Sugar feeds cancer, so if you cut sugar out of your diet you can starve a tumour and slow or stop the disease. It spreads through wellness blogs and worried relatives, and it sounds biologically sensible: tumours love sugar, so take away the sugar.
Every cell in your body runs on glucose, a sugar, including your brain, muscles, and healthy tissue, not just cancer cells. Your body keeps blood glucose within a tight range no matter what you eat, breaking down other foods into glucose when needed, so you cannot selectively deny sugar to a tumour by avoiding the sugar bowl; the cancer will get glucose regardless, and so will the rest of you. Major cancer bodies, including Cancer Research UK and major cancer research centres, are clear that there is no good evidence cutting sugar starves cancer or that sugar uniquely causes it. The grain of truth that fuels the myth is indirect: regularly eating lots of sugar can lead to weight gain, and obesity is a genuine risk factor for several cancers, so a balanced diet matters. But that is about overall health and weight, not starving a tumour. So a sugar-free diet will not fight an existing cancer, and selling it as a cure can do real harm by steering people away from treatment.