But the researchers can't say for sure which comes first, the obesity or the misery.
"It's a complex relationship. We don't know the answer to that," says study co-author Dr. Jeffrey Schwimmer, a pediatric gastroenterologist at the University of California, San Diego.
Schwimmer, who works with sick, obese kids at Children's Hospital & Health Center in San Diego, says he was inspired to do the research after he noticed the children seemed unhappy. But he couldn't find any research in the United States to suggest if fat children suffered from a lower quality of life than other children.
The study researchers surveyed 106 patients who were treated at the hospital for obesity. The patients, aged 5 to 18, had an average body mass index (BMI) of 34.7.
The index is a ratio that includes weight and height. To reach a BMI of 34 or higher, a 12-year-old boy who's 5-foot-3 would have to weigh at least 190 pounds.
The results of the study appear in the April 9 issue of the Journal of the American Medical Association.
The children took a test that measured their quality of life by asking questions about issues such as their health, stamina, athletic and school activities, personal relationships and general feelings -- such as whether they were happy or sad, angry or calm.
On a 100-point scale, the obese children reported their quality of life at 67, compared to 83 for healthy, non-obese children surveyed in another study. Parents of both sets of kids were also questioned about the quality of life of their children, and they reported an even larger gap -- 63 versus 88.
Looked at another way, the obese children were 5.5 times more likely to have a poor quality of life than the healthy kids, the researchers say.
"The obese children reported lower quality of life in every single domain," Schwimmer says. "These differences held up even when we looked at issues of gender, race, age and socioeconomic status."
The children also reported poor quality of life regardless of whether they suffered from illnesses such as diabetes and heart disease.
To put the figures in context, the researchers compared them to quality of life surveys of children undergoing chemotherapy.
"They've been demonstrated to have the lowest quality of life of any pediatric health condition," Schwimmer says. "That's why we selected them. It's a tough benchmark."
The quality of life reported by children undergoing chemotherapy was actually a slight bit better than the obese children, the researchers found.
The study doesn't offer any new information about why very heavy children have less fulfilling lives than their skinnier contemporaries. However, Schwimmer suspects social stigma plays a large part.
"From an early age, even at the kindergarten level, children see obese children as different and many respond to them differently," he says. "And certainly by junior high, children who are still obese are clearly ostracized in many places."
Dr. James Rosen is a professor of psychology at the University of Vermont and an expert in weight control among children. He says other stressful obstacles confronting heavy children include "not participating in sports, physical discomfort moving around, and the stress of trying to eat healthy when facing a junk food school cafeteria."
Food and weight control can become a "battleground" at home, he adds. And divorced parents may give their obese children conflicting messages about the importance of controlling weight.
An estimated 15 percent of American children are obese, or one in seven, according to the U.S. Centers for Disease Control and Prevention. The rate has tripled since the 1970s.
Schwimmer acknowledges the study didn't look at very heavy children in general, but only those who visited a hospital for treatment of obesity.
In another study on obesity and children in the April 9 Journal of the American Medical Association, researchers at the University of Pennsylvania report that obese children who have behavior-modification therapy can lose even more weight if they take a weight-loss medication for adults called sibutramine.
Children who were treated with sibutramine for six months lost an average of 17 pounds, compared to just seven pounds for those who also had the behavior-modification therapy but took a placebo instead of the drug.
The researchers caution, however, that more testing needs to be done before doctors should recommend the drug to young patients.
To learn more about obesity and children, visit the National Institute of Diabetes and Digestive and Kidney Diseases. To calculate a body mass index, check with the National Institutes of Health.