Left panel: while breast primary tumors rely on glucose and glutamine metabolism, metastatic tumors use pyruvate (lung metastasis), serine and acetate (brain metastasis) to sustain the TCA cycle. Right panel: within the same organ, a tumor can develop at different sites. The level of perfusion can dictate metabolic activity of tumor cells. Indeed, lung cancer cells in highly perfused areas consume glucose to sustain both glycolysis and OXPHOS, while cancer cells in lowly perfused areas rely on other carbon sources. Yellow arrows indicate upregulation.