Steven L. Britton of the University of Michigan Health System and his colleagues bred 11 generations of laboratory rats that were selected for their exercise capacity, so that the latest litters differ significantly from one another in their ability to use oxygen effectively. "Essentially, we are breeding for genes that code for low levels of proteins involved in mitochondrial function," Britton explains. "Our hypothesis is that impaired mitochondrial function may be what links low aerobic capacity and disease." The scientists subjected the rats to treadmill testing (without training them extensively) and found that one group tired after 14 minutes of running, whereas the group that was the best at utilizing oxygen could be on the move for 42 minutes before exhaustion set in. In addition to tuckering out more easily, the rats with low aerobic capacity had higher levels of blood fat disorders and insulin resistance, and more abdominal fat than the long-distance runners did, as well as having higher blood pressure and more vascular dysfunction.
The results provide yet more impetus to get back to the gym. "The reality of having a genetic determinant of our existence is that there are some people who are born with less ability to take up oxygen and transfer energy than others," Britton remarks. "These people may have to work harder and will never reach the status of a professional athlete, but almost everyone can improve their aerobic capacity and health status with regular exercise."