The haze shrouded skyline of New York is a backdrop to a jogger running along the Hudson River in Hoboken, New Jersey August 3, 2006. [Reuters]

Tests on overweight people show that a calorie is just a calorie, whether lost by dieting or by running, they said.

They found there is no way to selectively lose belly fat, for instance, or trim thighs. And their carefully controlled study added to evidence that adding muscle mass does not somehow boost metabolism and help dieters take off even more weight.

"It's all about the calories," said Dr. Eric Ravussin of the Pennington Biomedical Research Center, part of Louisiana State University in Baton Rouge.

"So long as the energy deficit is the same, body weight, fat weight, and abdominal fat will all decrease in the same way."

Ravussin said the study, published in the Journal of Clinical Endocrinology & Metabolism, is one of the few done under controlled conditions that can actually demonstrate what happens to a human body while dieting and exercising.

Ravussin's team has been testing volunteers for another reason -- to see if taking in fewer calories helps people live longer. Strict diets have been shown to help animals from worms to dogs live longer, but it takes longer to study monkeys and humans.

They tested 24 people, 12 who ate a calorie-restricted diet, and 12 who dieted and also exercised five times a week for six months.

The dieters ate 25 percent less than normal, while the exercisers reduced their calorie intake by 12.5 percent and increased their physical activity to lose an extra 12.5 percent in calories.

Another 10 volunteers acted as controls. All food was provided by the university in carefully measured portions for most of the study.

The volunteers in both groups lost about 10 percent of their body weight, 24 percent of their fat mass, and 27 percent of their abdominal visceral fat. Visceral fat is packed in between the internal organs and is considered the most dangerous type of fat, linked with heart disease and diabetes.

The distribution of the fat on the body was not altered by either approach -- helping prove that there is no such thing as "spot reducing," Ravussin said in a telephone interview.

This suggests that "individuals are genetically programmed for fat storage in a particular pattern and that this programming cannot easily be overcome," he added.

Ravussin has published other studies that also dispute the idea that exercise builds muscle that helps people lose weight.

"If anything, highly trained people are highly efficient, so they burn fewer calories at rest," Ravussin said.

Dieting alone also did not appear to cause the volunteers to lose muscle mass along with fat, Ravussin's team found.

"There is a concept that if you exercise, you are going to lose less of your muscle," he said. But his team found no evidence this is true.

Ravussin believes exercise is crucial to health, however.

"For overall health, an appropriate program of diet and exercise is still the best," he said.

His team found some small suggestion that cutting 25 percent of calories by either diet or diet and exercise might extend life.

"We found that 2 of the biomarkers of aging were improved -- core temperature was 0.4 to 0.5 degrees C less," he said. "Insulin, which has been shown to be a biomarker of aging, was reduced," Ravussin said. That finding was published in the Journal of the American Medical Association last April.