Normal cells obtain their energy by oxidation of glucose. Cancer cells use instead a less efficient way: glycolysis, even in the presence of oxygen. Although inefficient, this provides biomass for the synthesis of new cells. However, tumors are complex metabolic entities with different nutrients and oxygen being used in different possible ways and with a complex interplay of different tumor cell subpopulations (and normal cells). We use mathematical models to better understand the interplay of the different nutrients, cellular pathways and tumor cell subpopulations to find vulnerabilities in cancer cells.