THE TWO MAIN SOURCES OF ENERGY PRODUCTION DURING EXERCISE ARE FAT AND CARBOHYDRATE STORED WITHIN THE BODY. IN CONTRAST, BODILY PROTEIN OXIDATION DOES NOT CONTRIBUTE SIGNIFICANTLY TO ENERGY PRODUCTION. THEREFORE, THE MAJOR SOURCES OF ENERGY PRODUCTION IN THE BODY FOR EXERCISE ARE BLOOD GLUCOSE, MUSCLE GLYCOGEN, PLASMA FATTY ACID AND INTRAMUSCULAR TRIACYLGLYCEROLS. HOWEVER, ENERGY FUEL MOBILIZATION AND UTILIZATION ARE LARGELY DETERMINED BY THE DURATION AND INTENSITY OF THE ACTIVITY. DURING PROLONGED LOWER-INTENSITY EXERCISE PERFORMANCE, LIPID OXIDATION BECOMES PREDOMINANT, WHILE HIGH-INTENSITY EXERCISE UTILIZES CARBOHYDRATE METABOLISM. DESPITE THE VAST STORES OF ENDOGENOUS TG, THERE IS LIMITED CAPACITY FOR FA OXIDATION, ESPECIALLY DURING EXERCISE 65% OF MAXIMAL O2 UPTAKE (VO2MAX). UNDER PHYSIOLOGICAL CONDITIONS, THE RATE OF FA OXIDATION DEPENDS ON THE INTERACTION OF A VARIETY OF FACTORS THAT INCLUDE THE RELATIVE EXERCISE INTENSITY, TRAINING STATUS, PLASMA FFA CONCENTRATION, AND ENDOGENOUS AND EXOGENOUS CHO AVAILABILITY.FATTY ACIDS (FFAS) OXIDATION DURING SUSTAINED PROLONGED PHYSICAL ACTIVITY DELAYS GLYCOGEN DEPLETION AND HYPOGLYCEMIA. UTILIZATION OF FATTY ACIDS REQUIRES THE HYDROLYSIS OF TRIACYLGLYCEROLS FROM ADIPOSE TISSUE, MUSCLE, AND PLASMA. THE INCREASED HYDROLYSIS FROM ADIPOSE TISSUE REQUIRES THE DELIVERY OF FATTY ACIDS TO SKELETAL MUSCLE MITOCHONDRIA FOR OXIDATION. HOWEVER, THE RELEASE OF FREE FATTY ACIDS INTO BLOOD AND THE CONCENTRATION OF PLASMA FFAS ARE NOT CLOSELY MATCHED TO ENERGY NEED. THE USE OF TRIACYLGLYCEROLS AS AN ENERGY SOURCE REQUIRES THE COORDINATION AND REGULATION OF LIPOLYSIS IN ADIPOCYTES, FFA MOBILIZATION AND DELIVERY TO SKELETAL MUSCLE, ADIPOSE TISSUE AND SKELETAL MUSCLE BLOOD FLOW. HOWEVER, TRIACYLGLYCEROL’S ROLE AS AN ENERGY SUBSTRATE IS DEPENDENT NOT ONLY ON EXERCISE INTENSITY AND DURATION BUT IS ALSO MODULATED BY OTHER FACTORS, SUCH AS ENDURANCE TRAINING, DIET, AGE, AND GENDER.IN CONTRAST, AN IMPAIRED CAPACITY TO OXIDIZE FAT APPEARS TO BE AN IMPORTANT FACTOR IN THE ETIOLOGY OF OBESITY, INSULIN RESISTANCE AND TYPE II DIABETES. AS A RESULT FATTY ACIDS MAY BE STORED IN THE MUSCLE AND IN OTHER TISSUES. THIS ACCUMULATION OF LIPID IN THE MUSCLE AND ITS METABOLITES MAY INTERFERE WITH THE REGULATION OF METABOLISM. IT IS ALSO ATTRACTIVE TO THINK THAT AN INCREASED CAPACITY TO OXIDIZE FAT MAY AID THOSE WHO WANT TO LOSE WEIGHT AND IN PARTICULAR BODY FAT IF USED IN COMBINATION WITH A NEGATIVE ENERGY BALANCE. HERE, WE WILL DISCUSS SOME FACTORS THAT CAN AFFECT FAT OXIDATION INCLUDE EXERCISE TYPE, INTENSITY & DURATION, DIET AND RELATED SUPPLEMENTS, GENDER DIFFERENCES, ENVIRONMENT, TRAINING AND FITNESS LEVEL, EXERCISE PROGRAMMES TO LOSE WEIGHT OR BODY FAT, HORMONAL CHANGES.