Ketosis or nutritional ketosis is the production and build-up of ketones. Ketones are produced as a byproduct of your body burning its fat stores. It is merely the metabolic state of elevated levels of ketones in the body. When you are in ketosis, your body has adapted from burning glucose for energy to burning your body's fat stores. In the context of the Keto Diet, this is primarily caused by the restriction of the body’s primary energy source, carbohydrates. Carbohydrates are readily broken down into glucose and are used immediately as fuel. When carbohydrates are limited, your body resorts to another source of energy, fat. The breakdown of fat produces ketones that the body uses for energy. This is ketosis.