Aplicabilidade do ângulo de fase e análise dos vetores da bioimpedância em adolescentes esportistas

Abstract

The phase angle (PA), determined by the bioimpedance´s vectors: resistance (R) and reactance (Xc) is associated with the hydration and nutritional status. The positive correlation between AF and body mass index (BMI) has been interpreted as appropriate, without considering that the high BMI indicates overweight. To minimize misinterpretation, was developed the body impedance vector analysis (BIVA) that provides information of hydration and cell mass. Few studies have measured the nutritional status in adolescent athletes considering the phase angle and BIVA as nutritional index. This study aimed to test the discriminatory power of the PA for overweight pre-defined by percentile BMI/age and also testing different reference standards for the analysis of impedance vectors. 304 adolescent athletes were evaluated (n=123, female, n=181, male, 12±1 years). The adolescents were separated by gender and classified as with overweigth (BMI/age > 85th percentile) or without overweight (BMI/age < 84 percentile). Partial correlation between PA and BMI with the other variables were adjusted for sex and BMI/age in the case of PA; and only for sex to BMI. To test the discriminatory power of the PA for overweight, was calculated ROC curve (Receiver Operating Characteristic). A specific software was used to design the BIVA´s graphics. As expected, when compared to the group without overweight female (n=33, 27%) and male (n = 59, 33%) adolescents with overweight showed higher values (p= 0.001) of fat mass (FM, kg) (85% and 103%, respectively), fat percentage (F% , 41% and 52% , respectively) and sum of skinfolds (ΣDC) (83% and 89%, respectively). Only female adolescents overweight had a higher value PA (7%, p=0.001). The highest frequencies of the PA ranged from 5.9 to 6,3o. Considering the necessary adjustments, the PA correlated (p < 0.05), with BMI (r=0.178), fat free mass (r= 0.138) and FM% (r= -0.121). BMI also presented correlations with all the variables tested. The PA showed a good discriminatory power for prediction of overweight, and the largest area under the curve (AUC) (0.718) was for the range of 13 years. The analysis of the vector impedance standardized by height showed that adolescents aged 11 to 12 had hypohydration´s state while individuals of other ages had overhydration, possibly due to increased fat-free mass. The Z-score´s graph of the vectors, when standardized by a sample of sedentary adolescents, showed a higher concentration points in overweight quadrant. However, when considering only male adolescent boys, standardized by a group of male athletes, a greater clustering of points in dehydration´s quadrant was observed. In conclusion, the PA is able to discriminate the excess weight. The development of a specific BIVA for adolescent athletes of both sexes is necessary