Ketone bodies occur as the result of metabolism of fat (specifically fatty acids) rather than sugar. They can be thought of as soluble fatty acids. Production is mainly in the liver, whilst it is principally the heart and the brain that are able to use them as an energy source. They are produced in situations of glucose/glycogen depletion, e.g. fasting/starvation or intentionally in certain diets for weight loss or against some forms of epilepsy (where it is deliberately induced). Ketosis is a cardinal sign of trouble in insulin-deficient diabetes and vomiting in pregnancy.
Qualitative measurement may be made with the nose, sticks for urine are semi-quantitative and modern blood glucose meters may have sticks to determine ketosis from a blood drop. Many dipsticks detect acetoacetate rather than β-hydroxybutrate and may therefore give false negative (e.g. in alcoholic ketoacidosis where the biochemical equilibrium favours β-hydroxybutrate). Urine should be tested fresh as acetoacetate can degrade into acetone.
Biochemically, the main forms of ketone bodies are acetone, acetoacetic acid and beta hydroxybutrate, often in a degree of equilibrium. In severe ketosis, ketone bodies may be present in sufficient numbers to create an anion gap.