Answer:
Fats are the principal source of energy in many organisms as well as carbohydrates, and other nutrients. Although, the molecular structures are significantly different, the catabolism is surprisingly similar. In other words, a variety of molecules with different structures eventually break down to carbon dioxide (CO₂), water (H₂O), and ATP, which is the currency of energy.
 
Catabolism follows three common steps for all the nutrients including fats and carbohydrates. The first step is the conversion of small molecules of fats and carbohydrates into acetyl-CoA. Next, oxidation of acetyl-CoA in the citric acid cycle (tricarboxylic acid cycle ,TCA cycle) yields water, carbon dioxide, and electrons, simultaneously, acetyl-CoA changes its form to various intermediates such as citrate and fumarate. Carbon dioxide formed in the second step is released to the outside by the respiration system and electrons are utilized in the final step, the electron-transport system. In this step, a number of ATPs and water are produced by chain reactions. The chemical reaction of the formation of water can be simply written as 2H⁺ + 1/2 O₂ + 2 e^- → H₂O.  In conclusion, no matter what the starting products are, the basic steps described above are always the same.
 
Fats and carbohydrates, like two trains converging into one station, turn to a common compound, acetyl-CoA. However, the difference between fats and carbohydrates is the process of formation of acetyl-CoA. Carbohydrate first decomposes to glucose or other monosaccharides, and then changes to pyruvate, and finally to acetyl-CoA. On the other hand, lipid turns to fatty acids and glycerol by digestion, after that, acetyl-CoA is formed from fatty acids through a process known as b-oxidation.
 
Carbohydrates → glucose → pyruvate → acetyl-CoA
Lipids → fatty acids + glycerol → acetyl-CoA
 

  
Excess energy in organisms is usually stored in fat cells in the form of triacylglycerol. When energy is needed, for example, when exercise is performed, hormones will trigger activities of triacylglycerol lipase, which is an important enzyme for fat decomposition. Fats will be broken down to fatty acids and glycerol by the enzyme, and then carried by serum albumins in the blood stream to the cells where the fuel is needed. At the destination, fatty acids enter mitochondria and the three basic steps (b-oxidation to acetyl-CoA, TCA cycle, and electron-transfer) will be followed in order to yield ATP.

It is generally said that twenty-minutes of exercise is necessary to burn fats efficiently. This is not a myth but a scientifically understood scenario. The key concept is that enzymes need proper temperature to work efficiently. In this case, triacylglycerol works best at the temperature that can be achieved by twenty-minutes of exercise. Before that, only fatty acids floating in the blood stream will be used, therefore, at least twenty minutes of exercise are needed to burn fats in the body.
 
 

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(C.H)