You're standing in the supermarket contemplating a nice warm-weather meal — maybe grilled fish or chicken and salad. But you worry: Is there any local or organic produce, or does that even matter? Is the salmon wild, or does it come from those fish farms that you hear might not be clean?

In America these days, deciding what to eat is a real problem, says writer Michael Pollan. An abundance of foods tempts us. Multimillion-dollar food marketing and the latest scientific finds (or fads) muddle our thinking. And we don't have centuries of traditional eating patterns to help guide our choices. In the midst of that confusion, we've become heavier and less healthy every year.

To figure out guidelines for what we should eat, Pollan set out on a five-year journey to learn more about where the foods we eat come from and just how safe they are. The result is his new "The Omnivore's Dilemma" (Penguin Press, $26.95). We asked Pollan to interpret some of his findings.

Q. Your initial question was "what should we eat?" How did you go about your research?

A. Before I could figure that out, I had to know what I was eating. So I did the food detective work to trace what was on my plate.

Q. You went all over the country tracking American food — to a cornfield in Iowa, a feedlot in Kansas, organic farms, McDonald's. You even hunted, gathered and grew food for a single meal. Was there anything in particular that surprised you?

A. All that food, that seeming cornucopia of variety, kept taking me back to the cornfield in Iowa. So I followed a bushel of corn to see what you can make from it. I had no idea how much of our industrial system was based on corn, and turning corn into meat and processed food.

Q. What are the implications of having such a corn-based diet?

A. Environmentally, it's dangerous to eat so much of one thing. Nature teaches us not to put all our eggs in one basket. The only way to keep a food monoculture going is with lots of chemicals — they need more pesticides and fertilizer than mixed-cropped farms.

The other side of it is our health. We need a great variety of food. If we don't get it, we're not getting what we need.

Q. You're very critical of the foods made from corn, and processed foods in general. Why?

A. Corn products are not fresh food. To make them, you're using corn as an industrial raw material.

Q. Did researching the book make you re-evaluate your diet?

A. I avoid foods with more than five ingredients on the label, and I eat as few processed foods as I can — in particular, anything made with high-fructose corn syrup. It's not evil, but it's a marker of a highly processed food.

Q. You don't eat fast foods now. Did you ever?

A. I had a kid who liked to go to McDonald's. What turned me off was visiting a feedlot and spending time with a steer through his life. I saw how they live and how we make them into meat. Once you've seen that, it changes the way you eat. And my talking about it turned my son off fast foods.

Q. Do you eat any meat now?

A. I don't eat any industrial meat. I only eat beef that's been fed grass from start to finish.

Q. What about chickens?

A. I do tend to buy organic chickens, but they don't do a lot of free-ranging. And when I can, I buy pasteurized chickens and eggs.

Q. Which is more important: buying locally made or grown foods or organic foods?

A. Given the choice, buy local over organic. Often local food is organic, but farmers may not have the capital to deal with all the paperwork involved.

But I do buy organic chickens, because they aren't fed antibiotics or growth hormones. And I buy organic milk — but I look for milk from cows that have been grass-fed. Sometimes you can find that information on the label.

Q. So what's an ordinary supermarket shopper to do?

A. Go to farmers' markets, where the food is fresh, is more nutritious, and you know it hasn't been processed.

Q. But buying everything at farmers' markets isn't realistic for most people. So what specific advice can you give supermarket shoppers?

A. Read the labels.

And don't eat anything your great-grandmother wouldn't recognize as food.