Objective This study aimed to 1) analyze the criterion validity of the?Slaughter equations (Slg‐Eq) and bioelectrical impedance analysis (BIA) to estimate body fat percentage (BFP) and fat mass index (FMI) at different degrees of obesity in children compared with dual‐energy x‐ray absorptiometry (DXA) and 2) determine their agreement over time. Methods Ninety‐two children with overweight or obesity (10.0 [SD?1.2] years; 34.8% girls) participated in this 20‐week study. Anthropometric, BIA, and DXA measurements were performed. Results Both Slg‐Eq and BIA methods underestimated BFP and FMI against DXA, and the bias was markedly larger with BIA (mean absolute percentage error = 11% for Slg‐Eq vs. 18%‐21% for BIA); a larger underestimation was observed in girls compared with boys for Slg‐Eq (P ≤ 0.001), and the observed underestimation in adiposity was reduced as weight status increased. Systematic errors were kept constant over time, so no large differences between methods were observed in the change in adiposity. Conclusions At the group level, Slg‐Eq provides a more valid estimation of BFP and FMI than BIA. At the individual level, Slg‐Eq shows larger estimation errors. The validity of these methods might differ in sex and weight status. Nevertheless, both methods seem to be valid for monitoring changes in adiposity.
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