So, what are these 'ketone fuels' anyway?
The ketone bodies that we produce when on a ketogenic diet, when fasting, or when taking MCTs, are an alternate, brain- and body-friendly fuel derived from fatty acids (and some amino acids).
The ketone bodies are Acetoacetate, ß-hydroxybutyric acid (BOHB) and Acetone. These ketone bodies are produced through a process called 'ketogenesis' in the liver. Acetoacetate is the primary ketone body, and this is converted to BOHB, which functions as the main ketone 'fuel'.
BOHB can be used by most tissue in the body and most importantly is easily used by heart tissue as a preferred fuel, muscle tissue, and the brain and central nervous system (which usually relies on glucose for fuel.) This ability of the brain and nervous system to use ketones is pretty amazing...
In a natural state (think of a caveman running around on the Savannah!) there'd be times when carbohydrate foods would be really scarce. A restriction of carbohydrate results in reduced insulin levels, reduced lipogenesis (the creation of fats) and fat accumulation. When glycogen (your stored carb reserve) become insufficient to supply the glucose necessary for normal fat oxidation and for the supply of glucose to the CNS, an alternative fuel source is needed.
The CNS typically doesn't use fat for fuel for two reasons:
- The common dietary fats (consisting of long chain fatty acids) are usually bound to a protein called 'albumin' and can't cross the blood-brain barrier.
- Use of fats in the brain demands more oxygen than using glucose, which can starve brain cells of oxygen! This results in oxidative damage and impaired fuel provision to those neurons (not good!).
But, some dietary fats (such as short and medium chain triglycerides), and ketones, are able to easily cross the blood-brain barrier (as they are not bound to albumin) AND they do not promote the same raft of problems usually resulting from fat metabolism in the brain.
 Technically BOHB is not a ketone body as the ketone moiety has been reduced to a hydroxyl group