What is the biological nature of cancer? Is cancer a genetic disease or a metabolic disease? Controversy exists in history about this question which is a fundamental issue that determines the cancer treatment strategy. Genetic analysis has revealed that there are nearly 1,000 known cancer-associated genes in humans, including ~250 oncogenes and ~700 tumor suppressors. Many of these cancer genes play a key role in cell metabolism, mainly affecting five major metabolic pathways: 1) aerobic glycolysis, 2) glutaminolysis, 3) one-carbon metabolism, 4) pentosephosphate pathway and 5) de novo synthesize fatty acids. Studies via metabolomics and oncometabolites indicate that the above pathways allow cancer cells to shift from simply producing ATP to generating large quantities of amino acids, nucleotides, fatty acids and other intermediates needed for rapid cell growth and division, which in turn serve for the five pathways, that ultimately lead to an increased cancer growth and a decreased cell apoptosis. Based on the existing findings, this paper views cancer as a metabolic disease and urges for a better adjustment of cancer therapies. Cancer nutrition and metabolic modulation therapy should be and must be the main efforts for anticancer treatment. Due to the highly metabolic adaptive ability, cancer cell can rewire or shift to another metabolic pathway in face of harmful stress. Metabolic modulation therapy should therefore target different metabolic pathways.