The purpose of the present study was to investigate the association between cardiorespiratory fitness (CRF) measured as peak oxygen uptake (VO_2peak, expressed in mL/min) and body mass index (BMI) in a large cohort of apparently healthy subjects.

BMI and VO_2peak were measured in a cross-sectional study of 8470 apparently healthy adults. VO_2peak (mL/min) was determined by an incremental cycle ergometer test to exhaustion. Linear regression analyses were performed to identify predictors of CRF.

There was no difference in CRF between adults with a normal weight (BMI between 18.5–24.9 kg/m²) and those who were overweight (BMI 25.0–29.9 kg/m²). Subjects who were underweight (BMI<18.5 kg/m²) as well as females who were obese (BMI≥30.0 kg/m²) showed a reduced CRF compared to the normal and overweight groups. Age, height, and gender were significant predictors of CRF (R²=0.467, P<0.0001); BMI did not add significantly to this relationship.

Our findings indicate that BMI was not associated with CRF in addition to age, height, and gender. In subjects with a BMI<18.5 kg/m², CRF was lower compared to subjects with a BMI between 18.5 and 29.9 kg/m². In obese subjects, CRF was only lower in females compared to females with a BMI between 18.5 and 29.9 kg/m². Correcting CRF for BMI may be beneficial for subjects with a low BMI, and females with a BMI≥30.0 kg/m². The outcome of this study might help to improve the interpretation of exercise testing results in individuals with a low or high BMI.