NASHVILLE, Tenn. — For 12 weeks, this has been the incredible shrinking city.

The equivalent of about 6,000 people simply has disappeared, and everyone is just tickled pink about it.

The “culprit” is the Rotation Diet, which has helped the Music City melt almost a million pounds of fat — and may squeeze Nashville into the Guin-ness Book of World Records for weight loss.

Since the diet’s introduction in January, local vegetable bins have been pillaged of lettuce and broccoli while triple-layer cakes languish on grocery shelves. In the wee hours, early risers are seen briskly walking through city streets. Nashvillians ask their neighbors: Are you rotating?

Such fervor is the work of a self-effacing, formerly fat Vanderbilt University psychologist, Martin Katahn, who a few months ago scarcely dreamed he could change the habits of thousands. Now he wants to put America on a diet.

“I never expected all this to happen, but it’s great fun,” Katahn said recently as he prepared for his weekly Rotation Diet update on a local television show. “Millions of people, you know, are going to start doing it — all at once, just as we did.”

Houston; San Antonio, Texas; Greenville, S.C.; Portland, Ore.; and other cities already have joined the ranks. Even Nottingham, England, has challenged Nashville to a weigh-off. And this week Rotation Diet representatives are coming to Florida to talk to Publix and Winn-Dixie supermarket chains, possible sponsors of a Sunshine State weigh-in.

Katahn’s book, The Rotation Diet, reached bookstores nationwide late last month , and a videotape of helpful hints and encouragement is due out soon.

The fervor began unexpectedly in January, when Katahn asked for participants in a seminar at his Weight Management Program and announced that he had developed a new diet. Ever an optimist, he planned for 80 people.

Soon, phone lines began lighting up and he requested a larger lecture hall. About 1,400 potential dieters showed up, lining the aisles and spilling out the doors. Two nights later, at a quickly arranged second seminar, another 1,500 turned out.

When a local television station put Katahn on a talk show, he offered free copies of the diet. Within a week, the station had received 10,000 requests.

Katahn, who lost 75 pounds 23 years ago and kept it off, can only guess at why the response was so stunning.

“Maybe there was an element of surprise,” he said. “For so many years I had written about the dangers of low-calorie diets, and then I introduced one.”

The 1,200-calorie diets he had advocated simply didn’t work well enough, Katahn said. Dieters lost only one pound a week and became discouraged. Those who follow the Rotation Diet can lose of a pound a day, he said.

During the diet’s first three days, women restrict themselves to 600 calories. They increase to 900 calories for four more days. The second week provides for 1,200 calories, and the 600/900 combination is repeated for the third week.

Dieters then must take a break from dieting the fourth week. For men, the same cycle calls for 1,200-, 1,500- and 1,800-calorie counts daily.

The diet also calls for 45 minutes of exercise a day and specifically recommends brisk walking. Katahn also advises his dieters to drink eight glasses of water a day and allows unlimited eating of certain vegetables and at least three fruit snacks a day. The extra fruit and vegetables are not included in the calorie counts.

Katahn said the short duration of the diet prevents a “starvation response,” which causes the body to store extra calories when dieting stops and creates sudden weight gain.

His method has made him the latest diet guru and has won him zealous converts.

“I can meet the man who saved my life,” 30-year-old Elaine Harper said as she clutched her Rotation Diet book at an autograph-signing. “This is a miracle, it really is. It’s something you can stay with.”

Nashville’s most convincing testimonial comes from a man whose job is temptation itself.

Burt Hummell, president of Robert Orr/SYSCO, a food distributor, spends several hours a day tasting delicacies that range from chocolate mandarin cheesecake to fried dill chips. His office freezers are stocked with frozen fruit juice bars, rich ice cream and breaded shrimp.

He distributes 10,000 items and must taste them all at some point.

Between professional and recreational eating, Hummell slowly grew to 281 pounds and could shop only at stores catering to “big and tall” men. His doctor told him there was nothing wrong with him — except that he ate too much.

A year ago, Hummell became one of the guinea pigs of Katahn’s new diet. He lost 71 pounds and 8 inches in his waist and kept it off. He went from virtually never exercising to walking for an hour at 4:30 a.m. every day.

“I still love food — it’s my life,” said Hummell, 53, who hopes to slim more on the diet. “What I’m trying to teach myself is to eat to live and not to live to eat.”

Hummell is at least partly responsible for the sudden run on vegetable carts in Nashville. He, Katahn and publicist George Schnitzer met shortly after the first signs of diet madness and decided to arrange a community-wide weigh-in.

With a few phone calls, Hummell persuaded Kroger management to carry doctor’s scales and diet pamphlets in its stores. They expected about 500 shoppers per store to participate.

Eventually, 18,000 people weighed in at Kroger stores and 6,000 at Vanderbilt, and about 76,000 pamphlets were sent out. Several restaurants began offering the Rotation Diet on their menus and other restaurants fielded special orders. An office tower downtown lighted a sign saying, “Diet, diet, diet,” and other billboards went up to promote a “Melt-a-Million” campaign.

Phone calls to the Weight Management Program flooded the Vanderbilt switchboard until special answering machines could be set up.

“I wish I could convince potential employers I sat down and planned this whole thing,” said Schnitzer, who carries apples in his briefcase and lost 15 pounds on the diet.

“If we had set out last fall to get 20,000 people to sign up in grocery stores to go on a diet, we never could have done it on purpose,” said David Schlundt, Vanderbilt psychologist and research director. “I think once we’ve done it here, it could be repeated elsewhere.”

The Rotation Diet’s success, Katahn said, grew from the overwhelming community support. Weinco Co., a promotions firm, is contracting with grocery stores in other cities to repeat the feat in a more commercial way, with the diet pamphlets still free to the public.

The sponsors also will try to get television stations and restaurants to back the diet, as they did in Nashville.

Calorie-counting has died down in much of Nashville. But many people say their eating habits and lifestyles have changed.

At his law firm, Walter Searcy and several co-workers leave promptly at 3:30 p.m. every day for a brisk, three-mile walk. Clients have learned they can’t schedule appointments during that time, Searcy said. His briefcase also contains several apples for workday snacks.

“It just makes good sense,” he said while changing from running shoes back to street shoes. “I probably haven’t felt as good about doing something for my body since I quit smoking cigarettes.”