[A] study, which involved rodents and people, 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.
We already have plenty of evidence, of course, 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, even if, in the early stages, their condition does not meet the criteria for those diseases.
They also tend to be out of shape. In epidemiological studies, people with elevated blood sugar often also have low aerobic fitness.
[T]he 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 [researcher] Sarah Lessard.