Before we go any further it is imperative that you understand that adenosine triphosphate (ATP) is the currency of life, the biological source of energy. Not ad is assumed, calories, carbs, fats or protein.
ATP is generated via oxidation of various metabolic fuels, namely;
- Glucose (that nasty substance that Jamie Oliver and various others currently hate)
- Fatty Acids (fats), that substance they hated in the 80's.
- Amino Acids (proteins), the current darling of the macro-nutrients.
It is beyond doubt that the Brain is the most important organ you have, and glucose is the most important fuel it uses. It can use ketone bodies for fuel, but this happens during prolonged starvation, so it may be a worthwhile point to consider if you are engaged in a diet attempts to utilise ketone bodies or one that leave you with bad breath. Your brain is not able to store fuel, therefore relies upon a consistant supply of glucose (that nasty stuff remember). Your brain is quite gluttonous, and on average consumes around 120g of glucose per day (approx 400+ calories). For your brain to work properly it needs a glucose concentration of around 1mM, ultimately if it drops below that we can expect to see coma and brain death.Whilst fatty acids are not able to be utilised during starvation, the previously mentioned ketone bodies can act as a source if the brain is under a great enough degree of stress (starvation).
Your heart muscle is able to function aerobically and primarily uses fatty acids, but ketone bodies may used if needed.
Muscle on the other hand is able to utilise glucose, fatty acids or ketone bodies, and it also has the ability to store glycogen, that can then be converted to glucose when energy is needed. As mentioned in the main article, during resting state, 85% of your muscles fuel needs are met by fatty acid catabolism. Which really makes you wonder why so many are draining themselves at the gym trying to burn fat.