Severely obese people who undergo weight-loss surgery may have a higher-than-average risk of suicide in the years following the procedure, a new study finds.

The report, in The American Journal of Medicine, adds to evidence that patients who have bariatric surgery to lose weight have an increased risk of suicide compared with the general population.

But the reasons for the pattern, researchers say, remain unknown.

A number of studies have found that while the absolute rate of suicide among bariatric surgery patients is quite low, it is still higher than the norm for the general population.

The benefits of weight-loss surgery are well-documented. The procedures, which alter the digestive tract to limit food intake and nutrient absorption, can help severely obese individuals shed a substantial amount of weight. They can also reverse obesity-related health problems like type 2 diabetes and high blood pressure.

The latest study, which tracked deaths among Pennsylvania residents who underwent bariatric surgery, examined a longer period than previous research — up to 10 years following the procedure.

Among 16,683 who had bariatric surgery between 1995 and 2004, 31 committed suicide by the end of 2006, the researchers found. The data translate into a suicide rate of nearly 14 per 10,000 men per year, and five per 10,000 women each year.

Those numbers are substantially higher than the suicide rates among Pennsylvanians in the same 35-to-64 age range, during the same period. Among all men in the state, the suicide rate in 2005 was 2.5 per 10,000, while the rate among women was 0.6 per 10,000.

Overall, 30 percent of suicides in the surgery group occurred within two years of the procedure, and 70 percent occurred within three years.

For now, the reasons for the higher suicide risk are unclear, according to lead researcher Dr. Hilary A. Tindle of the University of Pittsburgh in Pennsylvania. Her group's study, based on death certificate data, could not examine details surrounding the individual suicides.

But the study does not imply that bariatric surgery itself leads to suicides — something that is "critical" for patients to understand, she told Reuters Health in an e-mail.

Rather, she said, it may be that some bariatric surgery patients have depression or other mental health problems before the procedure — or develop them afterward — and that, in turn, increases their overall suicide risk.

Depression is common among severely obese adults, including those who seek weight-loss surgery, Tindle and her colleagues point out. One study of surgery candidates found that two-thirds had, at some point in their lives, experienced a so-called "Axis I" disorder, a group of mental health conditions that includes depression, anxiety disorders, eating disorders and substance abuse.

Bariatric surgery candidates do undergo screening to see whether the operation is appropriate for them, and that includes a mental health evaluation. But a history of depression or other psychiatric conditions does not necessarily preclude a person from having the surgery.

Instead, Tindle explained, a team of health professionals — including the surgeon, a dietitian, a psychologist and, often, an exercise physiologist — try to determine whether an individual is medically and psychologically ready for surgery, and if not, they attempt to address any underlying problems.

More research, according to Tindle's team, is needed to understand why bariatric surgery patients show an increased suicide risk. If there are pre-surgery characteristics connected to a higher risk, then some suicides might be prevented by more careful monitoring and treatment, the researchers say.

Also unclear is whether suicide risk depends on how much weight a person loses after surgery. It's possible, Tindle's team speculates, that any pre-existing psychological distress could be exacerbated if patients have disappointing weight-loss results — or if their overall quality of life does not improve as they had hoped.

According to Tindle, the findings indicate a general need for longer term monitoring of patients after surgery.

Bariatric surgery centers across the U.S. do post-surgery monitoring, with the recommended follow-up being six months.

But, "most of the suicides among individuals in this study occurred later than the time post-surgical follow up usually ends," Tindle said.

About 225,000 Americans are now having bariatric surgery each year, according to the American Society for Metabolic and Bariatric Surgery. Given those numbers, Tindle said, it will become increasingly important to understand the factors that contribute to patients' heightened suicide risk.