People over 55 who start an exercise program can expect to lose a significant amount of fat in six months, but not bone mass, according to new research.

These results contradict long-held concerns that losing fat might also mean losing bone, since studies show that people who carry more body fat also tend to have stronger bones, explained study author Dr. Kerry Stewart of Johns Hopkins University in Baltimore, Maryland.

Even if fat protects bone, exercise can help bone as well, and “offset the potential negative effects” of losing fat, Stewart told. Consequently, people who want to lose weight should do so using exercise, not just diet, he added.

During the study, Stewart and his team asked 115 men and women between the ages of 55 and 75 to either try to follow general government recommendations on exercise for 6 months, or participate in exercise training under supervision 3 times each week, doing stretching, resistance training and aerobics.

People who followed the supervised exercise program showed improvements in upper and lower body strength, total strength, lean mass, body weight, total and abdominal body fat.

The supervised exercise program appeared to have no effect on men’s bone density. For women, exercising was associated with a slight decrease in overall bone density. However, even women who didn’t exercise lost some bone mass from around the hip, the authors note in the American Journal of Preventive Medicine.

In an interview, Stewart noted that people who showed the greatest increase in fitness from their exercise program actually showed improvements in bone mass. “Those who worked the hardest also got the benefit of bone,” he said.

Stewart explained that research has shown fat can actually help bone because it creates a beneficial stress on bone. Also, fat produces estrogen, which contributes to healthier bone, and fat is also associated with higher levels of the hormone leptin, which animal studies show can help form new bone cells, he added.

SOURCE: American Journal of Preventive Medicine, June 2005.