NEW YORK (Reuters Health) -- Poor physical fitness may raise the odds of serious complications following obesity surgery, a new study suggests.

The findings, researchers report in the journal Chest, suggest that obese patients should achieve a certain level of fitness before having the surgery.

Weight-loss surgery is becoming an increasingly common treatment for people who are morbidly obese or at least 100 pounds overweight. The surgery can be performed in several ways, but the common goal is to change the structure of the digestive tract to limit the amount of food a person can ingest and absorb.

It's major surgery with a risk of complications, including death. There are, however, no uniform standards for evaluating surgery candidates' potential for complications."We believe ours is the first paper demonstrating that fitness should be a consideration," Dr. Peter A. McCullough, the study's lead author, told Reuters Health.

McCullough and his colleagues at William Beaumont Hospital in Royal Oak, Michigan, followed 109 patients who had their fitness gauged on treadmill tests before undergoing Roux-en-Y gastric bypass surgery.

The procedure involves stapling the stomach to create a small pouch, then attaching a portion of the small intestine to the pouch so that food bypasses the rest of the stomach and part of the intestines.

Of the study patients with poor fitness levels, nearly 17 percent suffered a serious short-term complication from the procedure, such as kidney failure or clots in the blood vessels. One patient suffered a stroke and another died. In contrast, of the patients who were more fit, less than 3 percent had complications and none died.

Fitness tests indicate how well the heart, lungs and blood vessels function under stress. The combination of morbid obesity and low fitness levels may make surgery patients particularly prone to short-term complications, the researchers conclude.

These findings, according to McCullough, could "change practice around the country." Once fitness tests spot potentially high-risk patients, the researchers advise that they be put on an exercise and weight-loss plan. After about three months, they can have their fitness re-evaluated to see if they're ready for surgery.

Though many morbidly obese adults do have poor fitness levels, McCullough said that in his experience, some have only mild impairments. For those in poor shape, the researchers note, even slow walking may spur improvements.