While the findings don't mean that people who don't have the variant should give up on exercising, it does show why some older adults may maintain better physical function than others.

More than a third of American adults age 70 and older report having difficulty walking a quarter of a mile or climbing 10 stairs. Those who have these difficulties are at four times the risk of entering a nursing home and three times the risk of dying within two years compared with those who report no such mobility problems.

Link to ACE-Regulating Gene

The new study, which includes more than 3,000 well-functioning adults (ages 70-79) still living in their community around Pittsburgh and Memphis, Tenn., found that those who were active enough to burn about 1,000 calories a week from exercise are about a third less likely to report mobility problems than those who did not exercise.

But researchers found that older exercisers who inherited a gene combination that gives them lower levels of an enzyme involved in regulating blood pressure were 45 percent more likely to still develop mobility problems than exercisers with a gene combo that produces higher levels of the enzyme.

"Our results reinforce the importance of exercise, but also may explain a mechanism for why it seems to benefit some individuals more than others," said Stephen Kritchevsky, a professor of gerontology at Wake Forest University School of Medicine and lead author of the study, published Wednesday in the Journal of the American Medical Association.

Specifically, the study shows that a gene that controls levels of the angiotensin-converting enzyme (ACE) in the body is associated with physical function in older adults.

Three Combinations

Millions of people take drugs called ACE inhibitors to block the effects of the enzyme that sometimes excessively narrows the arteries and causes blood pressure to rise and contributes to congestive heart failure.

But the research shows that various gene types guide the body's own tweaking of this system. The same ACE gene combination associated with better function in older adults has been linked to superior muscle strength and power in young athletes.

The new study was designed to test where ACE production also affects physical function in older exercisers.

Researchers followed the adults for up to four years, noting which ones had developed mobility limitations. "Overall, ACE genotype seems to be associated with how well activity helps preserve function," Kritchevsky said. "This finding may offer new opportunities to explore treatments to help older adults maintain their [physical] function."

The gene that controls ACE production can be inherited with three different combinations. Only about 19 percent of study participants had the combination that's associated with lower ACE production, and thus did not benefit as much from exercise.

Those with the subtype also had higher levels of total body fat and fat in their thighs, but still did better at maintaining mobility that those who did no significant exercise at all.