The issue has long been pondered by doctors working with diabetic patients, one expert said.

"For many years we have been under the impression that exercise helps decrease insulin resistance in muscles," boosting blood sugar control, said Dr. Maria Pena, director of the Center for Weight Management at North Shore-LIJ's Syosset Hospital in Syosset, N.Y.

"However, from clinical experience we are still puzzled by the discrepancies between patients that we see in varying weight loss, exercise, and changes in metabolic profile," she added.

"In other words," Pena said, "why is it that someone who walks 30 minutes every other day and loses 15 pounds is able to significantly reduce their hemoglobin A1c [a measure of blood sugar control], whereas another person who reports to exercising twice as much is unable to achieve the same success?"

In the new study, a group led by Lauren Marie Sparks of Florida Hospital and the Sanford-Burnham Medical Research Institute in Orlando examined clinical trials that looked at the effects of exercise among people with type 2 diabetes. They also looked at genetic research on the topic and research done in animals.

Their analysis revealed that in 15 percent to 20 percent of people with type 2 diabetes, exercise did not lead to improvement in blood sugar control, insulin sensitivity, or their body's ability to burn fat.

The animal and genetic studies suggest that this "resistance to exercise" among people with type 2 diabetes is genetic and can be handed down through generations, according to findings published Nov. 20 in the Journal of Clinical Endocrinology & Metabolism.

"Since obesity and lack of physical activity are two key risk factors for type 2 diabetes, physicians frequently recommend exercise and other lifestyle interventions to prevent or manage the disease," Sparks said in a journal news release.

"Most people benefit from an exercise regimen, but our research indicates that a significant minority of individuals with type 2 diabetes do not experience the same improvements in metabolism due to their genes," she said.

The issue is an important one, since about 40 percent of Americans will develop type 2 diabetes in their lifetime, according to the U.S. Centers for Disease Control and Prevention.

"More research is needed to determine which people with or at risk of developing type 2 diabetes will respond to an exercise program and which will not," Sparks said. "With that information in hand, we can target specific interventions and treatments to the individuals who will benefit most and identify novel treatment approaches to help those who do not respond to exercise."

One other expert stressed that this in no way means that people with type 2 diabetes -- which is often linked to obesity -- should give up on regular exercise.

"Ask yourself this: Are you among the 15 percent who purportedly do not benefit from exercise, or among the 85 percent who do?" said Dr. Ronald Tamler, clinical director of the Mount Sinai Diabetes Center in New York City.

"In my practice, I find that different people will respond to different types of exercise," he said. "When patients are matched with the proper type and intensity of exercise, I often see a need for less diabetes medication."