That's why I stated they don't tell you this, but that is what happens when you think about it. I experienced it, so I know this is what happens.
If I need to repair a fast twitch muscle, fat isn't going to be used as energy to fuel the repair, at least not efficiently, as carbs will. Glycogen comes from carbs. The enzymes within the fast twitch muscle don't break down fat.
When I workout my slow twitch, they don't breakdown as easy as fast twitch, BUT you can make them grow by feeding them excess fat, and protein. Remember excess calories allow growth of muscle, but all muscle isn't equal, so you have to eat the proper foods for the proper exercises, to grow certain types of muscles. I look at it in simple terms. Your body needs oxygen, and water, CONSISTENTLY so it needs something to hold onto them in the body. Fat absorbs oxygen efficiently, especially, polyunsaturated fat. Carbohydrates hold water efficiently, but starches help the body make glycogen WAY BETTER then simple sugars, and fructose is only good for the liver, as it lets the liver to help it preserve it's glycogen stores because once that goes problems occur.
High carbs, and low fat, is one of the reasons we have problems in this country as well as too much stressed out people. They eat too much carbs, and protein with very little good fat(they eat high saturated fat foods) to activate the relaxing parasympathetic, so they are sympathetic dominant which not meant to be highly active all day, just enough to keep you aware of your environment.
There is a lot more, but this is enough for now. You probably think it's bullshyt. I understand, but trust me it's not.
Check this out:
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000065
Polyunsaturated fatty acids (PUFAs) are important dietary components that mammals cannot synthesize de novo. Beneficial effects of PUFAs, in particular of the n-3 class, for certain aspects of animal and human health (e.g., cardiovascular function) are well known. Several observations suggest, however, that PUFAs may also affect the performance of skeletal muscles in vertebrates. For instance, it has been shown that experimentally n-6 PUFA-enriched diets increase the maximum swimming speed in salmon. Also, we recently found that the proportion of PUFAs in the muscle phospholipids of an extremely fast runner, the brown hare (Lepus europaeus), are very high compared to other mammals. Therefore, we predicted that locomotor performance, namely running speed, should be associated with differences in muscle fatty acid profiles. To test this hypothesis, we determined phospholipid fatty acid profiles in skeletal muscles of 36 mammalian species ranging from shrews to elephants. We found that there is indeed a general positive, surprisingly strong relation between the n-6 PUFAs content in muscle phospholipids and maximum running speed of mammals. This finding suggests that muscle fatty acid composition directly affects a highly fitness-relevant trait, which may be decisive for the ability of animals to escape from predators or catch prey.
If you go by my idea of fat needed in the body to absorb a lot oxygen, you can see why the animals with the most polyunsaturated fat were the fastest.