As exercise intensity is enhanced, the transition of energy resoutces in the body rises in orderly way. The selection of resources of energy occurs by shifting from free fatty acid (FFA) in serum at 20%V˙O2max to FFA from lipid cells and glucose from skeletal muscles and liver at 65%V˙O2max. At the highest level of exercise of 85%V˙O2max, utilization of FFA from lipid cells is suppressed, and energy resources mostly shift to more efficient and rapidly available blycolysis. This transition of energy resources is mediated by the acylcarnitine system and direct utilization of lactic acid in skeletal muscles instead of passing through the ‘Coli’ cycle. At moderate exercise, the acylcarnitine system operates in facilitating β-oxidation by providing available carnitine for transporting long-chain fatty acid acyl CoA from cytosole to mitochondria, but at much more strenuous exercise, the same system functions in the opposite direction-the suppression of β-oxidation by reducing available carnitine for long-chain fatty acid acyl CoA, where carnitine is largely transferred for buffering acetyl CoA. By so doing, due to overproductuon of acetyle CoA by β-oxidation and additionally metabolizing accumulated lactic acid, the suppression of PDC, a key enzyme of the TCA cycle, will be removed and an efficient and rapid pathway through glycolysis and the TCA cycle is preferentially selected. In the case of lactic acid, it is directly metabolozed in TCA cycle by using intramitochondrial LDH as well as by directly transfering it from white muscle to red muscle cells. Another supporing mechanism is provided by glycolysis in the liver and lipolysis in fat tissues conducted by IL-6 and other cytokines liberated from active skeletal muscles. These three exercise-induced mechanisms concertedly function for smooth transition of metabolic processes for carbohydrates and fat.