Interestingly, 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.
To better understand why, the scientists next looked inside muscles. And conditions there were telling. 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.
Finally, since rodents are not people, the scientists checked blood sugar levels and endurance in a group of 24 young adults. None had diabetes, although some had blood-sugar levels that could be considered prediabetic. 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 in mice and people suggest that “constantly bathing your tissues in sugar is just not a good idea” and could undercut any subsequent benefits from exercise, says Sarah Lessard, an assistant professor at the Joslin Diabetes Center and Harvard Medical School, who oversaw the new study.
In practical terms, 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, she says. (The control mice ate a high-carbohydrate chow, so carbohydrates, per se, are not necessarily the issue, she says; diet quality is.)
More fundamentally, the study intimates that “diet and exercise should be considered together” when we start thinking about how to improve our health, Dr. Lessard says. They affect each other and they influence how each affects us more than we might expect, she says.
But perhaps most important, the study contains some encouraging data, Dr. Lessard points out. The hyperglycemic mice gained little endurance from their weeks of working out, but they were beginning to show early signs of better blood-sugar control, she says. So, it might require time and gritty determination, but exercise eventually could help people with hyperglycemia to stabilize their blood sugar, she says, and then start feeling their fitness rise.