To understand the adaptations which occur as a result of ketosis, it is necessary to examine the physiology behind the production of ketone bodies in the liver. As well, an examination of what ketone bodies are and what ketosis represents is necessary. Finally, concerns about ketoacidosis as it occurs in diabetics are addressed.
What are ketone bodies?
The three ketone bodies are acetoacetate (AcAc), beta-hydroxybutyrate (BHB) and acetone. AcAc and BHB are produced from the condensation of acetyl-CoA, a product of incomplete breakdown of free fatty acids (FFA) in the liver. While ketones can technically be made from certain amino acids, this is not thought to contribute significantly to ketosis. Roughly one-third of AcAc is converted to acetone, which is excreted in the breath and urine. This gives some individuals on a ketogenic diet a ‘fruity’ smelling breath.
As a side note, urinary and breath excretion of acetone is negligible in terms of caloric loss, amounting to a maximum of 100 calories per day. The fact that ketones are excreted through this pathway has led some authors to argue that fat loss is being accomplished through urination and breathing. While this may be very loosely true, in that ketones are produced from the breakdown of fat and energy is being lost through these routes, the number of calories lost per day will have a minimal effect on fat loss.
Functions of ketones in the body
Ketones serve a number of functions in the body. The primary role, and arguably the most important to ketogenic dieters, is to replace glucose as a fat-derived fuel for the brain. A commonly held misconception is that the brain can only use glucose for fuel. Quite to the contrary, in situations where glucose availability is limited, the brain can derive up to 75% of its total energy requirements from ketone bodies.
Ketones also decrease the production of glucose in the liver and some researchers have suggested that ketones act as a ‘signal’ to bodily tissues to shift fuel use away from glucose and towards fat. These effects should be seen as a survival mechanism to spare what little glucose is available to the body.
A second function of ketones is as a fuel for most other tissues in the body. By shifting the entire body’s metabolism from glucose to fat, what glucose is available is conserved for use by the brain. While many tissues of the body (especially muscle) use a large amount of ketones for fuel during the first few weeks of a ketogenic diet, most of these same tissues will decrease their use of ketones as the length of time in ketosis increases. At this time, these tissues rely primarily on the breakdown of free fatty acids (FFA). In practical terms, after three weeks of a ketogenic diet, the use of ketones by tissues other than the brain is negligible and can be ignored.
A potential effect of ketones is to inhibit protein breakdown during starvation through several possible mechanisms, discussed in detail in the next chapter. The only other known function of ketones is as a precursor for lipid synthesis in the brain of neonates.
Lyle McDonald, The Ketogenic Diet: A Complete Diet for the Dieter and the Practitioner, published in 1998 by Lyle McDonald