Chemistry - Glycogen continued
With an energy content per gram more than twice that of glycogen, fat is a
more efficient form of energy storage.
Because it is non-polar and hence not soluble in water, fat can be stored
in much larger quantities than glycogen.
Carbohydrate reserves in the body are depleted after about one day
without food, but stored fat can provide energy for between 30 and 40 days.
When stored fats are needed for energy, hydrolysis  reactions release
the fatty acids and glycerol. 
The greater energy content of fats can be attributed to the fact that
fatty acids  are a more reduced form of fuel (i.e. contain less oxygen)
than carbohydrates such as glucose.
Since they require more oxygen for oxidation, fats release their energy
more slowly than glucose.
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