Active Voice: Muscle gets better with age - enhanced oxidative capacity in lifelong endurance athletes!

By F. Amati, M.D., Ph.D., FACSM, J.J. Dubé, Ph.D. and B.H. Goodpaster, Ph.D.
F. Amati, M.D., Ph.D.,
J.J. Dubé, Ph.D. B.H. Goodpaster, Ph.D.

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Francesca Amati, M.D., Ph.D., FACSM, is an associate professor in the Department of Physiology and Institute of Sports Sciences at the University of Lausanne, Switzerland. Her research focuses on metabolic adaptations to exercise in aging and chronic diseases. Her clinical duties combine activities in sports metabolism and diabetes clinics.

John J. Dubé, Ph.D., is an assistant professor of Biology at Chatham University in Pittsburgh, Pennsylvania. His research focuses on skeletal muscle substrate regulation, particularly intramyocellular lipids, as it relates to muscle function and metabolic disease.

Bret H. Goodpaster, Ph.D., is a senior investigator and professor at the Florida Hospital Sanford Burnham Prebys Translational Research Institute for Metabolism and Diabetes. His research and scholarship have made a broad impact across the disciplines of exercise physiology, metabolism, obesity, diabetes and aging.

This commentary presents the views of these three investigators on the topic of a research article which they had published with their colleagues in the March 2016 issue of
Medicine & Science in Sports & Exercise® (MSSE).

According to the United States Census Bureau, the population of Americans aged 65 years and older is expected to grow to 83.7 million by the year 2050. This has the potential to place an enormous burden on the health care system, as well as individuals, given the association between aging, declines in physical activity and increased rates of morbidity and mortality. While a significant body of literature has clearly demonstrated the capacity of older adults to respond to exercise interventions, less is known about effects of lifetime fitness on skeletal muscle.

Chronic aerobic exercise results in several positive adaptations within skeletal muscle, including a maintenance of oxidative fibers and overall fiber size, as well as enhanced mitochondrial function. These aspects of muscle physiology likely contribute to an improved quality of life and reduction in the risk for falls. Yet, there are gaps in our understanding of the effects of lifetime fitness, skeletal muscle metabolism and whole-body substrate oxidation.

In our recently published MSSE article, we examined several aspects of skeletal muscle physiology, substrate oxidation and insulin sensitivity in 14 young (age 18-39 years) and 13 older (age 60-75 years) endurance-trained athletes. There were several strengths to our study design and analytic approach, including subjects who were matched for mode and frequency of exercise and that we directly assessed skeletal muscle characteristics from biopsy samples. Also, using indirect calorimetry, we assessed whole-body carbohydrate and fat oxidation under different physiological conditions - including fasting, insulin-stimulation (mimicking what happens after a meal) and aerobic exercise. Finally, we measured insulin sensitivity using the hyperinsulinemic euglycemic clamp, known as the gold standard method to assess how the body responds to insulin, which is an important marker of metabolism.

The results of our study indicated that older endurance-trained athletes had greater intramyocellular lipid compared to the younger athletes - this was true of both the oxidative and glycolytic fiber types that we analyzed. Conversely, muscle from younger athletes contained more glycogen. In addition to greater lipid storage, we demonstrated that older athletes had a higher relative proportion of oxidative fibers. Importantly, we demonstrated that older endurance trained athletes have enhanced metabolic efficiency during exercise (i.e. the proportion of energy derived from fat oxidation). We could not detect any differences in metabolic flexibility (i.e. the capacity to switch from fat to carbohydrate after insulin stimulation) or insulin sensitivity between the older and younger athletes.

The results from our study suggest that lifetime aerobic exercise training results in key skeletal muscle adaptations that promote enhanced lipid storage and utilization at moderate exercise intensities. These data emphasize the importance of physical activity as a method to attenuate the age-related declines in skeletal muscle oxidative capacity.