The food pyramid is crumbling, but will most Americans notice?

Uncle Sam would like you to finish your vegetables - 2-1/2 cups a day, along with two cups of fruit for an average 2,000-calorie-per-day diet. Bonus points if you walk briskly to the store to buy them. Think 90 minutes of exercise, five days a week, if weight loss is a goal.

Last week, as it has every four years since 1980, the US government handed down a new set of dietary guidelines. Primary purpose: to provide a foundation for federal nutrition programs. They also are meant to nudge foodmakers toward providing choices deemed healthier, and - amid what is often called an obesity "epidemic" - to encourage Americans to pay more attention to the role of food and exercise in their lives.

Whether the guidelines will resonate with a culture awash in flavor-of-the-month diets is unclear. But this time, the government appears eager to alter old habits. Set to crumble is the vaunted food pyramid, which has promoted a set of recommended daily servings of assorted foods largely unchanged since 1992.

Government officials and outside medical advisers who helped shape the guidelines would not comment on the likely new look of the pyramid replacement - or "food guidance system," as it is being called - which is expected to be released in coming months.

But experts say it appears clear that, just as the pyramid represented a more comprehensive take on the basic four food groups that came before, whatever emerges next will reflect a fuller awareness of current nutritional science. Many hope that it also allows for more dietary flexibility.

For years, critics of the pyramid have hammered away at what they view as a system either too rigid or simply wrong. An alternative "Mediterranean" pyramid - with olive oil as a central component - has been suggested in recent years by such prominent players as Walter Willet, chairman of the department of nutrition at the Harvard School of Public Health.

Another critic, the late Dr. Robert Atkins of Atkins diet fame, endorsed the idea that protein-packed red meat belonged in that big area at the pyramid's base long occupied by breads and grains - the dreaded "carbs" in the parlance of many of today's dieters.

One result of all the variations has been public confusion. And despite the keep-it-simple proclamation Jan. 12 by Tommy Thompson, Health and Human Services secretary - "You lower your calorie intake, you lower your fats, your carbs, you eat more fruits and vegetables, more whole grain, and you exercise," he said in a press conference - some confusion is likely to remain.

"It's a complicated thing, because from a public- communications standpoint, the more specific you can be, the better," says Russell Pate, associate dean for research at the School of Public Health at the University of South Carolina in Columbia. "But the reality is, there is no single best diet."

"We can eat in various ways that will all conform to the guidelines," adds Professor Pate, who served on the government's advisory committee.

In many cases, others add, different dietary approaches are essential. "We cannot recommend the same diet to an active 20-year-old male as we can to a sedentary 70-year-old woman," says Elisabetta Politi, nutrition manager at the Duke University Diet and Fitness Center.

Health experts applauded the guidelines' new emphasis on physical activity to offset caloric consumption - and a long-awaited acknowledgment that nutrition is not an area where one size fits all. In a shift aimed at lessening confusion, recommended amounts of food are now expressed in more user-friendly "cups" instead of generic "servings."

Not everyone is satisfied. Some critics cite too much influence by corporate interest groups. And the organic-food movement - while delighted to see mention of "nutrient density" and whole grains - is disappointed that the quality and origin of food are not given more official emphasis.

Still others question whether government guidelines will be enough to alter sedentary lifestyle habits that reflect a culture of convenience in which a majority of the American populace is overweight.

Target numbers for calorie intake are fine. But "theapplication of these numbers is where it's going to come into play," says John Jakicic, director of the Physical Activity and Weight Management Research Center at the University of Pittsburgh.

Americans should not view the exercise targets - with which he agrees - as prescribed blocks of time, he says, but as a call for "lifestyle tweaks." Take the stairs instead of the elevator or escalator, he suggests. Exercise in multiple short bouts throughout the day.

In recent years Ms. Politi has discarded the pyramid as a teaching tool because it did not encompass the degree of detail that new nutrition research had revealed.

Carbohydrates are useful fuel for active adults, she says. Distinctions must also be made between foods traditionally lumped into that one maligned category. The nutritional value of a highly refined white bagel, she says, does not approach that of a fiber-packed sweet potato.

The new government guidelines indicate growing attention to such qualitative differences, its supporters say, pointing out that positive response by the food industry - already in motion - should trickle down to a growing number of mainstream consumers.

For example, in light of more detail on grains - the guidelines recommend that half of all grains consumed be whole grains - General Mills, a major cereal producer, recently introduced more whole-grain products. Sara Lee, the baked-goods giant most people probably associate with coffeecake, just launched a "bread nutrition" website.

Along with some other major firms, Kraft has reportedly taken action on fats in its products. (The new US guidelines distinguish among saturated fats, trans fats, and the "good" fats found in fish and nuts.)

Trans fats - artificial fats made when hydrogen gas reacts with oil - have already made it onto food labels required by the Food and Drug Administration. That kind of attention to nutritional science was built into the guidelines process.

"This was the first time the advisory committee was tasked with making evidence-based recommendations," says Pate. "Everything in our report had to pass the test of being adequately supported by scientific evidence."

Pate - the first expert on physical activity to land a spot on a guidelines-advisory committee - recalls a very public process of compiling and reviewing such evidence. The result, he says, was group interpretation, not a clamoring to advance viewpoints or give special interests sway.

"There was no direct contact between any industry or trade group and any member of the committee during the process," he says. He allows that the views of industry representatives did show up in the "boxload" of documents that arrived every couple of weeks - shipments that also included public input and the positions of such groups as the American Heart Association.

But critics still maintain that some of the recommendations point to the enduring power of lobbies.

"The USDA must take the Big Meat, Big Sugar, and Big Dairy industries' money and influence out of the guidelines process," said Neal Barnard, president of the Physicians Committee for Responsible Medicine, in a release hours after the guidelines were announced.

The group, which promotes a diet of food only from plant sources, maintains that the federal guidelines should "specifically recommend Americans avoid meat, dairy, and fish."

Less hard-line critics have other concerns. The recommendations that preceded the guidelines included some discussion of the trade-off between the positive aspects of Omega 3 fats in fish and the danger of pollutants in wild fish, for example. But that concern was not prominent in the guidelines.

The guidelines' recognition of the value of whole grains is a good start, says Sara Tedeschi of the Organic Valley Family of Farms in La Farge, Wis., but she would like to see federal incentives for the eventual use of organic produce in, for example, school lunches.

"[The government is] so careful to never cross that line and talk about organics even in the same realm as nutrition," she says.

"I do think there's a movement in society now toward knowing more about the foods you eat, and eating well," says Yvonne Bronner, director of the public-health program at Morgan State University in Baltimore and another advisory committee member.

"What [the new set of guidelines] does is clearly lay out foods that a person could choose from to have a diet at different calorie levels," she says. They introduce choice, she says, and stress balance.

Given Americans' fondness for convenience and leisure, Pate doubts the new guidelines are going to motivate most people to push back from computers and television sets and return to the eating habits and hard work of their ancestors. But he's hopeful.

"The solutions here are going to have to be more involved than just saying we have to get back to where we came from," he says. "On the other hand, we're pretty good at figuring out solutions to problems once we recognize that they exist - and that they are a threat to our well-being."