Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood or urine. Physiologic ketosis is a normal response to low glucose availability, such as low-carbohydrate diets or fasting, that provides an additional energy source for the brain in the form of ketones.
Ketosis and the Keto Diet
For healthy people who don’t have diabetes and aren’t pregnant, ketosis usually kicks in after 3 or 4 days of eating fewer than 50 grams of carbohydrates per day. That’s about three slices of bread, a cup of low-fat fruit yogurt, or two small bananas. You can start ketosis by fasting, too.
A diet high in fat and protein but very low in carbs is called a ketogenic or “keto” diet.
What is ketosis?
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 all the needs of your brain, which requires a constant fuel supply.
Fortunately, ketosis can provide you, and especially your brain, 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).
Your liver produces ketones on a regular basis even when eating a higher-carb diet. This happens mainly overnight while you sleep but only in tiny amounts. However, when glucose and insulin levels decrease, such as 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 people can potentially follow indefinitely.