High Blood Sugar Lowers Aerobic Fitness

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Although exercise is essential for good health, high blood glucose, or blood sugar, levels can undermine or even cancel out the benefits of aerobic exercise.

Hyperglycemia and Workout Capacity

Individuals with consistently high levels of blood sugar seem to derive less benefit from their workouts than their non-hyperglycemic counterparts, according to a cautionary new study of nutrition, blood sugar, and exercise. Consuming a diet high in processed foods could also alter how well our bodies respond to exercise.

Many clients open up to trainers during an initial assessment, not only regarding fitness goals but also other health struggles. Trainers have no doubt noticed throughout their careers that often clients who reveal issues with hyperglycemia tend to be deconditioned and overweight, placing them at a greater long-term risk for heart disease and Type 2 Diabetes, if these aren’t already factors. Epidemiological studies indicate that people with elevated blood sugar often also have a diminished aerobic capacity, closely linked to a high risk of premature death.

Blood Sugar Factor: Causation or Correlation?

Most studies linking blood sugar and fitness have identified correlations between the two; however, they fall short in clarifying how either condition influences the other. The answer to this key question has yet to be determined : Does hyperglycemia precede and lead to a reduced level of fitness, or does a lack of aerobic capacity foster high blood sugar levels?

The results of a recent study, documented and published in the journal Nature Metabolism, aim to shed light upon these very questions. Researchers at the Joslin Diabetes Center in Boston, in collaboration with other institutions, sought to explain how elevated blood sugar levels in mice affected their exercise capabilities, if at all.

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Vascular Remodeling

With regular aerobic exercise, the muscles of the control animals exhibited an abundance of healthy, new muscle fibers, along with increased blood vessels. Optimal vascularization such as this facilitates the shuttling of additional oxygen and fuel to the muscle tissues. In comparison, the same tissue in mice with elevated blood glucose levels displayed mostly new deposits of collagen, a rigid substance that crowds out new blood vessels, thereby preventing the muscles from “remodeling” and contributing to improved levels of fitness.

Finally, the scientists repeated this protocol with a test group of 24 young adults. During treadmill fitness testing, the subjects whose bodies displayed the worst blood-sugar control also exhibited diminished endurance. Upon microscopic examination of their muscle tissues following the exercise sessions, researchers noted the presence of proteins that actually inhibit improvements to aerobic fitness. The scientists propose that high levels of blood sugar may prevent muscle remodeling in part by modifying the “extracellular matrix” proteins in the region of blood vessel formation. This less-vascularized muscle tissue explains the observed lack of increase in aerobic fitness, in spite of diligent exercise.

Focus on Clients’ Overall Health

Sarah Lessard, an assistant professor at the Joslin Diabetes Center and Harvard Medical School and lead author of this study, advocates a combination of diet and exercise for optimal health. “The good news is that although our hyperglycemic mouse models did not improve aerobic fitness through exercise, they did achieve other important health benefits from exercise, including a reduction in body fat and improved glucose metabolism,” says Dr. Lessard. Therefore, regular aerobic exercise remains a key recommendation for maintaining health, regardless of blood glucose levels.

The adaptation of this muscle tissue points to the precise reason why the relative comfort of frequent and regular participation in exercise replaces that initial overwhelming exhaustion, Lessard says. With time, any vigorous aerobic exercise can alter muscle fibers towards a greater oxygen utilization during aerobic workouts. “We also grow new blood vessels to allow more oxygen to be delivered to the muscle, which helps to increase our aerobic fitness levels,” she says.

Caring, Sharing, and Fostering Results

Clearly, this new scientific data points to chronic hyperglycemia as a potential negative regulator of aerobic adaptation. Such knowledge leads us to realize the importance of dietary/metabolic health in conjunction with exercise. We can share this with clients when discussing food choices, raising awareness about today’s soaring incidence of high blood sugar, both in adults and children. Addressing this topic early in the trainer/client assessment can foster trust as well as the evolution of more successful outcomes of their exercise-related goals.







Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at [email protected] She welcomes your feedback and your comments!