In life, it’s helpful to have a plan B in case plan A doesn’t work out.
In terms of energy, the body’s plan A is to generate energy from carbohydrates, fats, and proteins - basically in that order.
But if these main fuels aren’t readily available, then plan B is to use an alternative fuel source - ketone bodies.
Ketone bodies are a group of carbon-containing molecules produced by liver mitochondria using a 2-carbon molecule called acetyl-CoA.
The liver makes ketone bodies in physiologic states like prolonged fasting or exercise, as well as in pathological states like type 1 diabetes mellitus or alcoholism.
Ketone bodies can be released into the circulation and get picked up by the majority of cells.
Inside the cells, they’re reconverted back into acetyl-CoA, at which point they can then enter the mitochondria and produce ATP.
The 3 primary ketone bodies are acetoacetate, beta-hydroxybutyrate, and acetone.
Alright, so let’s say you decide to go on a 5-day fast.
About 12 hours into your fast, your blood glucose levels start to dip.
In response, glucagon is secreted from the pancreas and stimulates hepatic glycogenolysis - meaning that the liver begins to break down glycogen into glucose and release that glucose into the blood.
About 24 hours into your fast, your liver begins running out of glycogen, so it starts the process of gluconeogenesis which is where it makes new glucose molecules from substrates like amino acids.
Then, around 1 to 3 days into your fast, your body begins to run out of the necessary substrates to make new glucose.
So, it switches to breaking down fatty acids for energy.
Fatty acids are mobilized from fat stores and are broken down to acetyl CoA through beta oxidation in the mitochondria of most cells - except for brain cells.