Every doctor recommends regular aerobic exercise, since greater aerobic fitness is important for achieving better overall health.
But in a new study, researchers have discovered that some benefits of aerobic exercise may be dampened by higher-than-normal blood sugar levels, a condition known as hyperglycemia.
These diminished gains are seen in mouse models and humans with chronic hyperglycemia that is in the “pre-diabetes” range.
The study also showed that this maladaptive trait is independent of obesity and insulin levels in the blood.
The research was conducted by Joslin Diabetes Center scientists.
Clinical studies have demonstrated that people with diabetes or chronically high levels of blood sugar struggle to improve their aerobic exercise capacity compared to people with normal blood sugar levels.
In the study, the team used two mouse models that reflect the two major causes of hyperglycemia in humans.
One group of mice consumed a Western diet, high in sugar and saturated fat, which caused some weight gain in addition to hyperglycemia.
The other group was modified to produce less insulin, which caused similar increases in blood sugar as the Western diet, even though the mice ate a diet lower in sugar and fat, and maintained normal body weight.
Both groups were subjected to a training protocol in which they ran in wheels in their cages to boost their aerobic fitness.
In both hyperglycemic groups, animals ran around 500 kilometers over the course of the study but, on average, failed to improve their aerobic exercise capacity compared to mice with lower blood sugar levels.
Looking in more detail at the skeletal muscle in these mice, the team saw that the muscle was not adapting to the aerobic challenge as muscle normally would.
Muscle tissue can remodel itself, which is one reason why exercise becomes easier when we do it regularly.
Over time, aerobic exercise such as running or swimming can alter muscle fibers to become more efficient at using oxygen during exercise.
People also grow new blood vessels to allow more oxygen to be delivered to the muscle, which helps to increase our aerobic fitness levels.
The scientists say that high levels of blood sugar may prevent muscle remodeling in part by modifying the “extracellular matrix” proteins in the space between the muscle cells, where blood vessels are formed.
Clinical tests with young adult volunteers found that those who had higher blood sugar levels in response to ingesting glucose, a condition known as impaired glucose tolerance, showed the lowest aerobic exercise capacity.
The team says although hyperglycemia may harm aerobic fitness with training, people still can achieve other important health benefits from the exercise including decreased fat mass and improved glucose metabolism.
So, regular aerobic exercise is still a key recommendation for maintaining health in people with or without hyperglycemia.
It is important to note that people with hyperglycemia also can benefit from other forms of exercise, such as strength training, which is recommended for maintaining health.
One author of the study is Sarah Lessard, Ph.D., a Joslin assistant investigator in the section of Clinical, Behavioral and Outcomes Research.
The study is published in Nature Metabolism.
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