The fight against obesity starts young in Singapore. Fat children are separated from their classmates and ordered to do more exercising until they lose weight.

Ten-year-old Mona Siow has been trying to lose weight for the last four years.

Instead of joining her friends at the canteen during recess every day, the fourth-grader and other chubby students gather in the hall and follow a teacher's instructions to jump-rope, run and dribble a basketball.

"I feel sad to be overweight when I look at people and they're so skinny and can wear so many clothes," said Siow, who needs to shed about 37 pounds before she can leave the program. At 4 feet 8, she weighs 128 pounds.

As a member of a Singapore primary school's Health Club -- where membership is compulsory for overweight kids -- Siow does special exercises on top of the regular physical education curriculum. Teachers monitor her height and weight every month.

While the school does not put restrictions on what Siow can eat, teachers meet her parents regularly to recommend healthier ways to prepare their daughter's meals at home. Siow said she used to hate eating vegetables but has grown to like them.

More than a decade ago, this tiny but modern city-state's leaders decided that the best way to fight the war on expanding waistlines -- and ballooning health care costs -- was to begin with the generation growing up on a diet of fast food, television and computer games.

The government created a school-based intervention program that includes rigorous exercise for plump children and recommendations on food sold in canteens, where the aromas of Western-style meals mingle with the sometimes spicy and exotic smells of local fare.

Most schools in tropical Singapore have open-air lunchrooms catered by up to a dozen different private vendors selling a variety of foods from their stalls. This allows kids from different ethnic and cultural backgrounds with varied dietary restrictions to choose between Chinese, Muslim, Indian or Western-style meals.

About three-quarters of Singapore's 4 million people are ethnic Chinese. About 15 percent are Malay Muslims, with the rest mostly of Indian descent.

Siow's vice principal, Ng Sock Hua, said no more than two of 10 dishes served to the children are preserved or canned.

"No deep frying, only grilled food allowed," she declared. "And I've asked them to hold back on selling caffeinated and soft drinks."

But reality isn't exactly as Ng describes it. As she gave a tour of the canteen, she noticed with some surprise that students were lining up for plates of French fries and chicken nuggets and snapping up cans of Coca-Cola at the drinks stall. She had a stern word with the stall-owners about the rules then walked away.

"We try to monitor the vendors but it's not easy to ensure that they're selling the right things," she said, shaking her head. "They tend to provide the food that kids like to eat."

Unlike the tight controls it places on many aspects of everyday Singapore life, the government issues recommendations rather than regulations about the food sold in school cafeterias. In turn, the school passes along the instructions to canteen operators but isn't always able to check on them.

The sheer abundance of food and major cultural shifts over the past few decades have contributed to young people's bad eating habits, said Gladys Wong, president of the Singapore Nutrition and Dietetics Association.

"In the 1950s, if you're hungry, you drink water until Grandma prepares the next meal, and have to wait for Dad to come home before everyone eats together at the table," Wong said.

"Nowadays, if the kid is hungry, taking a break from the computer can be a TV-dinner frozen pizza, from the freezer to the microwave within minutes," she said.

These new habits are proving hard to break. The so-called "health clubs" have reduced the proportion of overweight students from 14 percent in 1992 to 10 percent in 2003, Education Ministry statistics show, but many, like Siow, don't shed the pounds.

"It's quite disheartening to see students remain in the club for a number of years," said Lim Ee Kheng, the school's head of physical education at Siow's school, the elite Singapore Chinese Girls' Primary School. "To keep them in the club for a long time is bad for their self-esteem because there's a stigma tied to it."

For example, if Siow fails to lose her required weight, she is doomed to stay in the program until she completes her pre-university schooling.

The government is also trying to mobilize its adult population to fight the flab. Every September the city-state holds a month-long fitness campaign aimed at getting the entire population to eat better and stay active.

The theme for 2004 is "Fighting Obesity" and the campaign was launched with a mass aerobic workout class that had 12,000 people sweating it out together on the beach.

Even Singapore's Prime Minister Lee Hsien Loong, who is tall and trim at 52, took part, with TV cameras capturing him sweating, punching and stomping it out with the locals.

Compared to the United States and Western Europe, Singapore's vigilance seems remarkable. Two-thirds of Americans are overweight. Singapore Health Ministry figures show that one in three people need to lose weight, and about 6 percent of its 4 million people are obese -- at least 30 pounds too heavy.

But those numbers may soon expand dramatically as officials here prepare to revise their measurement standards to match new research indicating Asians suffer adverse health effects at a lower weight than Caucasians.

"We noticed that although Singaporeans are less obese compared to people of [other] developed countries, our heart disease trend is similar to theirs," said Mabel Yap, head of research and information at the government's Health Promotion Board.

Yap conducted a study that showed Singaporeans had 5 percent to 6 percent more fat in their bodies than European Caucasians of the same age and same body-mass index.

BMI is a calculation of weight and height commonly used as a benchmark for body fat.

To define obesity, most countries follow standards set by the World Health Organization, which defines overweight as a BMI of more than 25, and obesity as a BMI of 30 or over. These levels are largely based on data derived from research on Caucasians.

"If you truly want to define obesity, it should be about excessive body fat and related health risks," said Yap.

Yap has recommended that the country's health policy-makers lower the BMI at which a person would be considered overweight from 25 to 23. That could lead to as much as half of all the island's residents being too fat, instead of just one in three.

"It's enough to alarm," she said.

Schoolgirls wait during lunch break to take part in a jump-rope exercise. Mona Siow, 10, has been trying to lose weight for the past four years and must spend her lunch breaks playing sports. She belongs to a "health club," where membership is compulsory for overweight kids. Vendors sell healthy food and drinks to schoolchildren in Singapore. School officials try to monitor the vendors to prevent them from selling fatty foods.