Fats are the primary energy storage form in animals and in plant seeds because energy can be stored more densely in fats than in carbohydrates. That is, metabolic oxidation of fats yields 37 J/g, whereas carbohydrates and proteins yield only 17 kJ/g.

A typical 70-kg human may have the following fuel reserves:

400,000 kJ in total human body fat energy

100,000 kJ in total human protein energy

2500 kJ in total human glycogen energy

170 kJ in total human glucose energy

Brain cannot use fats for energy; instead, brain has a specific requirement for glucose. Under conditions of starvation, however, when blood glucose levels decrease, brain can adjust to use ketone bodies, which can be derived from fatty acids.