An overwhelming amount of evidence points to the benefit of low carb diets for treating conditions associated with insulin resistance such as metabolic syndrome and type-2 diabetes. New research suggests that a low carbohydrate diet may help reduce the risk of cancer and slow the growth of tumors.
Cancer cells rely on glucose for fuel. The reason a low carb diet may be effective against cancer is that it decreases reliance on glucose (sugar) in favor of fat. When carbs are restricted, the body produces ketones, which can be used to serve most cells in the body including the brain.
If you are primarily burning fat and ketones, the rest of the body can function fine while the cancer cells are without a viable source of fuel to grow.
This is a bit of an oversimplification to a complex issue, but the use of low carb diets is currently being investigated in human clinical studies based on positive findings in animal experiments and human case studies.