FRIDAY, Oct. 13, 2006 (HealthDay News) -- If you're a fitness buff, the increased amount of energy you burn during your lifetime probably won't shorten your life, new research shows.
That may seem counterintuitive, considering the many proven benefits of a physically active lifestyle. But one scientific theory, the "rate-of-living" theory, suggests that increased physical activity may actually result in a decreased lifespan.
The theory holds that every organism is born with a set amount of energy to expend. Once its allotment of calories is burned up, its health declines and it dies, according to the theory.
"We've found further proof, which adds to a growing body of evidence, that the rate-of-living theory is not valid," said Theodore Garland Jr., a biology professor at the University of California, Riverside and co-author of a study presented this week at the American Physiological Society conference in Virginia Beach, Va.
The study involved 300 mice, 200 of which had been bred over many generations to love to run on wheels. One hundred of these "runner" mice had access to wheels and 100 did not. The other 100 mice were regular laboratory mice with wheels in their cages.
The runner mice with wheels in their cages expended 25 percent more energy during their lifetimes than the other two groups of mice, the researchers found.
"According to the rate-of-living theory, the physically active mice should have died sooner than the sedentary mice," Garland said. But they didn't. The average lifespan of the physically active runner mice and the sedentary runner mice were virtually identical: 735 days vs. 725 days.
The regular laboratory mice lived the longest of all -- an average of 826 days. This also contradicts the rate-of-living theory, which would have predicted that the runner mice without the wheels and the lab mice with a wheel would have the same average lifespan because they expended the same amount of energy.
"The shorter lifespan cannot, therefore, be explained by a difference in metabolism," study author Lobke Vaanholt, of the University of Groningen, the Netherlands, said in a statement. "There must be something else going on that causes these animals to age and die."
Garland speculated that genetic dissimilarities between the runner mice and the laboratory mice might be responsible.
"These lines of mice have had separate breeding pools for over 30 generations," Garland said. "They differ in a lot of ways, so in a sense they're apples and oranges. The situation is similar to small, human populations that have been separated for many generations. Any differences in lifespan could be attributable to a variety of genetic differences."
During another portion of the study, the researchers selected 40 mice from each group to assess their energy expenditure, body composition and antioxidant enzyme levels in their hearts and livers.
Because an increased activity level is associated with a higher metabolic rate and more oxidative stress, the researchers expected to see increased levels of antioxidant enzymes in the runner mice with wheels. But they found that all three groups of mice had the same amount of antioxidant enzyme activity during assessments at age 2 months, 10 months, 18 months and 26 months.
"I was a little surprised by this, given some of the previous results we've seen," Garland said. In 2002, Garland and another group of researchers found higher levels of antioxidant liver enzymes in runner mice, especially in females.
New research must examine whether tissues outside the liver and heart generated additional antioxidants to help cope with the increased oxidative stress brought on with increased activity and metabolic rate, Vaanholt said in a statement. Future studies should also examine other possible mechanisms, one of which is that increased physical activity helps improve DNA repair rates.
As scientists learn more about the intricate and interrelated effects of exercise, diet and genes on aging, at least there's general agreement about one thing: Exercise is good for you.
"Aside from any possible benefits for lifespan, exercise in moderation has been shown to have many positive effects, including on health span, immune function, heart disease and mental health," Garland said.
For more on healthy aging, visit the U.S. Centers for Disease Control and Prevention.