Based on a recent study, a diet high in sugar and processed foods can change the way are bodies react to exercise and effect our long-term health.

It is widely understood that elevated blood sugar is unhealthy. People with hyperglycemia tend to be overweight and face greater long-term risks for heart disease and Type 2 diabetes. They are typically out of shape and have low aerobic fitness levels.

But most past studies of blood sugar and fitness have been epidemiological, meaning they have identified links between the two conditions 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.

Blood Sugar Study

The new study, which was published in Nature Metabolism, decided to raise blood sugar levels in mice and see what happened when they exercised. They started with adult mice, switching some from normal chow 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, a hormone that helps to control blood sugar, similar to when people have certain forms of diabetes. Those animals did not get fatter, but their blood sugar levels rose to the same extent as among the mice in the sugary diet group. Other animals remained on their normal chow, as a control group.

After four months, the scientists checked each mouse’s fitness by measuring how long it could run on a treadmill before exhaustion.The control group now ran for a much longer period of time on the treadmill before exhaustion; they were much fitter. But the animals with high blood sugar showed little improvement. Their aerobic fitness had barely budged.

Scientists next looked inside muscles. And conditions there were telling. The muscles of the control animals showed healthy, new muscle fibers and a network of new blood vessels carrying 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 study was also conducted on human beings. 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.

The findings suggest that, for those of us whose blood-sugar levels depend on our diets, we might want to “cut back on sugar” and the highly processed, fatty foods that also can raise blood sugar and blunt exercise effects. By stabilizing our blood sugar we may be able to improve our fitness levels.

Something to think about with the holidays fast approaching.