LOS ANGELES - Extremely obese people suffering from depression or anxiety tend to lose less weight after obesity surgery than mentally healthy people, researchers reported Thursday in a study that suggests such patients could benefit from treatment beforehand.

People diagnosed with mood or anxiety disorders on average lost 81 pounds six months after gastric bypass surgery compared to their counterparts who shed 86 pounds. Although both groups lost significant weight after surgery, people without mental health problems did slightly better.

"Patients with a lifetime history of mental health problems might benefit from closer surveillance," said lead author Melissa Kalarchian of the University of Pittsburgh Medical Center.

The federally funded research was presented at a meeting of bariatric surgeons in San Diego Thursday. Researchers plan to follow patients for up to two years to determine if there's a weight difference over time.

Many hospitals and insurers require surgery candidates to go through a psychological evaluation before obesity surgery to make sure they are mentally fit for the operation and the lifestyle change afterward. Depressed people aren't automatically disqualified for surgery, but those who are suicidal or abusing drugs and alcohol are usually ruled out.

How depression and other mental health disorders are handled before obesity surgery vary widely by medical center.

For example, surgery candidates at the University of California, San Diego, go through a rigorous mental health evaluation first.

Those with serious problems are usually treated before surgery. That could include antidepressants, psychotherapy or more family involvement, said the center's director William Perry.

"The success of surgery is only as good as the willingness of a patient to make a life change," Perry said.

In the new study, Pittsburgh researchers interviewed 207 surgery candidates and found two-thirds had a history of depression, bipolar disorder, post-traumatic stress syndrome or panic attacks. The vast majority were women with an average body-mass index of 51. A person with a BMI of over 40 is considered morbidly obese.

After adjusting for age, gender and race, researchers compared weight loss six months after surgery. Patients with a history of depression on average weighed 322 pounds before surgery and 241 afterward. Those with no mental health problems weighed 303 pounds before the operation and 217 pounds afterward.

Having a history of mental health problems should not prevent people from getting obesity surgery, even though they may not lose quite as much weight as mentally healthy people, said Dr. Philip Schauer, president of the American Society for Bariatric Surgery.

Gastric bypass, or stomach-stapling surgery, is the most common obesity operation performed in the United States with about 177,600 people undergoing the procedure last year. Federal guidelines say a person should be at least 100 pounds overweight and should have tried traditional weight-loss means before choosing surgery.

Gastric bypass involves reducing the size of the stomach and bypassing part of the small intestine, where digestion occurs. People eat less and absorb fewer calories as a result.

Meg Phelps, a 48-year-old social worker from Pennsylvania, struggled with major depression and low self-esteem for a decade. Often, she wouldn't go out or even make eye contact with people.

Four years ago, Phelps had obesity surgery at the Geisinger Medical Center in Danville, Pa. Beforehand, she had a medical evaluation and attended several months of counseling for depression.

Phelps, who weighed 335 pounds, shed 120 pounds after the surgery and no longer has to see a therapist. She even went back to college and got a degree in social work last year.

"It helped me to take charge of my life," Phelps said of the operation.