Once whole grains were the darling of back-to-nature types, consigned to hearty brown bread and healthy cereals.

What a difference a couple of years makes. Stroll down any aisle in the grocery store and you'll see an explosion in whole-grain foods and product claims in unexpected places, from whole-wheat English muffins to sugary kids' cereals and frozen entrees. Even makers of microwave popcorn are jumping on the bandwagon.

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Why the hunger for whole grains? New government nutrition guidelines, issued this winter, advise Americans to eat at least 3 ounces daily to improve their health and reduce the risk of chronic diseases such as heart disease and diabetes. A revamped Food Guide Pyramid, due this spring, is expected to put that advice into easier-to-understand visual form. And the South Beach diet, with its emphasis on good carbs and bad carbs, also encouraged many to give whole-grain foods another try.

Most Americans don't come close to eating the recommended three 1-ounce servings of whole grains daily. Just 10 percent of Americans consume that much, says Harry Balzer, who studies eating patterns as a vice president of the marketing research firm NPD Group.

To follow the new guidelines, most Americans will need to switch from the refined grains found in white bread, pasta, white rice and refined wheat flours used for crackers, cookies and cakes. Instead, they should choose such foods as whole-wheat bread or pasta, oatmeal, brown or wild rice, popcorn, whole corn, barley, rye and buckwheat.

Consumers are getting the message, some food manufacturers say. Some 66 percent of shoppers say they're looking for whole, unrefined grains, according to a survey released this month by the Grocery Manufacturers of America, an industry lobbying group. Bakeries have seen demand for whole-wheat bread increase, and sales of white bread slow.

Earth Grains and Sara Lee once downplayed whole grains on bread labels because consumers believed they didn't taste good, says Matt Hall, a spokesman for Sara Lee Bakery Group. Now labels trumpet "whole wheat" in large type, and the company has introduced several new varieties.

Yet persuading Americans to make the switch a lasting one might be tough, Balzer says.

"We don't eat foods that don't taste good," Balzer says. "I imagine we'll see a bump in whole grains for a while, but will there be a structural change? It's going to be slow."

Food manufacturers have been trying to make whole grains more palatable to those who continue to believe they won't like them. A variety of white wheat flour made from whole grains produces bread that's similar to refined white bread in taste and appearance, yet has the fiber and other nutrients of traditional whole-wheat breads made with red wheat flour. It's slowly starting to turn up in some packaged foods, as well as being available in flour form from King Arthur Flour and other millers.

Aside from taste, another stumbling block for choosing whole grains has been how tough it is to identify which products contain them. Wheat bread, for example, isn't the same thing as whole-wheat bread; you need to read the ingredient list and look for whole-wheat flour as the first item to ensure you're buying whole-wheat bread. The Whole Grains Council, an industry group, is trying to make it easier.

A new stamp from the Whole Grains Council identifies products as a good or excellent source of whole grains. The Food and Drug Administration is considering a petition to allow a similar claim on package labels; some manufacturers already are using it. To get 3 ounces of whole grains, you'd need to eat six servings of food labeled as a "good source," or three servings of an "excellent source."

The dietary guidelines recommend whole grains to boost fiber and other nutrients and to help with managing weight. When grain is refined, much of the dietary fiber is removed, along with vitamins, minerals and other beneficial nutrients. Unrefined, or whole, grain still contains the whole grain seed, or kernel — including the bran, the germ and the endosperm.

"Whole grains are a wonderful source of vitamins and minerals," says Dr. Jim Hill, director of the Center for Human Nutrition at the University of Colorado in Denver and an adviser to the Grain Foods Foundation, an industry group.

"This is wonderful fuel for physical activity. This is the kind of fuel your muscles need," says Hill, who co-founded America on the Move, a walking program. "The more whole grains you include in your diet, the better it is to manage your weight."