Physiological impact of in vivo stable isotope tracing on cancer metabolism

•In vivo isotope tracing is the state-of-the-art approach to study tumor metabolism.•In vivo tracer administration challenges the physiological metabolism of mice.•Interorgan conversion of the tracer might confound tumor labeling patterns.•Mouse fasting before in vivo tracing impacts on systemic and tumor metabolism.•Optimization is key to minimize physiological alterations linked to in vivo tracing.