On a standard diet, carbohydrates in the form of glucose are the primary fuel for most cells and organs in the body. To obtain their energy, the cells take up glucose from the blood and once the glucose enters the cells a metabolic reaction takes place releasing energy. Our bodies have the ability to store excess glucose in the form of glycogen which allows energy to be stored for later use. Glycogen is made up of long chains of glucose molecules and is primarily found in the liver and skeletal muscle. The stores of liver glycogen are used to maintain normal levels of glucose in the blood and muscle glycogen stores are used mainly to fuel muscle activity.
When you eat a Low Carb, High fat diet proteins and fats are used for most of your energy requirements. Most of the cells in the body are able to use fatty acids for energy except for brain cells and developing red blood cells which are dependent on glucose. A supply of glucose in the blood is essential, however our brain cells can be trained to adapt and use ketones from the breakdown of fat for energy. When carbohydrate intake is restricted, the liver does not break down fat completely and instead produces ketone bodies which are then used by most cells to provide energy.
Ketosis occurs on a Low Carb, High Fat diet when ketones are produced more quickly than the body needs and their levels increase in the blood. As the glycogen stores become depleted the body begins to burn body fat and this increases the availability of fatty acids as a fuel source. Many of these fatty acids cannot pass the blood brain barrier so the brain switches to using ketone bodies produced by the liver as fuel.
This normal and extremely efficient physiological response to carbohydrate restriction is generally termed nutritional ketosis.