Ketosis is a temporary physical condition marked by elevated levels of compounds known as ketone bodies in the body's tissues and fluids.
The term "ketone bodies" refers to three different biochemicals: acetoacetate, beta-hydroxybutyrate and acetone. The first two molecules transfer energy produced in the liver to the tissues throughout the body; acetone is a breakdown product of acetoacetate, and is responsible for the sweet odor on the breath of people undergoing ketosis.
The condition of ketosis typically represents a change in the way the body gets its energy.
Normally, the body gets most of its energy by metabolizing glucose (a simple sugar) obtained from carbohydrates or stored in the liver and muscles as glycogen. But when unable to convert glucose into energy, the body switches to breaking down fat and converting it into energy. When this happens, the liver metabolizes fatty acids, producing energy-rich ketone bodies.
The most common causes of ketosis are physiological, according to a 2000 article in the journal Diabetes/Metabolism Research and Reviews. Fasting, eating a low-carbohydrate/high-fat diet and engaging in high-intensity exercise can all lead to ketosis, because these activities deplete the body's stores of glucose.
Have a look at our guide to 'what fruits can you eat on keto' for a list of low-carb, keto-friendly fruits.
Because ketone bodies are acidic, a prolonged excess of the molecules in the blood can lead to a pathological form of ketosis, called ketoacidosis, in which the blood becomes acidic.
Most commonly, ketoacidosis is associated with type 1 diabetes (and type 2 diabetes to a lesser extent). A lack of insulin, a hormone necessary for blood glucose to enter cells, causes glucose and ketone body concentrations to spike, lowering the blood's pH as it becomes more acidic. If left untreated, this condition, called diabetic ketoacidosis, can lead to diabetic coma and death.
Chronic alcohol abusers and binge drinkers are at risk of developing so-called alcoholic ketoacidosis if they don't eat enough food. Here, alcohol metabolism combined with little to no glycogen reserves causes ketone bodies to increase to dangerous levels, resulting in dehydration, abdominal pain, nausea and vomiting.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.