Purpose:
The purpose of this study was to examine the agreement between body mass index and body fat estimate among adolescents with different physical activity (PA) levels, using the sum of skinfolds as our criterion.

Methods:
This was a cross-sectional study with 732 adolescent participants (63.4% of which were female), with an average age of 16.1 (±1.1) years old. The reference method used to estimate the body fat was the sum of triceps and subscapular skinfolds. BMI was calculated using body mass and height values. The PA was estimated using the International Physical Activity Questionnaire - IPAQ (short version). Bland-Altman diagrams were used to evaluate the agreement between BMI and the sum of skinfolds and were transformed into a Z score.

Results:
For males, BMI overestimated body fat among those who did (mean error = -0.67; p < 0.001) and those who did not (mean error = -0.45; p < 0.001) meet the recommended PA values with a greater error observed for those who met PA recommendations. For females, BMI underestimated body fat among those who met (mean error = 0.29; p < 0.001) and did not meet (mean error = 0.41; p < 0.001) the recommended PA values with a greater error observed for those who did not meet recommendations.

Conclusions:
Compared to sum of skinfolds, there was weak agreement between BMI and body fat among adolescents, especially among those with higher or lower PA levels. The use of other methods to estimate body fat in such cases is strongly recommended.