If 100 individuals complete the identical aerobic training program there will be a large variation in results. Although all will receive some benefit, some individuals will show huge improvements in aerobic capacity while others will only see modest improvements. The question is why? A fascinating study from researchers at the Joslin Clinic provides enormous insight on the biological mechanisms that explain these differences. The study investigated what genes were activated in two groups of laboratory animals that had been bred to respond either very well or very poorly to endurance exercise, specifically running on a treadmill.
The results of the study were quite unexpected. There was a strong correlation in the response to endurance exercise and activation of a gene called JNK for short. Activation of the JNK gene was associated with poor response to endurance exercise. To further prove the association, the researchers removed the JNK gene from the mice who were poor responders, repeated the exercise protocol and found the mice had a significant improvement in endurance capacity. Furthermore, there was an increase in muscle fibers associated with enhanced aerobic capacity.
The endurance limiting action of the JNK gene appears to be mediated by a protein called myostatin. Myostatin has been shown to limit muscle growth. Although animal studies are suggestive, researchers confirmed that the JNK gene works the same in human endurance runners. As one might expect, the JFK gene is not stimulated in runners with strong endurance capacity.
The results with the JNK gene may go a long way in explaining the large variability in individual response to aerobic exercise.