The fatty acids or fats provide a large amount of energy about 9 kcal g⁻¹ than glycogen which provides about t 4 kcal g⁻¹.
This large difference in the calorific value is due to the nature of the fatty acids as these are much more reduced than the glycogen as well as they are stored in the body in anhydrous form due to non-polar triacylglycerols in the adipose cells. The carbohydrates are highly hydrated compounds as they contain polar molecules in their structure which binds the water molecules.
So, the evolution selected the fatty acids over glycogen for the production of the chemical energy.
