THURSDAY, Jan. 20, 2005 (HealthDayNews) -- Rats that are carefully bred to have a low ability to exercise have a much higher incidence of the risk factors for heart disease and stroke than those bred to run longer and better, an American-Norwegian study finds.
But that doesn't mean that anatomy (or genetics) is destiny for either rats or people, said Steven Britton, a professor of physical medicine and rehabilitation at the University of Michigan Medical School, and leader of the U.S. wing of the study.
Rather, the part of the study done at the Norwegian University of Science and Technology showed that a relatively brief exercise training program can produce significant improvement in the performance of the low-capacity mice.
The study, whose results appear in the Jan. 21 issue of Science, was designed to look at the underlying molecular factors that lead to an increased incidence of cardiovascular disease for individuals who don't get enough physical activity, Britton said.
He and his colleagues started with 168 laboratory rats that could run for an average of 23 minutes on a treadmill. Then they singled out the 13 males and females who could run the longest and the 13 who pooped out earliest. Each group served as a mating population for the next generation.
Eleven generations later, the researchers had two decidedly distinctive populations -- one group of rats that could run on a treadmill for 42 minutes before they ran out of energy, the other a group whose treadmill time averaged 14 minutes.
Studies showed the low-capacity rats scored higher on cardiovascular risk factors such as high blood pressure, insulin resistance and abdominal fat. Molecular examination of mitochondria, the energy-producing components of cells, showed that the low-capacity rats had abnormally low levels of the molecules needed to promote physical activity.
"Compared to high-capacity rats, the low-capacity rats had lower levels of oxidative enzymes and proteins used by mitochondria to generate energy in skeletal muscles," said Sonia M. Najjar, an associate professor of pharmacology at the Medical College of Ohio, who did the mitochondrial studies.
On the bright side, experiments done by Ulrik Wisloff in Norway showed that training can improve the performance of the low-capacity rats. A six-week training program on the treadmill produced significant improvement in 11 of 12 measures of exercise capacity for the low-capacity rats, although they were still outperformed by their high-capacity cousins.
The idea of the rat studies, which are continuing, is to explore both the genetic and environmental factors that influence exercise performance, Britton said.
"Some people are born with less ability to take up oxygen and transfer energy than others," he said. "These people may have to work harder and will never reach the level of a professional athlete, but almost everyone can improve their aerobic capacity and health status with regular exercise."
"Almost" is a crucial word, Britton noted. "Not everyone has the capacity to respond to exercise training," he said. "Most people do improve, but there is a whole spectrum of responses."
Advice for people who are musing about starting an exercise program is offered by the American Council on Exercise.