Fitness fades with age, more so after 45

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Watch out for that mid-life speed bump—turns out age 45 is a doozy. A report published this week in the Archives of Internal Medicine upends conventional wisdom about fitness and aging.

Until now, most experts thought people’s fitness levels declined in a linear fashion as they aged. But the new report suggests the downward march isn’t quite so linear after all. Indeed, most people’s fitness levels stay relatively stable between the ages of 20 and 40. But, after age 40 (especially around 45), the plateau ends and fitness levels slope downward at an accelerated pace.

Why 45? There seems to be a biological factor at play, says Andrew Jackson, P.E.D., the study’s lead author and professor emeritus in the department of health and human performance at the University of Houston in Texas. “As we age, our heart rate slows, and our heart rate is a big part of our aerobic capacity.”

The report also reflected a gender gap. Men’s fitness levels declined faster than women’s—0.32 metabolic equivalents (METs) per one unit increase in body mass index (BMI), compared to 0.21 METs. Jackson chalks the disparity up to the fact that men generally start with greater aerobic capacity than women.”The higher you start, the further you fall.”

Although a certain decline in fitness level is a biological fact of aging, one can take steps to lessen the blow. The authors found that study participants who kept a healthy weight, didn’t smoke, and stayed physically active had much higher fitness levels than their sedentary, overweight peers, regardless of age. Likewise, inactivity and obesity were linked to higher rates of disease and disability.

The strength of the study is its size. The researchers crunched data from a cohort of 3,429 women and 16,889 men who completed a total of more than 70,000 fitness tests between 1974 and 2006 as part of the larger Aerobics Center Longitudinal Study.

“With this study we’ve quantified what everyone already knows,” says Jackson. “That keeping your weight down and staying active are essential to good health.”