A client asked: why does muscle burn more fat than actual fat. (the in depth physiological reason)? I said it is in part that glycogen stores in the muscles require more energy?
Muscle & Fat
This is a great question, but answering it can get rather complicated. I will try to give a simple answer without loosing the "working parts" of the information. As you state, muscle does require more energy. It is a very metabolic cell and therefore requires a constant flow of elements. Glycogen is a stored form of sugar and is found in the skeletal muscle cells, liver cells and heart cells,(1). Glycogen is used as an energy reserve found in the previously mentioned cells; Fat is also an energy reservoir.
Fat is stored in, you guessed it, fat cells. The most common form of fat is triglycerides,(2). Much like extra sugar gets stored as glycogen, extra triglycerides get stored as fat in the fat cell. This storage is very rich in energy(9kcal/gram vs. 4kcals/gram for carbs. and protein), but will remain there until called on for energy use by the muscle cells. The fat cell is mainly a "bag" that holds fats.
Now, to answer your question. One of the main differences between muscle cells and fat cells is an organelle called mitochondria. Mitochondria (mito.) is the energy producing powerhouse of our body's and are found through out our body; especially in skeletal muscle cells. In the muscle cell, mito. accept all fuels (carbs., proteins and fats) and "burn" (oxidize) them into ATP (adenosine triphosphate),(1). ATP is what we live for and on. ATP is life, without it we are dead meat.
Fats cells do not have mito. and therefore can not use the energy they store.
This subject area has been researched quite heavily and holds tons of information for health and fitness subjects. I hope this explanation helps.
- Astrand,P.and K.Rodahl. Textbook of Work Physiology.second ed.McGraw-Hill.New York
- Powers,S. and E.Howley. Exercise Physiology: theory and application to fitness and performance. Brown Publishing, IA.