People with consistently high levels of blood sugar could get less benefit from exercise than those whose blood sugar levels are normal, according to a new study of nutrition, blood sugar and exercise. The study suggests that eating a diet high in sugar and processed foods, which may set the stage for poor blood sugar control, could dent our long-term health in part by changing how well our bodies respond to a workout.

People with hyperglycemia tend to be overweight and face greater long-term risks for heart disease and Type 2 diabetes, even if, in the early stages, their condition does not meet the criteria for those diseases.

But most past studies of blood sugar and fitness have been epidemiological, meaning they have identified links between the two but not their sequence or mechanisms. They have not clarified whether hyperglycemia usually precedes and leads to low fitness, or the other way around, or how either condition manages to influence the other.

For the new study, published in Nature Metabolism, researchers at the Joslin Diabetes Center in Boston and other institutions started with adult mice, switching some to a diet high in sugar and saturated fat. These mice rapidly gained weight and developed habitually high blood sugar.

They injected other mice with a substance that reduces their ability to produce insulin, which helps control blood sugar. Those animals did not get fatter, but their blood sugar levels rose to the same extent as the mice in the sugary diet group.

Other animals remained on their normal chow, as a control.

When tested after four months, each mouse on average ran about 300 miles during a month and a half. But the control group ran for a much longer period of time before exhaustion; they were much fitter. The animals with high blood sugar showed little improvement.

Their exercise resistance was the same, whether their blood sugar problems stemmed from poor diet or lack of insulin, and whether they were overweight or slimmer. If they had high blood sugar, they resisted the benefits of exercise.

Scientists found that the muscles of the control animals teemed with healthy, new muscle fibers and a network of new blood vessels ferrying extra oxygen and fuel to them. But the muscle tissues of the animals with high blood sugar displayed mostly new deposits of collagen, a rigid substance that seems to have crowded out new blood vessels and prevented the muscles from adapting to the exercise and contributing to better fitness.

The scientists also checked blood sugar levels and endurance in a group of 24 young adults. During treadmill fitness testing, those volunteers with the worst blood-sugar control also had the lowest endurance, and when the scientists later microscopically examined their muscle tissues after the exercise, they found high activation of proteins that can inhibit improvements to aerobic fitness.

Taken as a whole, these results suggest that “constantly bathing your tissues in sugar” could undercut any subsequent benefits from exercise, said Sarah Lessard, an assistant professor at the Joslin Diabetes Center and Harvard Medical School, who oversaw the study.

But Lessard also noted that while the hyperglycemic mice gained little endurance, they were beginning to show early signs of better blood-sugar control. So, it might require time and grit, but exercise eventually could help people with hyperglycemia to stabilize their blood sugar, she said, and then start feeling their fitness rise.