(HealthDay News) -- Losing a lot of weight rejuvenates the physical structure of the heart, and it makes no difference whether the weight is lost by surgery or by dieting, a new British study shows.

The heart muscles of people who started with a body mass index (BMI) averaging 40 -- a BMI of 30 is the usual marker of obesity -- became noticeably thinner and more efficient when they brought their BMI down to 32.2 in a single year, according to a report in the Aug. 18 issue of the Journal of the American College of Cardiology.

"Both diet and bariatric surgery led to comparable, significant decreases" in heart structure abnormalities and malfunction, the University of Oxford researchers reported.

Bariatric surgery is designed to induce weight loss by reducing the amount of food people can eat, the amount of food they can metabolize or both.

Weight loss averaging 21 kilograms (about 45 pounds), achieved by the 37 obese people in the study, "is typically what is seen after bariatric surgery," said Dr. Philip R. Schauer, director of the Cleveland Clinic Bariatric and Metabolic Institute.

Noting that many of the obese people lost those kilograms by eating less, Schauer called it "quite unusual for someone to diet with that effect. These were a very special subset."

And the problem with weight loss by diet is that "weight regain is the norm, whereas with bariatric surgery there is ample evidence that the weight loss is maintained," Schauer said.

The Oxford researchers used cardiac MRI, a special X-ray technique, to obtain detailed information on the structure of the hearts not only of the 37 obese participants but also of 20 normal-weight volunteers, whose average BMI was 21. They found that the walls of the left and right ventricles, the blood-pumping chambers of the heart, were significantly thicker in the obese people. They also found impaired ability of the heart to hold blood at diastole, the resting point of the heartbeat, in the obese.

A year later, after weight loss, the heart muscles of the obese people were less overgrown and the hearts could also hold more blood. Thickening of the aorta, the main heart artery, was also greatly reduced after weight loss.

"These findings provide a potential mechanism for the reduction in mortality seen with weight loss," the researchers wrote.

And it thus helps explain something of a medical mystery -- why people who are grossly overweight are more at risk of heart attack and sudden death than their numbers show, said Dr. Christine Ren, a bariatric surgeon who is an associate professor of surgery at New York University Langone Medical Center.

"Most of them say they are pretty healthy, maybe with a slight elevation of blood pressure, but when you really drill down to it you can show an abnormal cardiac function," Ren said. "The point is that their heart is not normal and that already is having a negative effect on their health."

Losing weight by dieting is desirable, "but the problem with diets is that statistics show maintenance of weight loss by diet is extremely difficult and quite rare," Ren said.

Bariatric surgery is expensive, costing anywhere from $15,000 to $25,000, she said, and it is not perfect. "There is always going to be 5 percent of these people who gain most of their weight back, but it still is the best chance of having significant weight loss," Ren said.

However, health insurance coverage of bariatric surgery is uncertain, varying from company to company and state to state, she said, and many plans are starting to put more limits on coverage.

More information

Bariatric surgery is described by the U.S. National Library of Medicine.