By 12 hours, you’ve entered the metabolic state called ketosis (Anton et al., Obesity 2018). In this state, your body starts to break down and burn fat.
Some of this fat is used by the liver to produce ketone bodies. Ketone bodies, or ketones, serve as an alternative energy source for the cells of your heart, skeletal muscle, and brain, when glucose isn’t readily available. Did you know that your brain uses up some 60% of your glucose when your body is in the resting state? When you are fasting, ketone bodies generated by your liver partly replace glucose as fuel for your brain as well as other organs. This ketone usage by your brain is one of the reasons that fasting is often claimed to promote mental clarity and positive mood – ketones produce less inflammatory products as they are being metabolized than does glucose, and they can even kick-start production of the brain growth factor BDNF! Ketones have also been shown to reduce cellular damage and cell death in neurons and can also reduce inflammation in other cell types.
