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Obesity is a risk factor for many types of heart problems, including heart failure, atrial fibrillation (an abnormal heart rhythm) and death, explained senior study author Dr. Sheldon Litwin, chief of cardiology at the Medical College of Georgia.

The connection between obesity and cardiovascular disease isn't fully understood, but obese people often show signs of structural changes to the heart, including excess heart muscle mass in the left ventricle and enlargement of the right ventricular cavity. Both are linked to heart failure and other problems.

For this study, published in the Feb. 8 issue of the Journal of the American College of Cardiology, researchers followed more than 400 severely obese people with an average age of 42 who had gastric bypass surgery, which reduces stomach size. They were compared with a reference group of more than 300 severely obese individuals who did not have the weight-loss surgery.

Two years later, patients' body mass index (BMI) fell from an average of nearly 48 (morbidly obese) to about 32. Their average weight loss was about 100 pounds, Litwin said. (A BMI of 30 is obese, while a BMI of 25 is considered normal weight).

Participants also had smaller waistlines, lower blood pressure and heart rate, healthier cholesterol levels and less insulin resistance, a precursor to diabetes, the researchers found.

Echocardiograms, or ultrasounds of the heart, also showed a "remodeling" of their heart structure, including a reduction in the left ventricular mass and right ventricular cavity area.

Those reductions in heart mass and volume indicate a heart that's less stressed, isn't having to work as hard pumping blood and has returned to a more normal shape and function, Litwin said.

"We know obese people get cardiovascular disease more often than non-obese people," Litwin said. "One of the questions out there is: Is it reversible if they lose weight? The answer is yes."

Bariatric surgeries have increased steadily in the United States — from 16,800 in 1992 to about 205,000 in 2007, according to the American Society for Metabolic & Bariatric Surgery.

The proportion of obese people having the surgery is still small, however. Annually, less than one-fourth of 1% of all obese people get it done, said Dr. Lee Kaplan, director of the MassachusettsGeneral Hospital Weight Center.

That's in part because the procedure remains controversial. It's expensive and some insurance doesn't pay for it. In addition, like all operations, bariatric surgery is not without risk. Potential complications, for example, include blood clots, infection, respiratory arrest, gastrointestinal bleeding, small bowel obstruction and death, according to the American Society for Metabolic & Bariatric Surgery.

"Would it be better if people could lose weight by eating better and exercising more? Probably yes," Litwin said. "But in general those lifestyle changes are ineffective, particularly in people who are severely obese."

The study provides more strong evidence for the benefits of gastric bypass surgery on not only diabetes, hypertension, cholesterol and sleep apnea, but on the heart muscle itself, Kaplan said.

"There are innumerable known benefits to gastric bypass surgery on most of the 60-odd diseases associated with obesity a patient may have," Kaplan said. "The question isn't if gastric bypass works. It does work. The issue has been at what cost, both in terms of health risks and financial? This study provides evidence there are additional substantial benefits with respect to heart muscle function, and we should consider gastric bypass more often in the appropriate patients."

The next step is to follow patients beyond two years to see if they maintain their weight loss, improve their heart health, and if they have less heart trouble over the long term, said Litwin, who was at the University of Utah when he did the research.