Bariatric surgery is currently the most effective treatment for morbid obesity and its associated metabolic complications. To ensure long-term postop

Objective: Body mass index (BMI) is more commonly used than waist circumference as a measure of adiposity in clinical and research settings. The purpose of this study was to compare the associations of BMI and waist circumference with cardiorespiratory fitness. Methods: In a cross-sectional study of 403 healthy men and women aged 50 ± 8.8 years, BMI and waist circumference were measured. Cardiorespiratory fitness was assessed from estimated maximal O₂ uptake (VO₂max), as calculated from a maximal fitness test. Results: Mean BMI (kg/m²) was 27.8 ± 3.7 and 25.5 ± 4.6; and mean waist circumference (cm) 94.1 ± 9.7 and 84.3 ± 10.4 for men and women, respectively. Both men and women reported an average of 2.5 hours of weekly sports related physical activity, and 18% were current smokers. Correlation coefficients between both BMI and waist circumference, and VO₂max were statistically significant in men (r = −0.280 and r = −0.377, respectively, p > 0.05 for both) and in women (r = −0.514 and r = −0.491, respectively, p > 0.05 for both). In women, the contribution of BMI to the level of VO₂max in a regression model was greater, while in men waist circumference contributed more to the final model. In these models, age, hours of training per week, and weekly caloric expenditure in sport activity, significantly associated with VO₂max, while smoking did not. Conclusion: The differences observed between the sexes in the associations of BMI and waist circumference with VO₂max support the clinical use of both obesity measures for assessment of cardiorespiratory fitness.