Cardiorespiratory fitness is a health-related component of physical fitness that relates to the ability of the circulatory and respiratory systems to supply oxygen during physical activity.  Andrew S. Jackson, from University of Houston (Texas, USA), and colleagues studied data collected on 3,429 women and 16,889 men, ages 20 to 96 years, enrolled in the Aerobics Center Longitudinal Study from 1974 to 2006.  The team found that after age 45, cardiorespiratory fitness declines at an accelerated rate and is independently influenced by body mass index (BMI), physical activity, and smoking.  For each unit of increase in BMI, the cardiorespiratory fitness of women declined 0.20 metabolic equivalents (METs) as measured on a treadmill; for men the decrease was 0.32 METs for each unit of increase in BMI.  Further, women's physical activity index regression coefficients were 0.27 METs for those with little physical activity, reaching 1.22 METs in the most physically active (walking or jogging more than 20 miles per week); men's regression coefficients were 0.37 METs for the least active and 1.48 METs for the most active.  Additionally, current smoking also was associated with lower cardiorespiratory fitness, with a decline of 0.29 METs in women and a decline of 0.41 METs in men.  Conclude the researchers: “Cardiorespiratory fitness in men and women declines at a nonlinear rate that accelerates after 45 years of age. Maintaining a low BMI, being physically active, and not smoking are associated with higher [cardiorespiratory fitness]  across the adult life span.