Fat in Japan? You’re breaking the law.

Editor's note: David Nakamura also contributes to The Atlantic's excellent Food blog. Read his personal take on this story, which includes the shocking revelation of his own waist size.

TOKYO, Japan — In Japan, being thin isn’t just the price you pay for fashion or social acceptance. It’s the law.

So before the fat police could throw her in pudgy purgatory, Miki Yabe, 39, a manager at a major transportation corporation, went on a crash diet last month. In the week before her company’s annual health check-up, Yabe ate 21 consecutive meals of vegetable soup and hit the gym for 30 minutes a day of running and swimming.

“It’s scary,” said Yabe, who is 5 feet 3 inches and 133 pounds. “I gained 2 kilos [4.5 pounds] this year.”

In Japan, already the slimmest industrialized nation, people are fighting fat to ward off dreaded metabolic syndrome and comply with a government-imposed waistline standard. Metabolic syndrome, known here simply as “metabo,” is a combination of health risks, including stomach flab, high blood pressure and high cholesterol, that can lead to cardiovascular disease and diabetes.

Concerned about rising rates of both in a graying nation, Japanese lawmakers last year set a maximum waistline size for anyone age 40 and older: 85 centimeters (33.5 inches) for men and 90 centimeters (35.4 inches) for women.

In the United States, the Senate and House health care reform bills have included the so-called “Safeway Amendment,” which would offer reductions in insurance premiums to people who lead fitter lives. The experience of the Japanese offers lessons in how complicated it is to legislate good health.

Though Japan’s “metabo law” aims to save money by heading off health risks related to obesity, there is no consensus that it will. Doctors and health experts have said the waistline limits conflict with the International Diabetes Federation’s recommended guidelines for Japan. Meantime, ordinary residents have been buying fitness equipment, joining gyms and popping herbal pills in an effort to lose weight, even though some doctors warn that they are already too thin to begin with.

The amount of “food calories which the Japanese intake is decreasing from 10 years ago,” said Yoichi Ogushi, professor of medicine at Tokai University and one of the leading critics of the law. “So there is no obesity problem as in the USA. To the contrary, there is a problem of leanness in young females.”

One thing’s certain: Most Japanese aren’t taking any chances.

Companies are offering discounted gym memberships and developing special diet plans for employees. Residents are buying new products touted as fighting metabo, including a $1,400 machine called the Joba that imitates a bucking bronco. The convenience store chain Lawson has opened healthier food stores called Natural Lawson, featuring fresh fruits and vegetables.

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Under Japan’s health care coverage, companies administer check-ups to employees once a year. Those who fail to meet the waistline requirement must undergo counseling. If companies do not reduce the number of overweight employees by 10 percent by 2012 and 25 percent by 2015, they could be required to pay more money into a health care program for the elderly. An estimated 56 million Japanese will have their waists measured this year.

Though Japan has some of the world’s lowest rates of obesity — less than 5 percent, compared to nearly 35 percent for the United States — people here on average have gotten heavier in the past three decades, according to government statistics. More worrisome, in a nation that is aging faster than any other because of long life spans and low birth rates, the number of people with diabetes has risen from 6.9 million in 1997 to 8.9 million last year.

Health care costs here are projected to double by 2020 and represent 11.5 percent of gross domestic product. That’s why some health experts support the metabo law.

“Due to the check up, there is increased public awareness on the issue of obesity and metabolic syndrome,” said James Kondo, president of the Health Policy Institute Japan, an independent think tank. “Since fighting obesity is a habit underlined by heightened awareness, this is a good thing. The program is also revolutionary in that incentivizes [companies] to reduce obesity.”

Though the health exams for metabolic syndrome factor in blood pressure, cholesterol, blood sugar, weight and smoking, waist size is the most critical element in the Japanese law — and perhaps the most humiliating.

The hesitancy of some Japanese to expose their bare stomachs to the tape measure has led the government to allow the tape measures to be administered to clothed patients. Those who elect not to strip down are permitted to deduct 1.5 centimeters from their results.

The crudeness of the system has alarmed some doctors. Satoru Yamada, a doctor at Kitasato Institute Hospital in Tokyo, published a study two years ago in which several doctors measured the waist of the same person. Their results varied by as much as 7.8 centimeters.

“I cannot agree with waist size being the essential element,” Yamada said.

Perhaps more astounding, even before Japanese lawmakers set the waistline limits last year, the International Diabetes Federation (IDF) amended its recommended guidelines for the Japanese. The new IDF standard is 90 centimeters (35.4 inches) for men and 80 centimeters (31.5 inches) for women. But the Japanese government has yet to modify its limits.

On the day of her exam, Yabe arrived at the clinic at 8:30 in the morning. The battery of tests lasted an hour. The result: her waist was 84 centimeters — safely under the limit. She had shed 6.5 pounds thanks to her diet and exercise.

A week later, however, Yabe was back to eating pasta and other favorite foods.

“I want to keep healthy now, but I don’t know,” she said. “Maybe in December, I will have many bonenkai [year-end parties]. And next summer I will drink beer, almost every day.”