By Maggie Kuo
We are constantly encouraged, or nagged, to exercise to promote good health. But how much exercise is enough? We finally might be able to precisely measure this threshold.
The research group lead by Thomas Thum at the Hannover Medical School in Hannover, Germany has been studying the use of microRNAs (miRs), noncoding RNAs that repress gene expression, to treat cardiovascular diseases. As a side project, the group wondered if miRs could be used as an indicator of physical fitness. The researchers found that a single session of endurance exercise increased the amount of miRs associated with the skeletal muscle. Moreover, the amount of these miRs did correlate with fitness level.
Physical endurance is generally measured by maximum oxygen uptake (VO2max), the amount of oxygen used by the skeletal muscles during maximum effort exercise. VO2max increases with fitness level. How much an exercise regimen will raise VO2max depends greatly on the individual’s genes so the outcomes in one person cannot necessarily be applied to another person. As a result, researchers in the exercise physiology field have been exploring the effects of exercise directly on gene expression.
miRs can be specific to the organ, like the heart, or to the biological process, like inflammation. miRs are also stable outside of the cell and can therefore be detected in the blood plasma. Several studies reported changes in plasma miR levels after exercise. The miRs corresponded with skeletal and heart muscle adaptations and the levels were influenced by the intensity, type, and duration of the exercise protocol.
Thum and colleagues measured the amount of heart and skeletal muscle-associated miRs in blood samples from experienced marathon runners. Blood was drawn two days before the race and immediately and 24 hours after the runner finished the course. The researchers also recorded VO2max and anaerobic lactate threshold (VIAS), another physical fitness indicator, for each runner. They found that the skeletal muscle miRs, miR-1, -133a, and -206, were higher shortly after the run and remained elevated 24 hours after the race. The heart muscle miRs, miR-208b and -499, were also higher immediately after the run but returned to pre-race levels after 24 hours. What intrigued the researchers the most was that the levels of all three skeletal muscle miRs trended with both VO2max and VIAS. Runners with greater VO2max and VIAS, meaning they were more fit, tended to have higher amounts of skeletal muscle miRs in their blood plasma. The researchers then compared the gene targets of the five miRs they measured against a list of 56 genes associated with physical endurance. The heart miRs only modulated one gene on the list. However, the skeletal muscle miR-1 regulated 8 genes and -206 regulated 10 genes, suggesting that enhanced fitness was due to increased regulation of physical endurance genes from the higher miR levels. The researchers concluded that these skeletal muscle miRs could potentially be used as biomarkers of physical fitness.
More work needs to be done to confirm the accuracy of miRs as biomarkers, including measuring the amount of the miRs directly in the skeletal muscle, studying how the miR levels change with different exercise protocols, and determining if other miRs are influenced by endurance. Nevertheless this study provides a potential new metric to evaluate physical fitness. This metric could be used to establish more precise recommendations on exercise routines to promote better health.