One thing that is never mentioned in nutrition in general is the true role of protein. Protein cannot be burned for energy. It's mainly used for growth and repair processes. It can be used as a substrate for the process of hepatic gluconeogenesis, where the liver produces glucose, which can be used. The other substrate for GNG is the glycerol that results when the fat molecule is broken, the other substance from the molecule being fatty acids, which can be used by most cells in the body, with the brain being the notable exception. Fortunately, glycerol is also used to make ketones, which the brain can use.
This all is why high protein diets are unhealthy. Energy sources are primarily carbs and fat. Gluconeogenesis is the reason that there is no nutritional requirement to eat carbs, (recommended daily allowance) as long as enough fat is eaten.
