Does Cardio After an Overnight Fast Maximize Fat Loss? Brad Schoenfeld, MS, CSCS Global Fitness Services, Scarsdale, New York

SUMMARY First and foremost, it is shortsighted to enhance fat loss is flawed even when look solely at how much fat is burned examining its impact on the amount of THIS ARTICLE WILL REVIEW THE during an exercise session. The human fat burned in the exercise session alone. EFFICACY OF A COMMON FAT body is very dynamic and continually True, multiple studies show that con- BURNING STRATEGY EMPLOYED adjusts its use of fat for fuel. Substrate sumption of carbohydrate before low- BY BODYBUILDERS, ATHLETES, utilization is governed by a host of intensity aerobic exercise (up to AND FITNESS ENTHUSIASTS _ factors (i.e., hormonal secretions, en- approximately 60% Vo2max) in un- BASED ON CURRENT RESEARCH. zyme activity, transcription factors, trained subjects reduces the entry of THIS STRATEGY IS TO PERFORM etc), and these factors can change by long-chain fatty acids in the mitochon- CARDIOVASCULAR EXERCISE the moment (27). Thus, fat burning dria, thereby blunting fat oxidation EARLY IN THE MORNING ON AN must be considered over the course of (1,14,18,28). This is attributed to an EMPTY STOMACH. THE THEORY days—not on an hour-to-hour basis—to insulin-mediated attenuation of adi- GIVEN FOR THIS STRATEGY IS get a meaningful perspective on its pose tissue lipolysis, an increased THAT A SHIFT IN ENERGY UTILIZA- impact on body composition (13). As glycolytic flux, and a decreased expres- TION AWAY FROM CARBOHY- a general rule, if you burn more sion of genes involved in fatty acid DRATES OCCURS, THEREBY carbohydrate during a workout, you transport and oxidation (3,6,15). How- ALLOWING GREATER MOBILIZA- inevitably burn more fat in the post- ever, both training status and aerobic TION OF STORED FAT FOR FUEL. exercise period and vice versa. exercise intensity have been shown to mitigate the effects of a pre-exercise It should be noted that high-intensity meal on fat oxidation (4,5,24). Recent interval training (HIIT) has proven to research has shed light on the com- common fat burning strategy be a superior method for maximizing employed by bodybuilders, ath- plexities of the subject. fat loss compared with a moderate- letes, and fitness enthusiasts is to A intensity steady-state training Horowitz et al. (14) studied the fat perform cardiovascular exercise early in (10,26,29). Interestingly, studies show burning response of 6 moderately trained the morning on an empty stomach. This that blood flow to adipose tissue individuals in a fed versus fasted state to strategy was popularized by Bill Phillips diminishes at higher levels of in- different training intensities. Subjects in his book, ‘‘Body for Life’’ (23). tensity (24). This is believed to entrap cycled for 2 hours at varying intensities According to Phillips, performing 20 free fatty acids within fat cells, on 4 separate occasions. During 2 of the minutes of intense aerobic exercise after impeding their ability to be oxidized trials, they consumed a high-glycemic an overnight fast has greater effects on while training. Yet, despite lower fat carbohydrate meal at 30, 60, and fat loss than performing an entire hour oxidation rates during exercise, fat 90 minutes of training, once at a low of cardio in the postprandial state. The loss is nevertheless greater over time intensity (25% peak oxygen consump- rationale for the theory is that low tion) and once at a moderate intensity in those who engage in HIIT versus glycogen levels cause your body to shift (68% peak oxygen consumption). During training in the ‘‘fat burning zone’’ energy utilization away from carbohy- the other 2 trials, subjects were kept drates, thereby allowing greater mobili- (29), providing further evidence that zation of stored fat for fuel. However, 24-hour energy balance is the most KEY WORDS: although the prospect of reducing the important determinant in reducing body fat. fat burning; fat oxidation; lipolysis; body fat by training in a fasted state may aerobic exercise; cardiovascular The concept of performing cardiovas- sound enticing, science does not support exercise; interval training its efficacy. cular exercise on an empty stomach to

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fasted for 12–14 hours before exercise It should also be noted that consump- more experienced trainees (12,16,22,31). and for the duration of training. Results in tion of food before training increases It is estimated that nonplasma fatty acid the low-intensity trials showed that the thermic effect of exercise. Lee et al. utilization during endurance exercise is although lipolysis was suppressed by (19) compared the lipolytic effects of approximately twice that for trained 22% in the fed state compared with the an exercise bout in either a fasted state versus untrained individuals (24,32). fasted state, fat oxidation remained or after consumption of a glucose/milk Hurley et al. (17) reported that the similar between groups until 80–90 (GM) beverage. In a crossover design, contribution of IMTG stores in trained minutes of cycling. Only after this point 4 experimental conditions were stud- individuals equated to approximately was a greater fat oxidation rate observed ied: low-intensity long duration exer- 80% of the total body fat utilization in fasted subjects. Conversely, during cise with GM, low-intensity long during 120 minutes of moderate- moderate-intensity cycling, fat oxidation duration exercise without GM, high- intensity endurance training. was not different between trials at any intensity short duration exercise with The important point here is that IMTG time—this is despite a 20–25% reduction GM, and high-intensity short duration stores have no bearing on health and/or in lipolysis and plasma Free fatty acid exercise without GM. Subjects were appearance; it is the subcutaneous fat concentration. 10 male college students who per- stored in adipose tissue that influences More recently, Febbraio et al. (9) formed all 4 exercise bouts in random body composition. Consequently, the evaluated the effect of pre-exercise order on the same day. Results showed actual fat burning effects of any fitness and during exercise carbohydrate con- that ingestion of the GM beverage strategy intended to increase fat oxida- sumption on fat oxidation. Using resulted in a significantly greater excess tion must be taken in the context of a crossover design, 7 endurance- postexercise oxygen consumption the specific adipose deposits providing trained subjects cycled for 120 minutes compared with exercise performed in energy during exercise. at approximately 63% of peak power a fasted state in both high- and low- Another factor that must be considered output, followed by a ‘‘performance intensity bouts. Other studies have pro- when training in a fasted state is its cycle’’ where subjects expended 7 duced similar findings, indicating a clear impact on proteolysis. Lemon and kJ/(kg body weight) by pedaling as thermogenic advantage associated with Mullin (20) found that nitrogen losses fast as possible. Trials were conducted pre-exercise food intake (7,11). were more than doubled when training on 4 separate occasions, with subjects The location of adipose tissue mobi- while glycogen depleted compared given (a) a placebo before and during lized during training must also be taken with glycogen loaded. This resulted training, (b) a placebo 30 minutes into account here. During low-to- in a protein loss estimated at 10.4% of before training and then a carbohydrate moderate intensity training performed the total caloric cost of exercise after beverage every 15 minutes throughout at a steady state, the contribution of fat 1 hour of cycling at 61% Vo_ max. This exercise, (c) a carbohydrate beverage 2 as a fuel source equates to approxi- would suggest that performing cardio- 30 minutes before training and then mately 40–60% of total energy expen- vascular exercise while fasting might a placebo during exercise, or (d) diture (30). However, in untrained not be advisable for those seeking to a carbohydrate beverage both before subjects, only about 50–70% of this maximize muscle mass. and every 15 minutes during exercise. fat is derived from plasma Free fatty The study was carried out in a double- Finally, the effect of fasting on energy acids; the balance comes from intra- blind fashion with trials performed in levels during exercise ultimately has an muscular triglycerides (IMTG) (30). random order. Consistent with previous effect on fat burning. Training early in research, results showed no evidence of IMTG are stored as lipid droplets in the morning on an empty stomach impaired fat oxidation associated with the sarcoplasm near the mitochondria makes it very difficult for an individual consumption of carbohydrate either (2), with the potential to provide to train at even a moderate level of before or during exercise. approximately two-thirds the available intensity. Attempting to engage in energy of muscle glycogen (32). Similar Taken together, these studies show a HIIT style routine in a hypoglycemic to muscle glycogen, IMTG can only be that during moderate-to-high intensity state almost certainly will impair oxidized locally within the muscle. It is cardiovascular exercise in a fasted performance (33). Studies show that state—and for endurance-trained indi- estimated that IMTG stores are ap- a pre-exercise meal allows an individual viduals regardless of training intensity— proximately 3 times greater in type I to train more intensely compared with significantly more fat is broken down versus type II muscle fibers (8,21,31), exercise while fasting (25). The net than that the body can use for fuel. Free and lipolysis of these stores are max- result is that a greater number of calories fatty acids that are not oxidized imally stimulated when exercising at are burned both during and after _ ultimately become re-esterified in ad- 65% Vo2max (24). physical activity, heightening fat loss. ipose tissue, nullifying any lipolytic The body increases IMTG stores with In conclusion, the literature does not benefits afforded by pre-exercise consistent endurance training, which support the efficacy of training early in fasting. results in a greater IMTG utilization for the morning on an empty stomach as

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