BACKGROUND & AIMS:Weight gain in breast cancer patients during treatment is prevalent; the metabolic implications of this weight gain are poorly understood. We aimed to characterize glucose metabolism in breast cancer patients near the initiation of chemotherapy. METHODS:Stage I-II breast cancer patients (n = 8) were evaluated near the initiation of chemotherapy and compared with a group of age- and body mass index-matched, as well as a group of young healthy, non-malignant females. Fasting blood samples (analyzed for lipids and cytokines) were taken and an oral glucose tolerance test was performed. Body composition, waist circumference, diet, cardiovascular fitness and muscle strength were evaluated. RESULTS:Breast cancer patients were abdominally obese (mean ± SD: 94.6 ± 14.0 cm), overweight (28.8 ± 6.0 kg/m(2)) and dyslipidemic (triacylglycerides: 1.84 ± 1.17 mM; high-density lipoprotein cholesterol: 1.08 ± 0.23 mM). Compared to non-malignant matched females, fasting glucose and insulin concentrations were similar but fasting c-peptide was greater in patients (2.6 ± 1.2 ng/mL vs. 1.9 ± 0.8 ng/mL, p = 0.005). Glucose was elevated to a greater extent in patients during the oral glucose tolerance test compared with all non-malignant females. During the glucose tolerance test, c-peptide, but not insulin, remained elevated in patients compared with all non-malignant females. No differences in body composition, serum cytokines, nutrition or exercise capacity between patients and matched, non-malignant females emerged. CONCLUSIONS:Breast cancer patients present with unhealthy metabolic features early in the disease trajectory. Future investigations need to examine the underlying mechanisms and the potential longitudinal changes following chemotherapy.