Ketosis is a metabolic state in which your body uses fat and ketones rather than glucose (sugar) as its main fuel source.
Glucose is stored in your liver and released as needed for energy. However, after carb intake has been extremely low for one to two days, these glucose stores become depleted. Your liver can make some glucose from amino acids in the protein you eat via a process known as gluconeogenesis, but not nearly enough to meet the needs of your brain, which requires a constant fuel supply.
Fortunately, ketosis can provide you with an alternative source of energy.
In ketosis, your body produces ketones at an accelerated rate. Ketones, or ketone bodies, are made by your liver from fat that you eat and your own body fat. The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate, and acetone (although acetone is technically a breakdown product of acetoacetate).
Even when on a higher-carb diet, your liver actually produces ketones on a regular basis – mainly overnight while you sleep – but usually only in tiny amounts. However, when glucose and insulin levels decrease on a carb-restricted diet, the liver ramps up its production of ketones in order to provide energy for your brain.
Once the level of ketones in your blood reaches a certain threshold, you are considered to be in nutritional ketosis. According to leading ketogenic diet researchers Dr. Steve Phinney and Dr. Jeff Volek, the threshold for nutritional ketosis is a minimum of 0.5 mmol/L of BHB (the ketone body measured in blood).
Although both fasting and a keto diet will allow you to achieve ketosis, only a keto diet is sustainable over long periods of time. In fact, it appears to be a healthy way to eat that can be followed indefinitely.1