Marion Nestle swings through the turnstile of a supermarket, her observational antennas already on high alert.

Nestle, professor of nutrition, food studies and public health at New York University, loves food. In two books — "Safe Food" and "Food Politics" — the outspoken professor took on topics such as industry influences on government food guidelines. Now she has written a practical book for consumers: "What to Eat: An Aisle-by-Aisle Guide to Savvy Food Choices and Good Eating."

Cruising the aisles, Nestle points out that supermarkets carry about 40,000 products, and most of the center aisles are stocked with ones you don't need for a healthy diet.

"Oh, my God!" exclaims the nutrition researcher, food activist and self-styled supermarket anthropologist, abandoning her cart and practically running down the front of the store. She pulls down cans to read labels, and stares up at the mountains of sweet drinks that cascade from the end of the aisles like postmodern sculptures.

The petite New Yorker pauses to do some calculations, then pulls a tiny digital camera from her blazer pocket and snaps a shot.

"The real estate devoted to this!" she exclaims. "This entire double aisle ... colas! Sprites! Candies!"

Americans are confused about food, Nestle says, and in large part she blames the food industry's powerful marketing, federal agencies interested in supporting that industry, and supermarkets that have made a science of cluttering the path to healthful items you need (such as milk, always in the back of the store) with unhealthful items you don't (such as snacks, candy and sodas in the central aisles and at the front).

Her first tip: Stick to the periphery of the store. That is where you will find the necessities to eat well.

We begin on the outer edge, in the produce section.

START WITH PRODUCE

Nestle suggests that local produce be first choice — because the longer it takes to move a food to where you buy it, the less fresh it is. Country of origin isn't always easy to ascertain, she says, because produce companies aren't required to state this. (She says pressure from the food industry led Congress to postpone until 2008 a law making such information mandatory.)

Second choice would be certified organic — not for nutritional reasons (so far, scientific evidence for nutritional differences are negligible) but because organic farmers use fewer pesticides and keep the water cleaner and the soil richer. She points out the tiny "9" in front of a product's price look-up number (or PLU, used by clerks to identify produce) that identifies a food as certified organic.

Our produce purchases done, we move on to the dairy section.

"Oh, dear, it's gotten so difficult," Nestle says, looking at the wall of colorful milk cartons of 1 percent, 2 percent, whole, skim, organic, half-gallons, quarts, cream, nondairy creamers, soy milk and Lactaid. She recommends 1 percent or skim to get the benefits of milk without the saturated fats.

Again, she recommends organic. "If you don't want your kids to have milk with pesticides in the feed, or hormones from genetically modified cows, or all the other stuff that gets put in there that is bad, or not bad, you buy it," she says.

I buy it, but I pay about $1 more for a quart. That difference could quickly add up in a house with two small milk drinkers.

Over to the yogurt — a handy snack that, she says, has "an aura of health." Maybe so, Nestle says, if it is plain, low-fat or nonfat and with no added sugar. That type is pretty hard to find among the 40 or so brands.

"The amount of sugar in these things is staggering," she says. "I call these desserts."

LOAVES AND FISHES

Next up: The seafood counter, probably the most nerve-racking part of the store for a health-conscious shopper.

Nestle spends more than 50 pages on fish in her book, discussing methyl mercury in fish, fish farming, labeling, safety and sustainability. Her basic rules boil down to: Find a fishmonger you trust, try to buy wild — not farm-raised — fish and carry a fish information card to help make your selection. (The Seafood Choices Alliance, www.seafoodchoices.com, has such a guide.)

Nutritionists largely agree with what Nestle says — but express reservations about the practicality.

Susan Bowerman, of the Center for Human Nutrition at the University of California-Los Angeles, says Nestle's recommendations are certainly thorough, but "the reality is that consumers struggle with so many issues when it comes to food purchases. ... So many choices will ultimately depend on individual priorities" such as taste, convenience, and, for many, cost.

For example, wild fish are expensive — and the benefits of eating fatty fish, farmed or wild, outweigh not eating it at all, says Andrea Giancoli, a Los Angeles-based spokeswoman for the American Dietetic Association.

We head to the bakery for bread — and encounter, yet again, a huge selection.

For whole-grain bread, Nestle says, find loaves stating they're 100 percent whole wheat, listing whole-wheat flour as the first ingredient, and with at least 2 grams of fiber per ounce. Many commercial bakers add back wheat bran or cracked wheat to make white bread look like whole wheat.

CHOOSY WITH CEREAL

We finally venture into the middle of the store for cereal.

She grabs a box of whole-grain Peanut Butter Cookie Crisp cereal from General Mills, with a cartoon character on the box, along with the manufacturer's proclamation that it's a good source of calcium and whole grains, with 12 vitamins and minerals.

She flips to the nutrition label and counts off at least seven kinds of sugar.

The most healthful choice, she says, is unsweetened cereal, and the place to look is the top shelves. Companies often pay to get their wares placed at eye level, and those that can afford that are usually large companies selling cereals with added sugar in attractive boxes.

I buy old-fashioned shredded wheat.

Stumbling into the daylight, Nestle swings a bag of souvenir cereals she has purchased. She will take the best (and most outrageous) back to her office to add to her academic cereal box collection.

"I am just staggered by how the marketers dream this stuff up," she says, "by how they get away with this. And why people fall for it."