What causes ketones? And why are high ketone levels dangerous?

Ketones are formed when fat is burned for energy by the muscles. If you don’t have glucose available for fuel (because you are on a low-carbohydrate diet, for example), you will form ketones when your muscles burn fat. Having measurable amounts of ketones in the urine (or blood) is cause for concern. The problem with high levels of ketones is that they are relatively strong acids, and because of their acidity, their presence in the blood can reduce the ability of oxygen to bind to hemoglobin, the molecule that transports oxygen to cells that need it.