In parts I and II of this series, we spoke about the background structure of the macronutrients, and the way in which energy is derived from foods. In part III of the series, I will attempt to explain how energy can be stored in the body.
As previously explained, the two ways in which energy can be stored within the human body are glycogen and body fat (muscle tissue can technically be used as energy under extreme circumstances such as starvation, but that is outside the scope of this post).
Glycogen is merely a long strand of glucose molecules attached to each other. It may be branched.
(each hexagon is a different molecule of glucose, the lines in between them are bonds)
We have glycogen stores in both our muscles and in our liver. They are both structurally exactly the same. However, liver glycogen can be transported to the muscles and used to replenish muscle glycogen, but muscle glycogen cannot leave the muscle once it is inside. Muscle glycogen is more abundant than liver glycogen by about a 4:1 ratio, with a total storage of about 400-450 grams. When it is needed, glycogen is chopped up into individual glucose molecules, which are then used by the body for energy. It does not take very long to utilize glycogen, but at the same time, it does not yield a lot of energy. (Note: remember that carbohydrates yield 4 kcal/gram).
Once our glycogen stores are full, any remaining energy intake will be stored as body fat (note: this is oversimplifying things a lot). As opposed to glycogen, with finite storage capacity, body fat stores are essentially limitless. Body fat is stored in cells called adipocytes, which can be located all over the body. Body fat takes a bit longer than glycogen to be oxidized (have the bonds broken and energy released), but there is far more of it, and has a much greater energy potential.