PurposeThe study aimed to analyze the influence of chemotherapy on health biomarkers and examine the relationship between phase angle (PhA) and oxidative stress.MethodsA prospective study was performed. Women who were starting chemotherapy were recruited. Also, this study included a control group of women without cancer. Bioelectrical impedance multiple-frequency (BIS) analysis, 24h food recall, and blood samples were collected at 2-time points: diagnosis (T0) and after one month of completion of therapy (T1) for the main study group and one-time point for the control group. T-tests or Mann-Whitney Wilcoxon Test was used to compare variables. Linear regression analysis was conducted to test if PhA is related to the dependent variables after adjusting for age and body mass index.Results119 women were included (61 with breast cancer and 58 healthy). There was no difference between the groups concerning anthropometrics, fat mass, and fat-free mass. Breast cancer patients had a worsening in PhA (p<0.001) after chemotherapy completion. PhA was positive statistically correlated with extracellular water, albumin, and the antioxidant markers at both times. The linear model showed that PhA was significantly predicted by C reactive protein, 2,2-Diphenyl-1-picrylhydrazyl (DPPH), Malondialdehyde (MDA), total body water/extracellular water, and body mass index fat mass. This model explained 58% of PhA variability (p<0.001).ConclusionOur findings show that PhA is an easy and affordable tool that correlates oxidative stress markers in breast cancer patients, regardless of age or body mass index.