Almost 20 percent of people with type 2 diabetes who participate in exercise plans do not see improvements in metabolism due to stubborn genes, according to a new scientific review.

Obesity and lack of physical activity are two key risk factors for type 2 diabetes, so doctors often recommend exercise and other lifestyle interventions to prevent or manage the disease. But as many as 1 in 5 people with type 2 diabetes do not see any improvement in blood sugar management when they take part in a supervised exercise program, according to new scientific review published in the Endocrine Society’s Journal of Clinical Endocrinology & Metabolism.

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A review of scientific literature showed that 15 to 20 percent of study participants with diabetes didn’t see any improvement in their blood sugar control, insulin sensitivity, or muscle mitochondrial density brake, a measurement of the ability to burn fat, study co-author Lauren M. Sparks, Ph.D., of the Translational Research Institute for Metabolism and Diabetes at Florida Hospital and the Sanford-Burnham Medical Research Institute in Orlando, told Healthline.

One study of 1,700 people with type 2 diabetes found that 7 percent actually had an adverse response to exercise in the form of greater cardiovascular risk factors, Sparks explained.

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The research tracked 45 African American and Caucasian adults in their mid-50s who had type 2 diabetes. The volunteers all exercised for nine months, doing weight training and aerobic activities.

About 20 percent of participants saw no changes in key measurements like HbA1c (a measure of blood glucose concentration over long periods of time), fat burning ability, Body Mass Index (BMI), and body fat after nine months of exercising.

Although the study had a limited sample size, Sparks believes that if the results were scaled to the 30 million people estimated to have type 2 diabetes, they would show that a significant number of people with diabetes are not benefitting from exercise.

Researchers are beginning to understand that when a person exercises, there are immediate changes in parts of their genes in response to the experience, explained Sparks.

“What happens to these people who don’t respond is that they are not changing the way their DNA is expressed in response to that exercise,” Sparks said. “They are not able to release that brake.”

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Since DNA is passed from one generation to another, this so called “brake” in the DNA of some type 2 diabetes patients, which halts the response to exercise, is also inherited.

Sparks believes researchers may be able to use people’s DNA to discover which genes aren’t responding. Drug companies could ultimately find compounds or drugs to release the “brake.”

“The idea is not to replace exercise. The message is, let’s find these people, find out where the brake is, and find a way to release that brake. It could be drugs, other kinds of exercise, a supplement, or a change in diet,” said Sparks, adding that more studies are needed, especially in larger patient populations.

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Dr. Gerald Bernstein, director of the diabetes management program at the Friedman Diabetes Institute at Mount Sinai medical center in New York, told Healthline that although Sparks’ study was conducted on a small population, it “opens a lot of doors to finding solutions through exercise.”

Bernstein said the study raises the following questions for future research: Is the data consistent over a lifetime? Would an exercise non-responder at age 30 be the same at 60, even if he or she continued to exercise over those 30 years? Can repetitive behavior over many years result in gene modification a decade or two later?

Bernstein said it is still very important for patients to exercise, provided they work with their doctors.

“The better part of therapeutic valor would be for everyone at risk to be in an exercise program for their lifetime, as we do when we brush and floss our teeth,” he said.