When glucose availability is reduced, as it occurs during fasting or in prolonged efforts, the production of ketone bodies is activated in the mitochondria of the liver, starting from acetyl- CoA coming from fat stored in subcutaneous deposits . 
These ketones are transported in the blood to various organs (brain, muscles, heart, etc. ), where they are converted back to acetyl- CoA and used in the Krebs cycle to produce energy. 

The 1,500 grams of the human brain normally require 100-150g of glucose per day. 
If this is not available, for instance in fasting conditions, muscle catabolism demolishes up to a maximum of 55-60g of protein per day, from which it can derive only 32-35g of glucose, amount far below the needs of the brain alone. 
And that's when Ketones come into play: a healthy liver can produce up to 150g of them per day, and the brain is able to use only glucose and ketones for its metabolism. 

Ketones have a higher metabolic efficiency than glucose itself, providing more energy for the same amount of consumed oxygen: the beta-hydroxybutyrate (one of the three ketone bodies produced by man, along with acetoacetate, and acetone) releases 30% more energy per mole compared to pyruvate derived from glycolysis, hence the name of "SUPERFUEL", especially in situations of hypoxia (extreme efforts, altitude, coronary insufficiency, peripheral arterial disease, etc.). 
Furthermore, Ketones inhibit glycolysis and increase the synthesis of glycogen in the heart and skeletal muscles, with an effect similar to that of insulin. 

Recently it has been proposed to artificially reproduce a moderate ketosis capable of providing an extra AcetylCoA to increase the production of energy, improving cognitive functions and physical performance, especially in extreme conditions of fatigue (military combat situations, athletes, etc.) . 
The classic Ketogenic Diets rich in fat and low in CHO are difficult to maintain, pose high atherogenic risk and are ill-suited to athletes. 
Coconut oil is particularly rich in medium-chain fatty acids (MCT) which can be quickly utilized as fuel and a part of which are transformed into ketone bodies. 

Coconut oil has long been used by athletes as a supplement with the purpose of being able to save muscle glycogen and therefore to prolong the effort. 
In addition, the intake of coconut oil is an effective suppressor of appetite, reducing the hunger of sugar in the brain. 

The researcher Kieran Clarke, University of Oxford (British, always them...) synthesized in 2005 a KETONE monoester : ®-3-hydroxybutyl®-3- hydroxybutyrate, which is able to reproduce a ketosis similar to that of fasting. 
The molecule, administered orally, is rapidly absorbed and immediately split into beta-hydroxybutyrate and acetoacetate: the maximum peak concentration in the blood is 2-3 hours after intake and the return to baseline comes after 8-10 hours (Regul Toxicol Pharmacol 2012 , 63 (3)) . 
This Ketone monoester was recently patented and should be marketed in 2014 by a company called TdeltaS, even though the substance, according to claims by the company itself, has already been used by athletes from 2011 up to now...