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    Default Interesting new article on Fasted AM Cardio

    Does Cardio After anOvernight Fast MaximizeFat Loss?

    Brad Schoenfeld, MS, CSCSGlobal Fitness Services, Scarsdale, New York


    A common fat burning strategyemployed by bodybuilders, athletes,and fitness enthusiasts is toperform cardiovascular exercise early inthe morning on an empty stomach. Thisstrategy was popularized by Bill Phillipsin his book, ‘‘Body for Life’’ (23).According to Phillips, performing 20minutes of intense aerobic exercise afteran overnight fast has greater effects onfat loss than performing an entire hourof cardio in the postprandial state. Therationale for the theory is that lowglycogen levels cause your body to shiftenergy utilization away from carbohydrates,thereby allowing greater mobilizationof stored fat for fuel. However,although the prospect of reducing thebody fat by training in a fasted state maysound enticing, science does not supportits efficacy.

    First and foremost, it is shortsighted tolook solely at how much fat is burnedduring an exercise session. The humanbody is very dynamic and continuallyadjusts its use of fat for fuel. Substrateutilization is governed by a host offactors (i.e., hormonal secretions, enzymeactivity, transcription factors,etc), and these factors can change bythe moment (27). Thus, fat burningmust be considered over the course ofdays—not on an hour-to-hour basis—toget a meaningful perspective on itsimpact on body composition (13). Asa general rule, if you burn morecarbohydrate during a workout, youinevitably burn more fat in the postexerciseperiod and vice versa.

    It should be noted that high-intensityinterval training (HIIT) has proven tobe a superior method for maximizingfat loss compared with a moderateintensitysteady-state training(10,26,29). Interestingly, studies showthat blood flow to adipose tissuediminishes at higher levels of intensity(24). This is believed to entrapfree fatty acids within fat cells,impeding their ability to be oxidizedwhile training. Yet, despite lower fatoxidation rates during exercise, fatloss is nevertheless greater over timein those who engage in HIIT versustraining in the ‘‘fat burning zone’’(29), providing further evidence that24-hour energy balance is the mostimportant determinant in reducingbody fat.

    The concept of performing cardiovascularexercise on an empty stomach toenhance fat loss is flawed even whenexamining its impact on the amount offat burned in the exercise session alone.True, multiple studies show that consumptionof carbohydrate before lowintensityaerobic exercise (up toapproximately 60% Vo_ 2max) in untrainedsubjects reduces the entry oflong-chain fatty acids in the mitochondria,thereby blunting fat oxidation(1,14,18,28). This is attributed to aninsulin-mediated attenuation of adiposetissue lipolysis, an increasedglycolytic flux, and a decreased expressionof genes involved in fatty acidtransport and oxidation (3,6,15). However,both training status and aerobicexercise intensity have been shown tomitigate the effects of a pre-exercisemeal on fat oxidation (4,5,24). Recentresearch has shed light on the complexitiesof the subject.

    Horowitz et al. (14) studied the fatburning response of 6 moderately trainedindividuals in a fed versus fasted state todifferent training intensities. Subjectscycled for 2 hours at varying intensitieson 4 separate occasions. During 2 of thetrials, they consumed a high-glycemiccarbohydrate meal at 30, 60, and90 minutes of training, once at a lowintensity (25% peak oxygen consumption)and once at a moderate intensity(68% peak oxygen consumption). Duringthe other 2 trials, subjects were kept fasted for 12–14 hours before exerciseand for the duration of training. Results inthe low-intensity trials showed thatalthough lipolysis was suppressed by22% in the fed state compared with thefasted state, fat oxidation remainedsimilar between groups until 80–90minutes of cycling. Only after this pointwas a greater fat oxidation rate observedin fasted subjects. Conversely, duringmoderate-intensity cycling, fat oxidationwas not different between trials at anytime—this is despite a 20–25% reductionin lipolysis and plasma Free fatty acidconcentration.

    More recently, Febbraio et al. (9)evaluated the effect of pre-exerciseand during exercise carbohydrate consumptionon fat oxidation. Usinga crossover design, 7 endurancetrainedsubjects cycled for 120 minutesat approximately 63% of peak poweroutput, followed by a ‘‘performancecycle’’ where subjects expended 7kJ/(kg body weight) by pedaling asfast as possible. Trials were conductedon 4 separate occasions, with subjectsgiven (a) a placebo before and duringtraining, (b) a placebo 30 minutesbefore training and then a carbohydratebeverage every 15 minutes throughoutexercise, (c) a carbohydrate beverage30 minutes before training and thena placebo during exercise, or (d)a carbohydrate beverage both beforeand every 15 minutes during exercise.The study was carried out in a doubleblindfashion with trials performed inrandom order. Consistent with previousresearch, results showed no evidence ofimpaired fat oxidation associated withconsumption of carbohydrate eitherbefore or during exercise.

    Taken together, these studies showthat during moderate-to-high intensitycardiovascular exercise in a fastedstate—and for endurance-trained individualsregardless of training intensity—significantly more fat is broken downthan that the body can use for fuel. Freefatty acids that are not oxidizedultimately become re-esterified in adiposetissue, nullifying any lipolyticbenefits afforded by pre-exercisefasting.

    It should also be noted that consumptionof food before training increasesthe thermic effect of exercise. Lee et al.(19) compared the lipolytic effects ofan exercise bout in either a fasted stateor after consumption of a glucose/milk(GM) beverage. In a crossover design,4 experimental conditions were studied:low-intensity long duration exercisewith GM, low-intensity longduration exercise without GM, highintensityshort duration exercise withGM, and high-intensity short durationexercise without GM. Subjects were10 male college students who performedall 4 exercise bouts in randomorder on the same day. Results showedthat ingestion of the GM beverageresulted in a significantly greater excesspostexercise oxygen consumptioncompared with exercise performed ina fasted state in both high- and lowintensitybouts. Other studies have producedsimilar findings, indicating a clearthermogenic advantage associated withpre-exercise food intake (7,11).

    The location of adipose tissue mobilizedduring training must also be takeninto account here. During low-tomoderateintensity training performedat a steady state, the contribution of fatas a fuel source equates to approximately40–60% of total energy expenditure(30). However, in untrainedsubjects, only about 50–70% of thisfat is derived from plasma Free fattyacids; the balance comes from intramusculartriglycerides (IMTG) (30).

    IMTG are stored as lipid droplets inthe sarcoplasm near the mitochondria(2), with the potential to provideapproximately two-thirds the availableenergy of muscle glycogen (32). Similarto muscle glycogen, IMTG can only beoxidized locally within the muscle. It isestimated that IMTG stores are approximately3 times greater in type Iversus type II muscle fibers (8,21,31),and lipolysis of these stores are maximallystimulated when exercising at65% Vo_ 2max (24).

    The body increases IMTG stores withconsistent endurance training, whichresults in a greater IMTG utilization formore experienced trainees (12,16,22,31).It is estimated that nonplasma fatty acidutilization during endurance exercise isapproximately twice that for trainedversus untrained individuals (24,32).Hurley et al. (17) reported that thecontribution of IMTG stores in trainedindividuals equated to approximately80% of the total body fat utilizationduring 120 minutes of moderateintensityendurance training.

    The important point here is that IMTGstores have no bearing on health and/orappearance; it is the subcutaneous fatstored in adipose tissue that influencesbody composition. Consequently, theactual fat burning effects of any fitnessstrategy intended to increase fat oxidationmust be taken in the context ofthe specific adipose deposits providingenergy during exercise.

    Another factor that must be consideredwhen training in a fasted state is itsimpact on proteolysis. Lemon andMullin (20) found that nitrogen losseswere more than doubled when trainingwhile glycogen depleted comparedwith glycogen loaded. This resultedin a protein loss estimated at 10.4% ofthe total caloric cost of exercise after1 hour of cycling at 61% Vo_ 2max. Thiswould suggest that performing cardiovascularexercise while fasting mightnot be advisable for those seeking tomaximize muscle mass.

    Finally, the effect of fasting on energylevels during exercise ultimately has aneffect on fat burning. Training early inthe morning on an empty stomachmakes it very difficult for an individualto train at even a moderate level ofintensity. Attempting to engage ina HIIT style routine in a hypoglycemicstate almost certainly will impairperformance (33). Studies show thata pre-exercise meal allows an individualto train more intensely compared withexercise while fasting (25). The netresult is that a greater number of caloriesare burned both during and afterphysical activity, heightening fat loss.

    In conclusion, the literature does notsupport the efficacy of training early inthe morning on an empty stomach as a tactic to reduce body fat. At best, thenet effect on fat loss associated withsuch an approach will be no better thantraining after meal consumption, andquite possibly, it would produce inferiorresults. Moreover, given thattraining with depleted glycogen levelshas been shown to increase proteolysis,the strategy has potential detrimentaleffects for those concerned with musclestrength and hypertrophy.


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    "The only way you can hurt the body is not use it. Inactivity is the killer and, remember, it's never too late."
    ~Jack Lalanne

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    Managing Dir., Rx Muscle Forums Curt James's Avatar
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    If Sassy posted it then you should cherish every word.
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    Managing Dir., Rx Muscle Forums Curt James's Avatar
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    But this looks like the take-home message:

    results showed no evidence of impaired fat oxidation associated with consumption of carbohydrate either before or during exercise. [...] studies show that during moderate-to-high intensity cardiovascular exercise in a fasted state—and for endurance-trained individuals regardless of training intensity—significantly more fat is broken down than that the body can use for fuel. Free fatty acids that are not oxidized ultimately become re-esterified in adiposetissue, nullifying any lipolytic benefits afforded by pre-exercise fasting.

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