Fat-Free Adipose Tissue Mass: Impact on Peak Oxygen Uptake (VO2peak) in Adolescents with and without Obesity.

Fat-free body mass (FFM) is a surrogate for skeletal muscle mass and is often used for the normalization of several physiological variables (e.g., oxygen uptake). However, FFM includes non-skeletal muscle components such as the fat-free component of adipose tissue fat cells. As the amount of adipose tissue mass increases, the FFM will also increase and be included in the measurement of FFM. Therefore, FFM may not be an appropriate indicator of muscle mass when targeting individuals with a large amount of body fat. This review aimed (1) to examine the fat-free adipose tissue (FFAT) in adolescents with and without obesity and (2) to investigate the impact of FFAT on peak oxygen uptake (VO2peak) by comparing VO2peak expressed relative to FFM and FFM minus FFAT between adolescents with and without obesity. Calculated FFAT was threefold higher in adolescents with obesity compared to those without obesity (mean ± SD; 6.94 ± 1.40 vs. 2.22 ± 0.58 kg). The VO2peak expressed relative to FFM-FFAT was similar between the two groups (56.5 ± 6.7 vs. 57.4 ± 8.9 ml min-1 kg-1 FFM-FFAT, respectively). On the other hand, VO2peak expressed relative to FFM was approximately 8% different between groups. When normalizing VO2peak by FFM, eliminating the FFAT component from FFM may be needed, especially when comparing groups with different body fat mass.