Not everyone with type 2 diabetes will benefit from exercise, according to a fresh look at past research. Exercise is widely recommended to help prevent and manage type 2 diabetes, but as many as one in five patients may not benefit, researchers suggest.

“But at this stage, we do not know who will and who will not respond to an exercise program,” said review author Lauren M. Sparks of the Sanford-Burnham Medical Research Institute in Orlando.

Sparks believes this phenomenon is rooted in genetics. She hopes more research programs committed to investigating the underlying mechanisms will identify the individuals before they begin an exercise program.

“We can then ultimately develop novel strategies, while still using exercise, to help them in their quest to either prevent the onset of type 2 diabetes or to get their diabetes under control,” she said.

In type 2 diabetes, sometimes referred to as adult-onset diabetes, the body's cells are resistant to insulin or the body doesn't make enough of the hormone, so glucose remains in the bloodstream and can climb to dangerously high levels. Insulin gives glucose - or blood sugar - access to the body's cells to be used as fuel.

Regular exercise plays a role in preventing the disease as well as in helping to manage it. Thus, regular exercise programs along with diet and medication are often part of a person’s treatment plan.

In their new study, published in the Journal of Clinical Endocrinology and Metabolism, Sparks and her colleague Natalie A. Stephens reviewed both published and unpublished research in an attempt to understand why exercise fails to benefit some people with diabetes.

The review, which included some of Sparks' own studies, showed that a minority of individuals did not improve their glucose control after nine months of supervised exercise.

“We now know that about 15 to 20 percent of individuals do not respond to supervised exercise interventions - at least in terms of glucose control and muscle metabolism,” she said. “And for many, it’s not from a lack of trying. They are making an effort.”

Sparks said some participants used treadmills three to four days a week for nine months and still ended up no better than when they started.

“We did not intervene with their diets, and these individuals were on their prescribed medications," she said. "I have also seen similar data from some of my colleagues’ studies.”

Even with these results, doctors should continue to prescribe exercise to their patients with diabetes as it has a lot of benefits - including improving quality of life, Sparks said.

“And for those that simply are not responding, don’t tell them to stop exercising, but just continue to work with them,” she added.

The review's results should be a "call to action” for more research to identify why exercise is not helping and how to better identify the people who seem to be “exercise resistant,” Sparks said.