Ketone bodies are three separate water soluble compounds that are produced by the liver from fatty acids by a process called ketosis. These compounds are acetoacetate, beta-hydroxybutyrate and their spontaneous break-down product acetone.
They are produced by the cells of the liver particularly under conditions such as fasting and low carbohydrate states. They are released into the blood, together with newly produced glucose from gluconeogenesis, after the liver glycogen stores have been depleted. Gluconeogenesis is a separate process where glucose is produced from non-carbohydrate sources, but not fatty acids.
Fats stored in adipose tissue are released from fat cells as both free fatty acids and glycerol. This happens when insulin levels are negligible and when glucagon and adrenalin levels are high and this occurs when blood glucose levels are likely to fall. So, in addition to stored body fat being used directly as an energy source it is also used to produce ketone bodies – this explains why intermittent fasting works and is a win-win situation for people wanting to lose weight using the Nysteia Formula.
Fatty acids themselves are taken up by all cells that have mitochondria simply because fatty acids can only be metabolised in the mitochondria. The one notable exception is red blood cells which are therefore entirely dependent upon glucose as their energy source.
Unlike free fatty acids, ketone bodies can cross the blood brain barrier and are therefore available as fuel for the cells of the central nervous system, acting as a substitute glucose. Ketone bodies cannot be used for fuel by the liver because the liver lacks the necessary enzyme. Acetone in low concentrations is taken up by the liver and undergoes detoxification which ends with lactate.
The brain can obtain a portion of its fuel requirements from ketone bodies when glucose is less available than normal such as in fasting. However, it does have an obligatory requirement for some glucose. If the blood glucose is lowered chronically, after about three days the brain gets 25% of its energy from ketone bodies and then after about 4 days this goes up to 70%. During the initial stages the brain does not burn ketones because they are an important substrate for lipid synthesis in the brain.
The heart preferentially utilises fatty acids as fuel under normal physiological conditions. However, under ketosis the heart can effectively utilise ketone bodies for this purpose.