Impact of body mass index, physical activity, and other clinical factors on cardiorespiratory fitness (from the Cooper center longitudinal study)

Cardiorespiratory fitness (CRF) is widely accepted as an important reversible cardiovascular risk factor. In the present study, we examined the nonmodifiable and modifiable determinants of CRF within a large healthy Caucasian population of men and women. The study included 20,239 patients presenting to Cooper Clinic (Dallas, Texas) for a comprehensive medical examination from 2000 through 2010. CRF was determined by maximal treadmill exercise testing. Physical activity categories were 0 metabolic equivalent tasks (METs)/min/week (no self-reported moderate or vigorous intensity physical activity), 1 to 449 METs/min/week (not meeting physical activity guideline), 450 to 749 METs/min/week (meeting guideline), and <750 METs/min/week (exceeding guideline). Linear regression modeling was used to determine the most robust clinical factors associated with achieved treadmill time. Age, gender, body mass index (BMI), and physical activity were the most important factors associated with CRF, explaining 56% of the variance (R² = 0.56). The addition of all other factors combined (current smoking, systolic blood pressure, blood glucose, high-density and low-density lipoprotein cholesterol, health status) were associated with CRF (p <0.05) but additively only improved R² by 2%. There was a significant interaction between BMI and physical activity on CRF, such that normal-weight (BMI <25 kg/m²) subjects achieved higher CRF for a given level of physical activity compared to obese subjects (BMI <30 kg/m²). Percent body fat, not lean body mass, was the key factor driving this interaction. In conclusion, BMI was the most important clinical risk factor associated with CRF other than nonmodifiable risk factors age and gender. For a similar amount of physical activity, normal-weight subjects achieved a higher CRF level compared to obese subjects. These data suggest that obesity may offset the benefits of physical activity on achieved CRF, even in a healthy population of men and women.