Associations between fatigue and cellular metabolism in breast cancer patients: A longitudinal study

•Cellular metabolic function may be an important mechanism of cancer-related fatigue.•Mitochondrial and glycolytic activity of PBMCs was tested in breast cancer patients.•Associations of metabolic function and fatigue depended on treatment status.•Fatigue prior to treatment was associated with increased metabolic activity.•Fatigue after treatment completion was associated with decreased metabolic function.•Improving metabolic function after cancer treatment may prevent persistent fatigue.