oakland, California.