Does Cardio After an Overnight Fast Maximize Fat Loss?

Abstract

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.

Full text

Does Cardio After an
Overnight Fast Maximize
Fat Loss?
Brad Schoenfeld, MS, CSCS
Global Fitness Services, Scarsdale, New York
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 UTILIZA-
TION AWAY FROM CARBOHY-
DRATES OCCURS, THEREBY
ALLOWING GREATER MOBILIZA-
TION OF STORED FAT FOR FUEL.
Acommon fat burning strategy
employed by bodybuilders, ath-
letes, 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 carbohy-
drates, thereby allowing greater mobili-
zation 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, en-
zyme 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 post-
exercise 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 moderate-
intensity steady-state training
(10,26,29). Interestingly, studies show
that blood flow to adipose tissue
diminishes at higher levels of in-
tensity (24). This is believed to entrap
free fatty acids within fat cells,
impeding their ability to be oxidized
while training. Yet, despite lower fat
oxidation rates during exercise, fat
loss is nevertheless greater over time
in those who engage in HIIT versus
training in the ‘‘fat burning zone’’
(29), providing further evidence that
24-hour energy balance is the most
important determinant in reducing
body fat.
The concept of performing cardiovas-
cular exercise on an empty stomach to
enhance fat loss is flawed even when
examining its impact on the amount of
fat burned in the exercise session alone.
True, multiple studies show that con-
sumption of carbohydrate before low-
intensity aerobic exercise (up to
approximately 60%
_
Vo
2
max) in un-
trained subjects reduces the entry of
long-chain fatty acids in the mitochon-
dria, thereby blunting fat oxidation
(1,14,18,28). This is attributed to an
insulin-mediated attenuation of adi-
pose tissue lipolysis, an increased
glycolytic flux, and a decreased expres-
sion of genes involved in fatty acid
transport and oxidation (3,6,15). How-
ever, both training status and aerobic
exercise intensity have been shown to
mitigate the effects of a pre-exercise
meal on fat oxidation (4,5,24). Recent
research has shed light on the com-
plexities of the subject.
Horowitz et al. (14) studied the fat
burning response of 6 moderately trained
individuals in a fed versus fasted state to
different training intensities. Subjects
cycled for 2 hours at varying intensities
on 4 separate occasions. During 2 of the
trials, they consumed a high-glycemic
carbohydrate meal at 30, 60, and
90 minutes of training, once at a low
intensity (25% peak oxygen consump-
tion) and once at a moderate intensity
(68% peak oxygen consumption). During
the other 2 trials, subjects were kept
KEY WORDS:
fat burning; fat oxidation; lipolysis;
aerobic exercise; cardiovascular
exercise; interval training
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fasted for 12–14 hours before exercise
and for the duration of training. Results in
the low-intensity trials showed that
although lipolysis was suppressed by
22% in the fed state compared with the
fasted state, fat oxidation remained
similar between groups until 80–90
minutes of cycling. Only after this point
was a greater fat oxidation rate observed
in fasted subjects. Conversely, during
moderate-intensity cycling, fat oxidation
was not different between trials at any
time—this is despite a 20–25% reduction
in lipolysis and plasma Free fatty acid
concentration.
More recently, Febbraio et al. (9)
evaluated the effect of pre-exercise
and during exercise carbohydrate con-
sumption on fat oxidation. Using
a crossover design, 7 endurance-
trained subjects cycled for 120 minutes
at approximately 63% of peak power
output, followed by a ‘‘performance
cycle’’ where subjects expended 7
kJ/(kg body weight) by pedaling as
fast as possible. Trials were conducted
on 4 separate occasions, with subjects
given (a) a placebo before and during
training, (b) a placebo 30 minutes
before training and then a carbohydrate
beverage every 15 minutes throughout
exercise, (c) a carbohydrate beverage
30 minutes before training and then
a placebo during exercise, or (d)
a carbohydrate beverage both before
and every 15 minutes during exercise.
The study was carried out in a double-
blind fashion with trials performed in
random order. Consistent with previous
research, results showed no evidence of
impaired fat oxidation associated with
consumption of carbohydrate either
before or during exercise.
Taken together, these studies show
that during moderate-to-high intensity
cardiovascular exercise in a fasted
state—and for endurance-trained indi-
viduals 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 ad-
ipose tissue, nullifying any lipolytic
benefits afforded by pre-exercise
fasting.
It should also be noted that consump-
tion of food before training increases
the thermic effect of exercise. Lee et al.
(19) compared the lipolytic effects of
an exercise bout in either a fasted state
or after consumption of a glucose/milk
(GM) beverage. In a crossover design,
4 experimental conditions were stud-
ied: low-intensity long duration exer-
cise with GM, low-intensity long
duration exercise without GM, high-
intensity short duration exercise with
GM, and high-intensity short duration
exercise without GM. Subjects were
10 male college students who per-
formed all 4 exercise bouts in random
order on the same day. Results showed
that ingestion of the GM beverage
resulted in a significantly greater excess
postexercise oxygen consumption
compared with exercise performed in
a fasted state in both high- and low-
intensity bouts. Other studies have pro-
duced similar findings, indicating a clear
thermogenic advantage associated with
pre-exercise food intake (7,11).
The location of adipose tissue mobi-
lized during training must also be taken
into account here. During low-to-
moderate intensity training performed
at a steady state, the contribution of fat
as a fuel source equates to approxi-
mately 40–60% of total energy expen-
diture (30). However, in untrained
subjects, only about 50–70% of this
fat is derived from plasma Free fatty
acids; the balance comes from intra-
muscular triglycerides (IMTG) (30).
IMTG are stored as lipid droplets in
the sarcoplasm near the mitochondria
(2), with the potential to provide
approximately two-thirds the available
energy of muscle glycogen (32). Similar
to muscle glycogen, IMTG can only be
oxidized locally within the muscle. It is
estimated that IMTG stores are ap-
proximately 3 times greater in type I
versus type II muscle fibers (8,21,31),
and lipolysis of these stores are max-
imally stimulated when exercising at
65%
_
Vo
2
max (24).
The body increases IMTG stores with
consistent endurance training, which
results in a greater IMTG utilization for
more experienced trainees (12,16,22,31).
It is estimated that nonplasma fatty acid
utilization during endurance exercise is
approximately twice that for trained
versus untrained individuals (24,32).
Hurley et al. (17) reported that the
contribution of IMTG stores in trained
individuals equated to approximately
80% of the total body fat utilization
during 120 minutes of moderate-
intensity endurance training.
The important point here is that IMTG
stores have no bearing on health and/or
appearance; it is the subcutaneous fat
stored in adipose tissue that influences
body composition. Consequently, the
actual fat burning effects of any fitness
strategy intended to increase fat oxida-
tion must be taken in the context of
the specific adipose deposits providing
energy during exercise.
Another factor that must be considered
when training in a fasted state is its
impact on proteolysis. Lemon and
Mullin (20) found that nitrogen losses
were more than doubled when training
while glycogen depleted compared
with glycogen loaded. This resulted
in a protein loss estimated at 10.4% of
the total caloric cost of exercise after
1 hour of cycling at 61%
_
Vo
2
max. This
would suggest that performing cardio-
vascular exercise while fasting might
not be advisable for those seeking to
maximize muscle mass.
Finally, the effect of fasting on energy
levels during exercise ultimately has an
effect on fat burning. Training early in
the morning on an empty stomach
makes it very difficult for an individual
to train at even a moderate level of
intensity. Attempting to engage in
a HIIT style routine in a hypoglycemic
state almost certainly will impair
performance (33). Studies show that
a pre-exercise meal allows an individual
to train more intensely compared with
exercise while fasting (25). The net
result is that a greaternumber of calories
are burned both during and after
physical activity, heightening fat loss.
In conclusion, the literature does not
support the efficacy of training early in
the morning on an empty stomach as
VOLUME 33 | NUMBER 1 | FEBRUARY 2011
24
Cardio After an Overnight Fast and Fat Loss

a tactic to reduce body fat. At best, the
net effect on fat loss associated with
such an approach will be no better than
training after meal consumption, and
quite possibly, it would produce in-
ferior results. Moreover, given that
training with depleted glycogen levels
has been shown to increase proteolysis,
the strategy has potential detrimental
effects for those concerned with mus-
cle strength and hypertrophy.
Brad
Schoenfeld is
president of Global
Fitness Services.
REFERENCES
1. Ahlborg G and Felig P. Influence of glucose
ingestion on fuel-hormone response during
prolonged exercise. J Appl Physiol 41:
683–688, 1976.
2. Boesch C, Slotboom J, Hoppeler H, and
Kreis R. In vivo determination of
intramyocellular lipids in human muscle by
means of localized H-MR-spectroscopy.
Mag Reson Med 37: 484–493, 1997.
3. Civitarese AE, Hesselink MK, Russell AP,
Ravussin E, and Schrauwen P. Glucose
ingestion during exercise blunts exercise-
induced gene expression of skeletal muscle
fat oxidative genes. Am J Physiol Endocrinol
Metab 289: E1023–E1029, 2005.
4. Coyle EF, Coggan AR, Hemmert MK, and Ivy JL.
Muscle glycogen utilization during prolonged
strenuous exercise when fed carbohydrate.
J Appl Physiol 61: 165–172, 1986.
5. Coyle EF, Hagberg JM, Hurley BF,
Martin WH, Ehsani AA, and Holloszy JO.
Carbohydrates during prolonged strenuous
exercise can delay fatigue. J Appl Physiol
59: 429–433, 1983.
6. Coyle EF, Jeukendrup AE, Wagenmakers AJ,
and Saris WH. Fatty acid oxidation is directly
regulated by carbohydrate metabolism
during exercise. Am J Physiol Endocrinol
Metab 273: E268–E275, 1997.
7. Davis JM. Weight control and calorie
expenditure: Thermogenic effects of pre-
prandial and post-prandial exercise. Addict
Behav 14: 347–351, 1989.
8. Essen B, Jansson E, Henriksson J, Taylor
AW, and Saltin B. Metabolic
characteristics of fibre types in human
skeletal muscle. Acta Physiol Scand 95:
153–165, 1975.
9. Febbraio MA, Chiu A, Angus DJ, Arkinstall MJ,
and Hawley JA. Effects of carbohydrate
ingestion before and during exercise on
glucose kinetics and performance. J Appl
Physiol 89: 2220–2226, 2000.
10. Gibala MJ, Little JP, van Essen M, Wilkin GP,
Burgomaster KA, Safdar A, Raha S, and
Tarnopolsky MA. Short-term sprint interval
versus traditional endurance training:
Similar initial adaptations in human skeletal
muscle and exercise performance.
J Physiol 15(pt 3): 901–911, 2006.
11. Goben KW, Sforzo GA, and Frye PA.
Exercise intensity and the thermic effect of
food. Int J Sport Nutr 2: 87–95, 1992.
12. Goodpaster BH, He J, Watkins S, and
Kelley DE. Skeletal muscle lipid content and
insulin resistance: evidence for a paradox in
endurance-trained athletes. J Clin
Endocrinol Metab 86: 5755–5761, 2001.
13. Hansen K, Shriver T, and Schoeller D. The
effects of exercise on the storage and
oxidation of dietary fat. Sports Med 35:
363–373, 2005.
14. Horowitz JF, Mora-Rodriguez R, Byerley LO,
and Coyle EF. Lipolytic suppression following
carbohydrate ingestion limits fat oxidation
during exercise. Am J Physiol Endocrinol
Metab 273: E768–E775, 1997.
15. Horowitz JF,Mora-Rodriguez R,Byerley LO, and
Coyle EF. Substrate metabolism when subjects
are fed carbohydrate during exercise. Am J
Physiol 276(5 Pt 1): E828–E835, 1999.
16. HowaldH,HoppelerH,ClaassenH,Mathieu
O, and Straub R. Influences of endurance
training on the ultrastructural composition of
the different muscle fiber types in humans.
Pflugers Arch 403: 369–376, 1985.
17. Hurley BF, Nemeth PM, Martin WH III,
Hagberg JM, Dalsky GP, and Holloszy JO.
Muscle triglyceride utilization during
exercise: Effect of training. J Appl Physiol
60: 562–567, 1986.
18. Ivy JL, Miller W, Dover V, Goodyear LG,
Sherman WM, Farrell S, and Williams H.
Endurance improved by ingestion of
a glucose polymer supplement. Med Sci
Sports Exerc 15: 466–471, 1983.
19. Lee YS, Ha MS, and Lee YJ. The effects of
various intensities and durations of exercise
with and without glucose in milk ingestion on
postexercise oxygen consumption. JSports
Med Physical Fitness 39: 341–347, 1999.
20. Lemon PW and Mullin JP. Effect of initial
muscle glycogen levels on protein
catabolism during exercise. J Appl Physiol
48: 624–629, 1980.
21. Malenfant P, Joanisse DR, Theriault R,
Goodpaster BH, Kelley DE, and Simoneau
JA. Fat content in individual muscle fibers of
lean and obese subjects. Int J Obes Relat
Metab Disord. 25: 1316–1321, 2001.
22. Martin WH III, Dalsky GP, Hurley B F, Matthews
DE, Bier DM, Hagberg JM, Rogers MA, King
DS, and Holloszy JO. Effect of endurance
training on plasma free fatty acid turnover and
oxidation during exercise. Am J Physiol
Endocrinol Metab 265: E708–E714, 1993.
23. Phillips B. Body for Life. New York, NY:
HarperCollins, 1999.
24. Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A,
Horowitz JF, Endert E, and Wolfe RR.
Regulation of endogenous fat and carbohydrate
metabolism in relationto exercise intensity. Am J
Physiol 265(3 Pt 1): E380–E391, 1993.
25. Schabort EJ, Bosch AN, Weltan SM, and
Noakes TD. The effect of a preexercise meal on
time to fatigue during prolongedcycling exercise.
Med Sci Sports Exerc 31: 464–471, 1999.
26. Schoenfeld B and Dawes J. High-intensity
interval training: Applications for general fitness
training. Strength Cond J 31: 44–46, 2009.
27. Sonko BJ, Fennessey PV, Donnelly JE,
Bessesen D, Sharp TA, Jacobsen DJ,
Jones RH, and Hill JO.Ingested fat
oxidation contributes 8% of 24-h total
energy expenditure in moderately obese
subjects. J Nutr 135: 2159–2165, 2005.
28. Spriet LL and Watt MJ. Regulatory mechanisms
in the interaction between carbohydrate and
lipid oxidation during exercise. Acta Physiol
Scand 178: 443–452, 2003.
29. Tremblay A, Simoneau JA, and Bouchard O.
Impact of exercise intensity on body
fatness and skeletal muscle metabolism.
Metabolism 43: 814–818, 1994.
30. van Loon LJ. Use of intramuscular
triacylglycerol as a substrate source during
exercise in humans. J Appl Physiol 97:
1170–1187, 2004.
31. van Loon LJC, Koopman R, Stegen JH,
Wagenmakers AJ, Keizer HA, and Saris
WH. Intramyocellular lipids form an
important substrate source during
moderate intensity exercise in endurance-
trained males in a fasted state. J Physiol
553: 611–625, 2003.
32. Watt MJ, Heigenhauser GJ, and Spriet LL.
Intramuscular triacylglycerol utilization in
human skeletal muscle during exercise: Is
there a controversy? J Appl Physiol
93: 1185–1195, 2002.
33. Wright DA, Sherman WM, and Dernbach
AR. Carbohydrate feedings before, during,
or in combination improve cycling
endurance performance. J Appl Physiol
71: 1082–1088, 1991.
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