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SUMMARY
THIS ARTICLE WILL REVIEW THE EFFICACY OF A COMMON FAT BURNING STRATEGY EMPLOYED BY BODYBUILDERS, ATHLETES, AND FITNESS ENTHUSIASTS BASED ON CURRENT RESEARCH. THIS STRATEGY IS TO PERFORM CARDIOVASCULAR EXERCISE EARLY IN THE MORNING ON AN EMPTY STOMACH. THE THEORY GIVEN FOR THIS STRATEGY IS THAT A SHIFT IN ENERGY UTILIZATION AWAY FROM CARBOHYDRATES OCCURS, THEREBY ALLOWING GREATER MOBILIZATION OF STORED FAT FOR FUEL.
KEY WORDS:
fat burning; fat oxidation; lipolysis; aerobic exercise; cardiovascular exercise; interval training
A common fat burning strategy employed by bodybuilders, athletes, and fitness enthusiasts is to perform cardiovascular exercise early in the morning on an empty stomach. This strategy was popularized by Bill Phillips in his book, "Body for Life" (23). According to Phillips, performing 20 minutes of intense aerobic exercise after an overnight fast has greater effects on fat loss than performing an entire hour of cardio in the postprandial state. The rationale for the theory is that low glycogen levels cause your body to shift energy utilization away from carbohydrates, thereby allowing greater mobilization of stored fat for fuel. However, although the prospect of reducing the body fat by training in a fasted state may sound enticing, science does not support its efficacy.
First and foremost, it is shortsighted to look solely at how much fat is burned during an exercise session. The human body is very dynamic and continually adjusts its use of fat for fuel. Substrate utilization is governed by a host of factors (i.e., hormonal secretions, enzyme activity, transcription factors, etc), and these factors can change by the moment (27). Thus, fat burning must be considered over the course of days-not on an hour-to-hour basis-to get a meaningful perspective on its impact on body composition (13). As a general rule, if you burn more carbohydrate during a workout, you inevitably burn more fat in the postexercise period and vice versa.
It should be noted that high-intensity interval training (HIIT) has proven to be a superior method for maximizing fat loss compared with a moderateintensity steady-state training (10,26,29). Interestingly, studies show that blood flow to adipose tissue diminishes at higher levels of intensity (24). This is believed to entrap free fatty acids within fat cells, impeding their ability to be oxidized while training....