MONDAY, Oct. 26 (HealthDay News) -- The declines in fitness that accompany growing old typically speed up after the age of 45, new research shows.

But people can slow the inevitable by staying lean, exercising and refraining from smoking.

The findings, appearing in the Oct. 26 issue of the Archives of Internal Medicine, are not so surprising in light of the piles of other research that have drawn similar conclusions.

But the new study has broad implications, given the rising number of older adults in the United States and the explosion in the sedentary, overweight and aging population.

"The Social Security Administration actually has an aerobic capacity threshold. If you're below the threshold, you are considered disabled," said study author Andrew Jackson.

This means more people could qualify for government disability benefits at a younger age, further draining an already strained economy.

This study group included 3,429 women and 16,889 men aged 20 to 96 who had undergone two to 33 health exams with lifestyle counseling between 1974 and 2006.

Reductions in cardiorespiratory fitness (CRF) were not seen as a straight downward line. Instead, after the age of 45, the slope became much steeper, accelerating even further with increases in body-mass index (BMI), smoking and lower levels of physical activity.

"We've known that, as you age, your aerobic capacity goes down, and the exercise physiology literature indicates it's a linear relationship. We found that this is not the case," said Jackson, who is professor emeritus of health and human performance at the University of Houston. "It makes sense to me. When things aren't working right, we tend to go down at faster rates. This was true for both men and women [although the rate of decline was faster for men than for women]."

Taking care of yourself could make you, in a sense, younger than your years.

"If you were overweight, inactive and smoked, your aerobic capacity would be lower at a given age as compared to other people who were healthy weight, active and nonsmokers," Jackson said. "The data showed that if people had that advantage when they were in their 30s and 40s and maintained that lifestyle, their aerobic capacity as they aged was, in fact, higher."

"It could delay the age when these health problems start to spring up," he continued. "If people are very overweight, inactive and smoke, they might see these health problems in their 50s and 60s, whereas people who maintain a healthy lifestyle, it's going to be more like their 70s, 80s and possibly even their 90s."

"You have to exercise. It's now becoming established fact, and if you don't incorporate it, you're going to see the effects. You will get sicker sooner," added Dr. Suzanne Steinbaum, director of women and heart disease for Lenox Hill Hospital in New York City and a spokeswoman for the American Heart Association. "Exercise is the most potent medication around, and the Social Security Administration agrees with me."

A second study in the same issue of the journal provides a measure of good news. Researchers at the University of Pennsylvania School of Medicine in Philadelphia found that blacks who partnered with a family member or friend to lose weight actually did lose pounds -- but only if the partner also lost weight.

More information

The U.S. Centers for Disease Control and Prevention has more on overweight and obesity.

SOURCES: Andrew S. Jackson, P.E.D., F.A.C.S.M., professor emeritus, health and human performance, University of Houston; Suzanne Steinbaum, M.D., director, women and heart disease, Lenox Hill Hospital, New York City, and spokeswoman, American Heart Association; Oct. 26, 2009, Archives of Internal Medicine

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