Ketones are energy molecules that are a source of caloric energy in the body that are used by the brain, heart, and muscles in the same manner as is glucose (sugar). They are produced in the liver as a by-product of fat metabolism when—owing to extreme restriction of dietary carbohydrates—insulin, blood sugar and liver glycogen levels are very low.
Ketones are a brilliant evolutionary adaptation that provides a critically important fuel for your body and brain during times of food scarcity. Without ketones, you wouldn’t be able to live more than a few weeks without food. Ketones are also an important component of a ketogenic lifestyle as their presence indicates that you are burning fat as your primary fuel instead of glucose.
Whenever your supply of carbohydrates from food is low or nonexistent, even after just a couple of days or so, your body is able to convert fat to ketones. This metabolic flexibility is an important reason why the human race has been able to survive; it helps up adapt to a wide variety of food sources.
When you are fully Fat-adapted and Keto-adapted, your muscles burn mostly fat for fuel, while the ketones produced by the liver are prioritized for use by the brain. The brain is a huge energy-demand organ (it’s around 2% of your total body weight, but the brain burns 20-25% of your daily calories!) that cannot burn fat and must burn either glucose or ketones. Ketones and fat (since the burning of these two caloric energy sources always go hand in hand) help minimize the inflammation and oxidative damage that come from eating the modern grain-based high-carbohydrate diet.
When your body receives limited carbohydrates, it can’t utilize glucose (sugar) to supply all your energy needs. As a result, your body’s fat cells will be broken down by your liver to produce ketones (a.k.a. ketone bodies). Your body will then utilize those ketones to fuel your body’s energy demands.
Many people consider a ketogenic lifestyle a low carbohydrate eating plan like Atkins. Strictly speaking, while an Atkins lifestyle can be ketogenic, it has traditionally emphasized low carbohydrates and high-fat (regardless of the quality of the fat source) while Modified-Ketogenic Diet emphasizes getting your body to have higher ketone levels. A Modified-Ketogenic Diet should be viewed as a healthy lifelong eating strategy rather than a rigid weight-loss protocol.