The 2009 child health and nutrition report says 64 percent of primary and middle school students don't get enough exercise to burn calories.

"Most of the food we eat is turned into energy to support daily activities," says Dr Tang. "When we take in more calories than we consume, fat is stored. Fat kids are eating too much and not burning it off."

Research shows that kids with overweight parents are more at risk of being fat than kids from normal-weight families, because their parents set bad examples. The rate is 80 percent for kids with both parents overweight, 40 percent for those with one fat parent and only 10 percent for kids from an ordinary-weight family.

Hereditary factors may play a role, but overeating and unhealthy lifestyle are the big problems. Parents should set examples of healthy eating and regular exercise.

Eating three nourishing meals on time is the first principle of a healthy diet. Kids who skip breakfast often over-eat at lunch; large late suppers mean fat accumulates and there's little calorie-burning during sleep. The same is true for adults.

For both children and adults, healthy diets should be balanced, including carbohydrates (which are turned to sugar to provide energy), protein, fat and fiber. Food should be rich in vitamins and minerals. Carbohydrates should represent at least 30 percent of each meal.

Fat kids should eat fewer fats, sweets and animal protein (cholesterol) and eat more fruits and vegetables.

Fish, eggs, shrimp and beans are good sources of protein. Meat and chicken should be lean.

Steaming, boiling and braising are better than frying in oil. A little sesame oil is good for flavoring as it adds few calories.

Children, and everyone, should eat slowly so they don't eat too much. It takes a while for the brain to send the "I am full" signal, so it's easy to overeat before feeling satisfied.

Chewing food 10 times before swallowing can help kids enjoy their food more, and help them eat less.

Protein and foods with fiber, like fruits and vegetables, make one feel full. Don't let kids snack or hide snacks. Offer healthy snacks and fruit.

Physical exercise is important. Kids should take 30-60 minutes of physical exercise every day. Exercising with parents can keep them motivated to continue.

"Losing weight is a tough task, and many kids give up halfway," says Dr Hong.

"It's best to set short-term, achievable goals for kids in advance, such as taking off 1-2 kilograms each month, and make detailed diet and exercise plans accordingly."

Praise and rewards (not food) are important when a child reaches each weight-loss goal.

"This can maintain the momentum and encourage them to keep going." BMI Index

Body mass index (BMI) is a statistical measurement that compares a person's weight and height. It's the weight divided by the square of the height.

Though it does not actually measure the percentage of body fat, it is used to estimate healthy body weight.

Since it's easy to calculate, BMI is the most widely used diagnostic tool to identify weight problems, including underweight, overweight or obese.

The formula universally used in medicine produces a unit of measure of kg/m2.

An adult with a BMI index over 24 is considered overweight; over 28 is considered obese.

Snacks

Snacking is a way of life in Asia and it can really pack on the pounds.

"The Guidelines on Snacks for Chinese Children and Adolescents" released by the Health Bureau in 2007 classifies foods into three categories - the ones that can be eaten often, eaten cautiously and eaten seldom.

It applies not just to snacks, but all foods.

Green light

Foods that can be eaten often are low fat, low salt and low sugar.

They include low-fat yogurt, no-sugar oatmeal, lean meat, shrimp, fish, eggs (not too many yolks), nuts (don't make a meal of them), fresh fruits and vegetables, boiled corn, plain soy milk, plain tofu, boiled corn, boiled or steamed potato and sweet potato, fresh fruit juice.

Yellow light

Foods that should be eaten with caution are high in fat and sugar.

They can be eaten in small amounts a few times a week, not daily. They include chocolate, cheese, and sugar-coated snacks.

Foods that should be eaten cautiously: chocolate, bacon, beef jerky, moon cake, cakes and pastry, sweet or salty soybean milk, dried fruits with sugar or salt coating, cheese, and juice with sugar additive.

Red light

Foods to avoid for anyone trying to lose weight are high in fat and sugar. They should be eaten no more than once a week.

They include sweets, deep-fried foods, canned fruit and condensed milk, hamburger, hotdogs, french fries, instant noodles, fried tofu or tofu with odor, potato and other chips, soda, and ice cream.

Junk fast-foods and sugary soft drinks, of course, should be avoided completely.

Water and pure fresh juice are best for kids, and everyone else.

Every child should drink about 1,000-1,500 ml water daily, more in summer and after vigorous physical exercise. Though fresh juice is nutritious, it cannot replace water. Too much juice will reduce kids' appetite for other foods, and cause problems.