What's the secondary energy source for our bodies?

Fats are the secondary energy source of the body. It is stored in the form of glycogen by the liver and is converted into usable energy when carbohydrate sources are no longer available. Glycogen can also be found stores in muscles and are burnt after more than two hours of continuous exercise/physical activity. Fats are burnt after all carbohydrates have been depleted and they are the body's emergency source of energy when certain situations call for it. Aside from this they provide cushioning and protection of the internal organs and some of the skeletal muscles. 

The body will always prefer carbohydrate as its primary energy source