After devoting decades to designing a food pyramid, then refining that design with colored stripes and steps, the nation’s nutrition experts have finally settled on what they believe is the perfect geometry to represent what we should eat: a plate.

Circular, with four colorful divisions to represent the four main food groups, the new plate looks just like a pie chart — a description experts shun because, well, pie isn’t good for you.

Indeed, arriving in the midst of an obesity epidemic, this new at-a-glance guide to healthful eating is meant to remind consumers to limit heavy foods like pie and beef up instead on the greens.

“MyPlate” promotes fruits and vegetables, which cover half the circle. Grains occupy an additional quarter, as do proteins such as meat, fish and poultry. A separate circle (looking remarkably like an aerial view of a cup) represents “dairy” and rests to the side. Desserts appear to have been banished — like the pyramid — to the desert.

The message is clear: “Make half your plate fruits and vegetables,” said Robert Post, an official at USDA’s center for nutrition policy and promotion.

The Obama administration has high hopes for establishing the brightly colored image as a ubiquitous consumer icon. Post said the USDA is targeting food producers, health insurers, restaurants and schools as partners in promoting the image.

At a media-heavy rollout Thursday morning at USDA headquarters, the famously foodie first lady presided, focusing on the obesity problem in children.

“Kids can learn to use this tool now and use it for the rest of their lives,” Obama said. “It’s an image that can be reinforced at breakfast, lunch and dinner.”

USDA will bring the image to “essentially all” schools in the country via the agency’s breakfast, lunch, afternoon snack and other nutrition programs, Post said.

Agriculture Secretary Tom Vilsack said the new “food icon” was designed to help slim Americans’ expanding girths: Two-thirds of American adults and one-third of children are overweight or obese.

“The costs associated with obesity are enormous,” Vilsack said, adding that the image popped into his head at just the right moment during dinner recently. A steak arrived covering “three-quarters” of his plate. “I didn’t eat it all,” he said.

Nutritionists pointed out that the plate image does not suggest portion sizes, only the ratios in which foods should be eaten.

Still, with the White House vegetable garden in full leaf, Obama armed her crusade against the country’s obesity problem with what nutritionists and food lobbyists are already calling a powerful image.

“It’s brilliant in its simplicity,” said Robb MacKie, head of the American Bakers Association, which represents bread makers. “It’s something the average American can look at and get a visual feel for how they can fill up a plate at a meal.”

But not all of the reviewers are raving. Some nutritionists would like to see more detail on USDA’s plate, which fails to direct consumers away from slathering their vegetables in butter or lard.

Walter Willett, a Harvard University nutrition researcher, took issue with the glass of milk on the side. “There really is no scientific basis” for encouraging three servings of dairy a day, he said. “I think the USDA has trouble telling people to consume less [dairy]. They have a whole division telling people to eat more cheese.”

While she lauded the plate as “a good first step,” Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest said that “one icon is not going to change the way Americans eat.” Consumers also need affordable healthy options at schools and restaurants, she added.

To avoid upstaging the first lady, the USDA made a select group of academics and food industry representatives sign non-disclosure forms at a private unveiling of the image three weeks ago, several sources said. Still, word leaked, leading to early rave reviews from hard-to-please corners of the foodieverse — and sighs of relief that the food plate’s predecessor, USDA’s confusing MyPyramid, had finally been dismantled.

As a 2005 update to the original food pyramid, MyPyramid eschewed words and depictions of food, featuring instead colored bars streaking down from a pyramid’s apex and a stick figure running up the side. “It was foodless and useless,” said Marion Nestle, a New York University nutritionist. And it was impossible to decipher without logging onto the USDA Web site.

But the meaning of the new image, a colorful wheel covered with grains, vegetables and meats, is instantly clear, said Wootan. “I saw it for 20 seconds maybe,” she said, referring to the private unveiling. “And I can picture it exactly.”

Already, the fruit and vegetable lobby is going bananas.

“This is a really, really big deal,” said Lorelei DiSogra, vice president of nutrition and health for the United Fresh Produce Association, the small but feisty lobby for fruit and vegetable producers. “It works on a plate, a bowl, a lunch bag,” she said of the concept of making half of every meal fruits and vegetables.

DiSogra has a special right to express delight: She oversaw the creation of the plate icon in 2004 while heading up a healthful-eating initiative at another government agency, the National Cancer Institute.

Seeking an easy visual cue to remind people to eat five fruits and vegetables each day — NCI’s so-called 5-A-Day program — DiSogra and her team hired a public relations firm, Porter Novelli. Advertising whizzes there sketched a plate half covered with carrots, lettuce, cucumbers and other greenery.

A test marketing campaign with “every cultural and ethnic group in the country” proved the image an effective and easily grasped promotional tool, DiSogra said.

NCI officials offered the image to USDA and the Department of Health and Human Services as a replacement for the original food pyramid.

The two agencies said no to the plate. “The mantra at USDA at the time was there is no such thing as a good food or a bad food,” Willett said. The plate broke that rule by banishing meat to one corner.

Seven years later, USDA has reversed course. The agency finally adopted the plate after additional market testing among some 4,000 consumers over the past year, said the USDA’s Post. Several other groups, including the American Diabetes Association and the American Institute for Cancer Research, already use very similar plate images in their healthy lifestyle campaigns.