At the best of times, they are called pleasingly plump. Or chubby. And it's said in polite, hushed tones that they haven't yet outgrown their "baby fat."

At the worst of times, they are taunted by classmates. They earn nicknames like Tubby or Fatso. They're the Fat Alberts and the Porkies -- labels that leave their psyches smarting well into adulthood.

But for the estimated 10 to 30 percent of American children, ages 8 to 18, who are overweight, being fat is more than a social inconvenience. Their extra pounds mean that they are vulnerable to a host of physical ailments that can shave years from their lives -- through premature heart disease, cancer, high blood pressure and diabetes.

Once experts advised parents to wait and let their overweight children "grow out of" their so-called baby fat. Today, pediatricians and others are taking a more aggressive approach, bolstered by scientific studies that show being an overweight child lays the foundation for becoming an overweight adult -- and increases the risk of dying early from heart attacks, stroke and other ailments.

"As late as 1974, people were concerned that if you did anything to change a child's diet, you would end up with a malnourished kid," says Robert Klesges, a psychologist at Memphis State University. "But that has changed dramatically in the last 10 years."

What concerns experts most is that obesity seems to be on the rise in the United States -- particularly among children. "We've shown over the past 15 years that obesity in children, 6 to 11 years, has increased 54 percent among U.S. children," says Dr. William Dietz, a pediatrician and director of Clinical Nutrition at the New England Medical Center in Boston. "Among youngsters, 12 to 17 years, obesity has increased 39 percent in the past 15 years."

In a consensus statement on obesity, the National Institutes of Health reported in 1985 that it "views with concern the increasing frequency of obesity in children and adolescents. Obese children should be encouraged to stay within normal {weight} limits."

Yet experts are still struggling to define what is normal and what isn't. Most now agree that obese children are those who are 20 percent or more above the ideal weight for their height, age and sex. Overweight children are generally children who are between 10 to 19 percent above their ideal weight for height, age and sex.

But if the definitions aren't totally clear, the research is. "Overweight children are at greater risk of becoming overweight adults than lean children are," says Leonard Epstein, an obesity researcher at the University of Pittsburgh School of Medicine. A growing number of studies have shown, Epstein says, "that a large number of overweight children do not outgrow being overweight."

Consider that:

At age 6 months, about one third of obese infants are destined to become overweight adults, according to a study by researchers at the University of Rochester.

By age 7, four of every 10 obese children become overweight adults.

At ages 10 through 13, about 70 percent of overweight children can count on carrying their extra pounds with them into adulthood.

Exactly which heavy children will go on to be heavy adults is not always clear, but there are some intriguing clues that suggest a variety of factors -- such as region of the country, family income, family size -- help determine who will become a fat adult.

For example, children from small families are more prone to putting on unwanted pounds than youngsters from large families. An only child has twice the chance of becoming heavy as does a child with three or more brothers or sisters.

Children from urban areas are more likely to be overweight than their rural counterparts, although no one knows why. Obesity experts think that the difference is due to less exercise and more opportunities for eating. City children rarely have yards in which to play. Their activities are likely to be confined indoors and burn up fewer calories that may help to contribute to extra pounds.

Youngsters who live in colder climates -- the Northeast and the Midwest -- also run a higher risk of added pounds than children who live in the South and the West. The reason, experts say, is that during the winter, people put on weight for extra warmth and lead more sedentary lives because of the inclement weather.

Income also plays a role in determining which children are likely to be fat for life. Lower income families are more apt to have youngsters with weight problems than upper-income families. Children from middle incomes households fall in between.

But the single most important predictor of whether a child is at risk for becoming obese is parent weight. Researchers like Dr. Albert Stunkard of the University of Pennsylvania are scientifically documenting what many people have always suspected: that being fat -- and being lean -- seems to run in families.

In a study of Danish adults who had been adopted into other families as children, Stunkard and his colleagues found that the body weight of adoptees much more closely matched the body weight of their biological rather than their adoptive parents. The findings point to the important role that the genes may play in obesity.

Other studies underscore the point. One of the most widely cited among obesity researchers is a report that found 80 percent of the children with two obese parents will become obese adults, while about 40 percent of the children with one obese parent will become fat, and only 20 percent of children whose parents are normal weight will be obese as adults.

Based on these results, some people have suggested that the battle is pointless. What's the use of trying to fight obesity, they say, if it's been preordained since birth?

"That's a wrong idea," says University of Pennsylvania's Stunkard. What these new results do "is give people the possibility of informed prevention, not just a shotgun approach."

It means, Stunkard says, that families that have a history of obesity "will really have to work on trying to prevent obesity in their kids." And once those children are adults, they, too, are going to have to "remain eternally vigilant against gaining weight."

The struggle is worth it, obesity experts say, for like their adult counterparts, overweight children pay a stiff emotional and physical price for their added pounds. "The more severe problem a child has, the less likely that he or she will be able to grow out of it," says New England Medical Center's Dietz.

"The biggest factor for overweight and obese children are the psychological problems," says Dr. Dorothy Richmond, a pediatrician who specializes in treating eating disorders at Georgetown University Medical School. "Those problems have more immediate effects than the medical problems and they're much more common."

The psychological suffering for fat children often translates to behavior problems: trouble in school; shyness; hyperactivity; being withdrawn or depressed. Fat kids are teased about their weight by their peers. They are the last chosen to play on the team. The style of clothes that is so important to children and adolescents often isn't even an option. "You don't get designer jeans in the super husky sizes," says Richmond.

But being fat also takes a physical toll. "They have a whole host of minor complaints," says Richmond. "Back pain, foot pain, inability to do exercise, shortness of breath."

The very obese child may have trouble sleeping at night because the chest can't expand normally. "Carbon dioxide builds up in the blood and they feel sleepy, and may fall asleep in class," says Richmond. Extra pounds can also cause sleep apnea -- the temporary cessation of breathing -- which forces them to wake up numerous times during the night to breathe.

Being overweight in childhood often sets the stage for a lifetime battle with excess pounds and an increased risk of heart disease, diabetes, cancer and orthopedic problems.

Obese children often suffer from "high blood pressure, abnormal glucose tolerance tests -- an indication of greater risk for diabetes -- and high blood cholesterol levels," says Dietz. "By treating children who are overweight, we are really preventing severe adult disease and avoiding the medical and social complications of obesity in childhood."

The other benefit of helping fat children lose weight is that it can change their self-image and make them feel better about themselves.

"The self-confidence that emerges from these children once they have lost weight just amazes me," says Debra DeVos, whose daughter lost 25 pounds through the Stoplight Diet, a program developed at the University of Pittsburgh. "You see grades excelling, you see children wanting to interact with peers. The aura of self-confidence just reeks from them."