This is a very interesting question. Every organ in the body has different preferences for energizing substances.
1. Why does the brain love sugar?
The brain does not have energy-storing substances, almost all use sugar to supply energy, and only in the long-term starvation state to supply energy by the fat metabolite ketone body. An adult brain consumes about 120g of sugar per day. Most of these sugars are powered by a nerve-electric impulse sodium-potassium pump, so it is necessary to continuously transport blood sugar to the brain.
It is easier to transport sugar to the brain. Sugar is transported to the brain via GLUT1 (glucose transporter 1), allowing glucose to be easily transported to the brain even at low blood glucose concentrations (lower than normal). The neurons rely on GLUT3 to use sugar.
It is more difficult to transport fat to the brain. Fat usually combines with albumin in serum to form macromolecules that do not easily cross the blood-brain barrier. Under starvation, the ketone bodies produced by the liver can supply energy to the brain through the blood-brain barrier.
2. Why does myocardium love fat?
Myocardium needs long-term stable and cheap energy
The myocardium is constantly moving, which requires long-term stable energy. Compared to sugar, fat supply is more stable, and fluctuations in blood sugar levels are common. In addition, the fat energy density is high (1 g fat 9 calories, and the storage of fat in the body does not need to store water like sugar). Myocardium does not store glycogen like skeletal muscle.
Myocardium has high density of mitochondria
The myocardium has a high density of mitochondria, which provides superior conditions for beta oxidation of fat.
The heart muscle uses fat as the main energy source, and in the special physiological state, ketone body, lactic acid and acetoacetic acid can also be used as energy sources.