Active Voice: Metabolic Adaptations in Older Men Following a 14-Day Regimen of Prolonged Exercise

By Thomas H. Morville, M.D., B.S.

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

Thomas H. Morville, M.D., B.S., is a Ph.D. student working in the Department of Biomedical Sciences at the University of Copenhagen in Denmark. He is a medical doctor and holds a bachelor’s degree in sport science. His current research areas include aerobic fitness, muscular strength and metabolic regulation in relation to cholesterol-lowering medication.

This commentary presents Dr. Morville’s views on the topic of a research article that he and his colleagues had pubished in the February 2017 issue of
Medicine & Science in Sports & Exercise® (MSSE).

Very prolonged exercise is becoming increasingly popular among both younger and older individuals who constantly seek to push the limits of physical performance and the associated environmental challenges. The regulation of energy metabolism during and after exercise of shorter duration has been investigated thoroughly, but less is known when it comes to exercise of very prolonged duration. Moreover, there is lack of knowledge as to how older individuals tolerate and adapt to performing these prolonged exercise challenges.

When old and young individuals are compared following acute exercise or after endurance training, the results often show great similarity, e.g., increases in aerobic fitness and metabolic adaptations to increased energy demands and delivery. But, as would be expected, the effects of acute exercise and endurance training are most often less pronounced in those who are older.

In recent years, it has been observed that after repeated bouts of very prolonged exercise young individuals respond with an increased capacity to oxidize fat during exercise. However, whether this adaptation is similar in older individuals is unknown. As reported in our article in the February 2017 issue of MSSE, we sought to answer this question. In this study, we examined six older men (mean age, 61 years) who performed 2706 km of outdoor cycling over 14 days. We hypothesized that their metabolic adaptations would be like those reported in younger individuals, i.e., that they would show both increases in aerobic fitness and maximal fat oxidation. We obtained muscle biopsies and blood samples at rest before and 30 hours after completion of the 14 days of cycling. Independently, before and after the 14 days of prolonged cycling, subjects also completed incremental cycle ergometer exercise tests to maximal effort, which included indirect calorimetric measurements and assessment of maximal oxygen uptake.

During the 14 days, the older men cycled 10.5 hours every day. Over that period, they more than doubled their daily average intakes of carbohydrates (931 grams) and total energy (just over 6000 kcal), but experienced no changes in body weight! Contrary to our hypothesis, we saw a reduction in the maximal fat oxidation, as well as a decreased aerobic capacity following the repeated prolonged exercise. Plasma levels of lipids were markedly lower, suggesting decreased lipid mobilization. Along with an increase in glucose transport capacity, these findings probably account for the observed decrease in maximal fat oxidation.

We saw a slight decrease in insulin sensitivity. Coupled with the observed decrease in aerobic fitness, this suggests that the extreme repeated prolonged exercise is probably at or exceeding the tolerable exercise limit and, thus, beyond the adaptive capacity in these older men.