Hindawi Publishing Corporation
Journal of Obesity
Volume 2011, Article ID 868305, 10 pages
High-Intensity Intermittent Exerciseand Fat Loss
School of Medical Sciences, Faculty of Medicine, University of New South Wales, Sydney, NSW 2052, Australia
Correspondence should be addressed to Stephen H. Boutcher, email@example.com
Received 3 June 2010; Accepted 5 October 2010
Academic Editor: Eric Doucet
Copyright © 2011 Stephen H. Boutcher. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
The effect of regular aerobic exercise on body fat is negligible; however, other forms of exercise may have a greater impact on
body composition. For example, emerging research examining high-intensity intermittent exercise (HIIE) indicates that it may be
more effective at reducing subcutaneous and abdominal body fat than other types of exercise. The mechanisms underlying the fat
reduction induced by HIIE, however, are undetermined. Regular HIIE has been shown to significantly increase both aerobic and
anaerobic fitness. HIIE also significantly lowers insulin resistance and results in a number of skeletal muscle adaptations that result
in enhanced skeletal muscle fat oxidation and improved glucose tolerance. This review summarizes the results of HIIE studies on
fat loss, fitness, insulin resistance, and skeletal muscle. Possible mechanisms underlying HIIE-induced fat loss and implications for
the use of HIIE in the treatment and prevention of obesity are also discussed.
Most exercise protocols designed to induce fat loss have
focused on regular steady state exercise such as walking and
jogging at a moderate intensity. Disappointingly, these kinds
of protocols have led to negligible weight loss [1, 2]. Thus,
exercise protocols that can be carried out by overweight,
inactive individuals that more effectively reduce body fat
are required. Accumulating evidence suggests that high-
intensity intermittent exercise (HIIE) has the potential to be
of overweight individuals.
HIIE protocols have varied considerably but typically
involve repeated brief sprinting at an all-out intensity
immediately followed by low intensity exercise or rest.
The length of both the sprint and recovery periods has
varied from 6s to 4min. Most commonly the sprints are
performed on a stationary cycle ergometer at an intensity
in excess of 90% of maximal oxygen uptake (˙VO2max).
Subjects studied have included adolescents, young men
and women, older individuals, and a number of patient
groups [3–12]. The most utilized protocol in past research
has been the Wingate test which consists of 30s of all-
out sprint with a hard resistance . Subjects typically
perform the Wingate test 4 to 6 times separated by 4min
of recovery. This protocol amounts to 3 to 4min of exercise
per session with each session being typically performed 3
times a week for 2 to 6 weeks. Insight into the skeletal muscle
adaptation to HIIE has mainly been achieved using this
type of exercise ; however, as this protocol is extremely
hard, subjects have to be highly motivated to tolerate the
accompanying discomfiture. Thus, the Wingate protocol
is likely to be unsuitable for most overweight, sedentary
individuals interested in losing fat. Other less demanding
HIIE protocols have also been utilised. For example, we have
cycling for a period of 20min . Thus, instead of 4 to
6 sprints per session, as used in Wingate protocol studies,
subjects using the 8s/12s protocol sprint 60 times at a lower
exercise intensity. Total sprint time is 8min with 12min of
low intensity cycling. For the HIIE Wingate protocols, total
exercise time is typically between 3 to 4min of total exercise
per session. Thus, one of the characteristics of HIIE is that it
involves markedly lower training volume making it a time-
efficient strategy to accrue adaptations and possible health
benefits compared to traditional aerobic exercise programs.
This review summarises results of research examining the
effect of different forms of HIIE on fitness, insulin resis-
tance, skeletal muscle, subcutaneous, and abdominal fat
2 Journal of Obesity
2.Acute Response and Chronic Adaptations to
Acute responses to HIIE that have been identified include
heart rate, hormones, venous blood glucose, and lactate
levels, autonomic, and metabolic reactivity. Heart rate
response is dependent on the nature of the HIIE protocol
but typically is significantly elevated during exercise and
declines during the period between sprint and recovery. For
example, Weinstein et al. , using the Wingate protocol,
recorded peak heart rates of 170bpm immediately after a 30-
second maximal all-out cycle sprint. Heart rate response to
the 8s/12s protocol typically averages around 150bpm after
5min of HIIE which increases to 170bpm after 15min of
HIIE . In this protocol, there is typically a small heart
rate decrease of between 5–8bpm during each 12-second
recovery period. A similar pattern of heart rate response
was found for an HIIE protocol consisting of ten 6-second
sprints interspersed with 30s recovery. Heart rate increased
to 142bpm after the first sprint and then increased to
173bpm following sprint ten .
Hormones that have been shown to increase during HIIE
include catecholamines, cortisol, and growth hormones.
Catecholamine response has been shown to be significantly
elevated after Wingate sprints for both men and women [17,
18]. Catecholamine response to HIIE protocols that are less
intensive than the Wingate protocol have also been shown
to be elevated. For example, Christmass et al.  measured
catecholamine response to long (24s/36s recovery) and
short (6s/9s recovery) bout intermittent treadmill exercise
and found that norepinephrine was significantly elevated
postexercise. Also Trapp et al.  found significantly
elevated epinephrine and norepinephrine levels after 20min
of HIIE cycle exercise (8s/12s and 12s/24s protocols) in
trained and untrained young women. Bracken et al. 
examined the catecholamine response of 12 males who
completed ten 6-second cycle ergometer sprints with a 30-
second recovery between each sprint. From baseline, plasma
epinephrine increased 6.3-fold, whereas norepinephrine
increased 14.5-fold at the end of sprinting (Figure 1). The
significant catecholamine response to HIIE is in contrast to
moderate, steady state aerobic exercise that results in small
increases in epinephrine and norepinephrine . The HIIE
catecholamine response is an important feature of this type
of exercise as catecholamines, especially epinephrine, have
been shown to drive lipolysis and are largely responsible for
fat release from both subcutaneous and intramuscular fat
stores . Significantly, more β-adrenergic receptors have
been found in abdominal compared to subcutaneous fat
 suggesting that HIIE may have the potential to lower
abdominal fat stores. Aerobic endurance training increases
β-adrenergic receptor sensitivity in adipose tissue .
Interestingly, in endurance trained women, β-adrenergic
sensitivity was enhanced, whereas the sensitivity of the anti-
lipolytic α2receptors was diminished . However, no data
are available concerning HIIE training effects on β or α2
adrenergic receptor sensitivity of human adipocytes.
Nevill et al.  examined the growth hormone (GH)
response to treadmill sprinting in female and male athletes
and showed that there was a marked GH response to only
and women but greater for sprint compared to endurance
trained athletes. GH concentration was still ten times higher
than baseline levels after 1 hour of recovery. Venous blood
cortisol levels have also been shown to significantly increase
after repeated 100m run sprints in trained males , after
five 15-second Wingate tests , and during and after brief,
all-out sprint exercise in type 1 diabetic individuals .
Venous blood lactate response to the Wingate test
protocols has typically ranged from 6 to 13mmol·L−1.
Lactate levels after the Wingate test are typically higher
in trained anaerobic athletes and have been shown to be
similar  and lower for trained women compared to
trained men . Lactate levels gradually increase during
longer, lower intensity HIIE protocols. Trapp et al. 
showed that 8s/12s HIIE for 20min increased plasma lactate
levels between 2 and 4mmol·L−1after 5min of HIIE for
both trained cyclists and untrained females. Lactate rose to
between 4 and 5mmol·L−1after 15min of HIIE. During a
12s/24s HIIE condition lactate levels of the untrained were
similar but were significantly higher for the trained female
cyclists (between 7 and 8mmol·L−1after 15min). Despite
increasing lactate levels during HIIE exercise, it appears that
free fatty acid transport is also increased. For example, a 20-
minute bout of 8s/12s HIIE produced increased levels of
glycerol indicating increased release of fatty acids  which
peaked for untrained women after 20 min and after 10 min
of HIIE for trained women.
HIIE appears to result in significant increases in blood
glucose that are still elevated 5min  and 30min
postexercise . HIIE appears to have a more dramatic
effect on blood glucose levels of exercising type 1 dia-
betic individuals. Bussau et al.  examined the ability
of one 10-second maximal sprint to prevent the risk of
hypoglycemia typically experienced after moderate aerobic
exercise in type I diabetics. Twenty minutes of moderate-
intensity aerobic exercise resulted in a significant fall in
glycemia. However, one 10-second sprint at the end of the
20-minute aerobic exercise bout opposed a further fall in
glycemia for 120 minutes, whereas in the absence of a sprint,
glycemia decreased further after exercise. The stabilization
of glycemia in the sprint trials was associated with elevated
levels of catecholamines, growth hormone, and cortisol. In
contrast, these hormones remained at near baseline levels
after the 20min of aerobic exercise. Thus, one 10-second
all out sprint significantly increased glucose, catecholamines,
growth hormone, and cortisol of type 1 diabetic individuals
10-second sprint after moderate intensity aerobic exercise
can reduce hypoglycemia risk in physically active individuals
who possess type 1 diabetes.
Autonomic function has been analyzed after HIIE by
assessing heart rate variability. Parasympathetic activation
was found to be significantly impaired in a 10-minute
recovery period after repeated sprint exercise  and a
1-hour recovery period  in trained subjects. Buchhiet
et al.  have suggested that parasympathetic or vagal
impairment is caused by the heightened sympathetic activity
Journal of Obesity3
Rest Sp1 Sp2 Sp3 Sp4 Sp5 Sp6 Sp7 Sp8 Sp9 Sp10
Plasma AD(nmol · L−1)
Rest Sp1 Sp2 Sp3 Sp4 Sp5 Sp6 Sp7 Sp8 Sp9 Sp10
Plasma AD(nmol · L−1)
Figure 1: Plasma noradrenaline (NA) and adrenaline (AD) concentration of subjects at rest (CON) and following each 6-second sprint(EX)
(mean ±SD, n = 12).∗Indicates a significant difference from equivalent CON value (P < .05). (Adapted from Bracken et al. ).
that occurs during HIIE exercise and the persistent elevation
of adrenergic factors and local metabolites during recovery
(e.g., epinephrine, norepinephrine, and venous blood lac-
With regard to metabolic response, HIIE initially results
in decreased adenosine triphosphate (ATP) and phospho-
creatine (PCr) stores followed by decreased glycogen stores
 through anaerobic glycolysis . Gaitanos et al. 
have suggested that towards the end of an HIIE session,
which consists of numerous repeat sprints (e.g., ten 6-second
bouts of maximal sprinting), an inhibition of anaerobic
glycogenolysis may occur. These authors have further sug-
gested that at the end of the HIIE bout, ATP resynthesis may
be mainly derived from PCr degradation and intramuscular
during HIIE has not been demonstrated in humans. After
hard, all-out HIIE exercise, complete phosphagen recovery
may take 3-4min but complete restoration of pH and lactate
to pre-exercise levels may take hours . The recovery of
the exercising muscle after HIIE to its pre-exercise state is
has typically been found to be biphasic with an initial rapid
phase of recovery lasting 10s to a few minutes followed by a
slower recovery phase lasting from a few minutes to hours
. During recovery, oxygen consumption is elevated to
help restore metabolic processes to baseline conditions. The
postexercise oxygen uptake in excess of that required at rest
has been termed excess postexercise oxygen consumption
(EPOC). EPOC during the slow recovery period has been
associated with the removal of lactate and H+, increased
pulmonary and cardiac function, elevated body tempera-
ture, catecholamine effects, and glycogen resynthesis .
Although EPOC does not appear to have been assessed after
HIIE, it is enhanced after split aerobic exercise sessions. For
example, magnitude of EPOC was significantly greater when
30-minute  and 50-minute  aerobic exercise sessions
were divided into two parts. Also an exponential relationship
between aerobic exercise intensity and EPOC magnitude has
been demonstrated . With regard to HIIE, it is feasible
that the significant increase in catecholamines (Figure 1)
and the accompanying glycogen depletion described earlier
could induce significant EPOC. However, aerobic exercise
protocols resulting in prolonged EPOC have shown that
the EPOC comprises only 6–15% of the net total exercise
oxygen cost . Laforgia et al.  have concluded that
the major impact of exercise on body mass occurs via the
energy expenditure accrued during actual exercise. Whether
HIIE-induced EPOC is one of the mechanisms whereby
this unique form of exercise results in fat loss needs to be
determined by future research. In summary, acute responses
to a bout of HIIE include significant increases in heart rate,
catecholamines, cortisol, growth hormone, plasma lactate
and glucose levels, glycerol, and a significant decrease in
parasympathetic reactivation after HIIE, and depletion of
ATP, PCr, and glycogen stores.
Chronic responses to HIIE training include increased
aerobic and anaerobic fitness, skeletal muscle adaptations,
Surprisingly, aerobic fitness has been shown to significantly
increase following minimal bouts of HIIE training. For
example, Whyte et al.  carried out a 2-week HIIE
intervention with three HIIE sessions per week consisting
of 4 to 6 Wingate tests with 4min of recovery. Previously,
untrained males increased their˙VO2maxby 7%. Increases in
˙VO2max of 13% for an HIIE program also lasting 2 weeks
have been documented . HIIE protocols lasting 6 to 8
weeks have produced increases in˙VO2max of 4%  and
6–8% . Longer Wingate-type HIIE programs lasting 12
to 24 weeks have recorded large increases in ˙VO2max of
41%  and 46%  in type 2 diabetic and older cardiac
rehabilitation patients. The less intense protocols (8s/12s)
coupled with longer duration conducted over 15 and 12
weeks resulted in a 24%  and 18% increase  in
˙VO2max. Collectively, these results indicate that participation
4 Journal of Obesity
Table 1: Effect of high-intensity intermittent exercise on subcutaneous and abdominal fat, body mass, waist circumference,˙VO2max, and
trunk fat (kg)
Type of HIIE
Boudou et al. 
⇓ .12kg (6%)
60 ×8s/12s R 12 weeks
Helgerud et al.
— 15s/15s R8 weeks
Helgerud et al.
—4×4min/4min R 8 weeks
Mourier et al.
Perry et al. ——
Talanian et al.
Tjønna et al. ——
4×4min/3min R 16 weeks
Tjønna et al. 
4×4min/3min R 12 weeks
Trapp et al. 
— 60 ×8s/12s R15 weeks
Tremblay et al.
— 15 ×30s 24 weeks
Warburton et al.
—7×2min/2min R 16 weeks
Whyte et al. ——
Note: ⇑ indicates increased; ⇓ decreased; ⇔no change; —not recorded;∗body fat was assessed by skin folds; # trunk fat; SSE=steady state exercise;
Wingate=30s flat out sprint; R=recovery.
in differing forms of HIIE by healthy young adults and older
patients, lasting from 2 to 15 weeks, results in significant
increases in˙VO2max from between 4% to 46% (Table 1).
Mechanisms underlying the aerobic fitness response to HIIE
are unclear although a major contributor is phosphocreatine
degradation during repeated HIIE. Using thigh cuff occlu-
sion to prevent PCr resynthesis during recovery, Trump et
al.  showed that PCr contributed approximately 15%
of the total ATP provision during a third 30-second bout
of maximal isokinetic cycling. Muscle glycogenolysis made
Journal of Obesity5
a minor contribution to ATP provision during the third
30-second bout indicating that aerobic metabolism was the
major source of ATP during repeated sprinting. Similarly,
Putman et al.  showed that repeated bouts of HIIE
the third out of five 30-second Wingate bouts, the majority
of ATP was generated oxidatively.
Other mechanisms underlying the HIIE increase in aero-
volume induced by enhanced cardiac contractility ,
enhanced mitochondrial oxidative capacity, and increased
skeletal muscle diffusive capacity . There is also evidence
occurring via AMPK activation ¡?bhlt?¿ . Harmer et al.
 have suggested that these marked oxidative adaptations
in the exercising muscle are likely to underlie the significant
increases in peak and maximal oxygen uptake documented
after regular HIIE.
Anaerobic capacity response to HIIE has typically been
assessed by measuring blood lactate levels to a standardized
exercise load or anaerobic performance on a Wingate test. A
number of studies have demonstrated that HIIE lasting from
2 to 15 weeks results in significant increases in anaerobic
capacity from between 5% to 28%. For example, Tabata et
al.  used a 20s/10s protocol and found that in previously
untrained males, anaerobic capacity, measured by maximal
accumulated O2 deficit, was increased by 28%. Whyte et
al.  carried out a 2-week HIIE intervention and found
that previously untrained males increased their anaerobic
capacity by 8%, whereas Burgomaster et al.  found that
Wingate test performance was increased by 5.4% after two
weeks of HIIE.
A number of studies have taken muscle biopsies after
Wingate test performance in order to examine skeletal
muscle adaptations. In a series of studies, Gibala et al. [13,
52] have consistently found increased maximal activity and
protein content of mitochondrial enzymes such as citrate
synthase and cytochrome oxidase after HIIE training. For
that consisted of 2 weeks of HIIE exercise performed seven
times with each session consisting of ten 4-minute bouts
at 90%˙VO2max separated by 2-minute resting intervals.
˙VO2maxwas increased by 13% and plasma epinephrine and
heart rate were lower during the final 30min of a 60-minute
cycling steady state exercise trial at 60% of pretraining
˙VO2max. Exercise whole body fat oxidation also increased by
36%, and net glycogen use was reduced during the steady
state cycling trial. HIIE significantly increased muscle β
Total muscle plasma membrane fatty acid binding protein
content also increased significantly after HIIE. Thus, seven
sessions of HIIE, over two weeks, induced marked increases
in whole body and skeletal muscle capacity for fatty acid
increases in citrate synthase maximal activity after 2 weeks
 and 6 weeks of HIIE . Similarly, β hydroxyacyl
coenzyme A dehydrogenase activity, which catalyzes a key
rate-limiting enzyme step in fat oxidation, also significantly
increased after HIIE training . Increases in oxidative
muscle metabolism (e.g., hexokinase and citrate synthase
activity) after 7 weeks of HIIE training with type 1 diabetic
individuals have also been documented . Collectively,
markers of muscle oxidative capacity have been shown to
significantly increase after six sessions of HIIE lasting as little
as 2 weeks. Glycolytic enzyme content and activity has also
been shown to increase after exposure to HIIE. Tremblay
et al.  have shown that 16 weeks of HIIE significantly
increased phosphofructokinase levels which is a key rate
limiting enzyme in glycolysis, whereas Macdougall et al.
 also showed increases in phosphofructokinase with a
Wingate-type protocol carried out for 7 weeks. In summary,
Wingate test HIIE protocols of between one and seven
capacity for fatty acid oxidation and glycolytic enzyme
content and activity.
The effect of HIIE training on fasting insulin and insulin
resistance is shown in Table 1. As can be seen all studies
that have assessed insulin response to HIIE have recorded
significant improvements of between 23% and 58% increase
in insulin sensitivity. Insulin sensitivity has typically been
assessed by measuring fasting insulin, HOMA-IR, and by
glucose tolerance tests. In healthy, nondiabetic individuals,
the improvement in fasting insulin and insulin resistance
has ranged from 23% to 33% [37, 39, 42, 45], whereas
in individuals possessing type 2 diabetes, two studies have
reported greater insulin sensitivity improvements of 46%
 and 58% . Babraj et al.  used a glucose tolerance
test to assess insulin sensitivity after an intervention that
consisted of 2 weeks of HIIE performed three times per week
with each session consisting of four to six 30-second all out
sprints separated by resting interval of between 2 to 4min.
Glucose (12%) and insulin areas under the curve (37%) were
significantly attenuated with a sustained improved insulin
action until at least three days after the last exercise session.
This was achieved without a change in body weight and
with a total exercise energy increase of only 500kcal for
the two weeks. Authors suggest that the small increase in
energy expenditure contrasts to the 2000–3000kcal per week
experienced during a typical aerobic training program. The
mechanism(s) underlying these large increases in insulin
muscle adaptations previously discussed involving marked
increases in skeletal muscle capacity for fatty acid oxidation
and glycolytic enzyme content . In summary, chronic
exposure to HIIE results in significant increases in aerobic
and anaerobic fitness, increased skeletal muscle capacity for
fatty acid oxidation and glycolytic enzyme content, and
increased insulin sensitivity.
The majority of research examining HIIE has focused on
short-term (2 to 6 weeks) programs on skeletal muscle
adaptation . However, some studies have utilized longer
6 Journal of Obesity
Change in fat mass(kg)
Change in central abdominal fat(kg)
Figure 2: Subcutaneous (a) and abdominal fat loss (b) after 15 weeks of high-intensity intermittent exercise. HIIE: high-intensity
intermittent exercise, SSE: steady state exercise, Cont: control.∗Significantly different from control and SSE groups (P < .05). (Adapted
from Trapp et al. ).
programs to determine the effect of HIIE on subcutaneous
and abdominal fat loss. For example, Tremblay et al. 
compared HIIE and steady state aerobic exercise and found
that after 24 weeks subjects in the HIIE group lost more
subcutaneous fat, as measured by skin folds, compared to
a steady state exercise group when exercise volume was
taken into account (Table 1). More recently, Trapp et al. 
conducted an HIIE program for 15 weeks with three weekly
20-minute HIIE sessions in young women. HIIE consisted of
an 8-second sprint followed by 12s of low intensity cycling.
Another group of women carried out an aerobic cycling
for 40min. Results showed that women in the HIIE group
lost significantly more subcutaneous fat (2.5kg) than those
in the steady state aerobic exercise program (Figure 2(a)).
Dunn  used a similar HIIE protocol together with a
fish oil supplementation and a Mediterranean diet for 12
weeks. In 15 overweight young women, the combination
of HIIE, diet, and fish oil resulted in a 2.6kg reduction
in subcutaneous fat (8%) and a 36% increase in insulin
sensitivity (Table 1). The amount of subcutaneous fat lost
was similar to that observed in the Trapp et al.  study
suggesting that shorter HIIE interventions (12 versus 15
weeks) are also effective for reducing subcutaneous fat.
With regard to abdominal fat, Trapp et al.  found that
15 weeks of HIIE led to significantly reduced abdominal fat
(.15kg) in untrained young women (Figure 2(b)), whereas
Dunn  found that 12 weeks of HIIE led to a.12kg
decrease in abdominal fat. As women in these studies
possessed relatively low abdominal fat levels, it is possible
that the greater abdominal fat of men may demonstrate
greater reductions after HIIE. For example, Boudou et
al. , in a study involving older type 2 diabetic males,
found that after 8 weeks of HIIE no change in body mass
occurred; however, abdominal adiposity was decreased by
44% (Table 1). Mourier et al.  found a 48% reduction in
visceral fat, measured by MRI, compared to an 18% decrease
in subcutaneous fat following an exercise regimen consisting
of steady state exercise two days per week and HIIE one day a
week for 8 weeks in type 2 diabetic men and women. Tjønna
et al. examined 32 middle-aged metabolic syndrome men
and women who performed 16 weeks of HIIE three times
per week.˙VO2max increased by 26% and body weight was
reduced by 2.3kg. Whyte et al.  examined ten overweight
males aged 32 years after two weeks of HIIE consisting of 6
sessions of a 4–6 repeats of a Wingate test.˙VO2maxincreased
(8%) and significant change in waist circumference was also
has not been extensively examined, one study using DEXA
found that trunk muscle mass was significantly increased
after 15 weeks , whereas another study using MRI showed
after HIIE .
A summary of the results of studies examining the effects
of HIIE on subcutaneous and abdominal fat, body mass, and
waist circumference is illustrated in Table 1. As can be seen
studies that carried out relatively brief HIIE interventions (2
to 6 weeks) only resulted in negligible weight loss. However,
the majority of subjects in these short-term Wingate test
studies have been young adults with normal BMI and body
mass. Studies that used longer duration HIIE protocols with
individuals possessing moderate elevations in fat mass 
have resulted in greater weight/fat reduction. Interestingly,
the greatest HIIE-induced fat loss was found in two studies
as subjects [8, 40]. Given that greater fat loss to exercise
larger initial fat mass , it is feasible that HIIE will have
a greater fat reduction effect on the overweight or obese.
Thus, more studies examining the effects of HIIE on obese
or overweight individuals are needed.
Possible mechanisms underlying the HIIE-induced fat
loss effect include increased exercise and postexercise fat
oxidation and decreased postexercise appetite. As men-
tioned, Gaitanos et al.  have suggested that towards the
end of an HIIE session that consists of numerous repeat
Journal of Obesity7
sprints (e.g., ten 6-second bouts of maximal sprinting)
an inhibition of anaerobic glycogenolysis occurs and ATP
resynthesis is mainly derived from PCr degradation and
intramuscular triacylglycerol stores. That increased venous
glycerol accompanied HIIE in both trained female cyclists
and untrained women  supports the notion that acute
Burgomaster et al.  and Talanian et al.  have shown
that 6 to 7 sessions of HIIE had marked increases in whole
body and skeletal muscle capacity for fatty acid oxidation.
As mentioned previously, the EPOC or postexercise
could influence postexercise fat metabolism. Increased fat
oxidation after HIIE may also occur as a result of the need
to remove lactate and H+and to resynthesize glycogen. The
elevated GH levels documented after a bout of HIIE 
may also contribute to increased energy expenditure and fat
It is also feasible that HIIE may result in suppressed
appetite. In rats, hard exercise has been repeatedly reported
to reduce food intake . The mechanisms underlying the
anorectic effects of exercise are not known but exercise may
reduce food intake by facilitating the release of corticotropin
releasing factor (CRF) a potent anorectic peptid . It
has been shown that hard running and swimming exercise
results in elevated levels of CRF in rats [57, 58] and increases
in indirect markers of CRF in humans . Rivest and
Richard  and Kawaguchi et al.  showed that injecting
a corticotropin-releasing factor (CRF) antagonist into the
hypothalamus of rats prevented the effects of exercise on
food intake and body weight reduction suggesting that CRF
plays a major role in the anorexia caused by exercise in
rats. Bi et al.  also provided evidence to support the
importance of CRF in mediating the long-term effects of
exercise on food intake and body weight in rats. Human
studies also show a considerable decrease in subjective
hunger after intensive aerobic exercise . However, this
exercise-induced anorexia has been observed only for a
short time after hard exercise (>60%˙VO2max). Mechanisms
include the CRF peptide effect previously discussed and an
example, a 60%–70% decrease in splanchnic blood flow in
humans exercising at 70%˙VO2max has been documented
 and at maximal exercise splanchnic blood flow is
reduced by approximately 80% . In summary, there is
evidence to suggest that regular HIIE results in increased fat
oxidation during exercise; however, the effects of HIIE on
postexercise fat oxidation and appetite suppression have not
Research examining the effects of HIIE has produced
preliminary evidence to suggest that HIIE can result in
modest reductions in subcutaneous and abdominal body fat
in young normal weight and slightly overweight males and
females. Studies using overweight male and female type 2
diabetic individuals have shown greater reductions in subcu-
taneous and abdominal fat. The mechanisms underlying the
fat reduction induced by HIIE, however, are undetermined
but may include HIIE-induced fat oxidation during and after
exercise and suppressed appetite. Regular HIIE has been
shown to significantly increase both aerobic and anaerobic
fitness and HIIE also significantly lowers insulin resistance
and results in increases in skeletal muscle capacity for fatty
acid oxidation and glycolytic enzyme content.
Some important issues for future HIIE research include
optimization of type and nature of HIIE protocols, indi-
vidual fat loss response to HIIE, and suitability of HIIE for
special populations. The most utilized protocol has been the
Wingate test (30s of all-out sprint). This protocol amounts
to 3 to 4min of cycle exercise per session with each session
being typically performed 3 times a week. This protocol,
although remarkably short in duration, is extremely hard
and subjects have to tolerate significant discomfiture. Thus,
the Wingate protocol is likely to be unsuitable for most
overweight, sedentary individuals interested in losing fat.
Other less demanding HIIE protocols have included an 8-
second cycle sprint followed by 12s of low intensity cycling
for a period of 20min , a 15-second cycle sprint followed
by 15s of low intensity cycling for a period of 20min
, and a 2-minute cycle sprint followed by 3min of low
intensity cycling for a period of 20min . A challenge for
maximum health benefit. As discussed earlier, reducing the
length of HIIE training from 15 to 12 weeks still resulted in
significant subcutaneous and abdominal fat loss . Thus,
more research is needed to identify the optimal length and
intensity of the HIIE protocol for achieving varying health
With regard to modality, studies have primarily utilized
a stationary cycle ergometer, thus, little is known about
the effects of other potential HIIE modalities such as
rowing, walking, running, stair climbing, and swimming.
That insulin resistance has recently been shown to primarily
be located in leg muscle  suggests that HIIE exercise
that focuses on the legs is likely to show the greatest insulin
sensitivity increases. How leg muscle adaptations to HIIE
impact on subcutaneous and abdominal fat loss and other
health markers compared to other regional adaptations is
It is unclear if the increase in insulin sensitivity following
HIIE training is simply a response to the last exercise session
or a result of more permanent skeletal muscle adaptations.
Whyte et al.  have provided evidence to suggest that for
sensitivity was largely a result of the last HIIE session. They
assessed insulin resistance 24 hours and 72 hours after the
sixth HIIE session in a two-week training program. Insulin
sensitivity had increased by 25% at 24 hours after HIIE
but had returned to preintervention levels after 72 hours.
In contrast to these results, Babraj et al.  used a glucose
tolerance test to assess insulin sensitivity after a similar
intervention and found that insulin sensitivity was improved
until at least three days after the last exercise session. Why
these similar HIIIE protocols produced differing results
8 Journal of Obesity
differ is not clear and also whether HIIE programs lasting
longer than two weeks display a similar effect has not been
Individual variability in fat loss to HIIE and other forms
of exercise is an important issue for future research. For
example, in the intervention previously described  there
were significant individual differences in the fat loss response
to HIIE. Fat response ranged from a loss of 8kg to a gain
of .10kg. If fat loss responders alone were examined in this
study (women who lost rather than gained fat), then average
fat loss was 3.94kg. As there are likely to be responders
and nonresponders in every exercise, fat loss trial calculating
mean fat loss alone hides the significant fat loss achieved
by some individuals. Thus, it is feasible that HIIE fat loss
programs are effective for producing a clinical decrease in
fat (greater than 6% of fat mass) for some but not all
participants. Boutcher and Dunn  have highlighted a
range of program design factors and individual factors that
are behavioral, inherited, and physiological in origin that
may affect individual fat loss response to exercise. Therefore,
research is needed to identify the major individual factors
that both enhance and impede fat loss response to HIIE-
A small number of studies have examined the effects of
HIIE fat loss and health of special populations and patients.
These have included overweight adolescents , older adults
, intermittent claudication , chronic obstructive pul-
Encouragingly, these studies have shown that HIIE appears
research needs to establish the most beneficial HIIE protocol
that is both optimal and sustainable for different types of
In conclusion, regular
increases in aerobic and anaerobic fitness and brings about
significant skeletal muscle adaptations that are oxidative and
glycolytic in nature. HIIE appears to have a dramatic acute
and chronic effect on insulin sensitivity. The effects of HIIE
on subcutaneous and abdominal fat loss are promising but
more studies using overweight individuals need to be carried
out. Given that the major reason given for not exercising
is time , it is likely that the brevity of HIIE protocols
should be appealing to most individuals interested in fat
reduction. The optimal intensity and length of the sprint
and rest periods together with examination of the benefits of
other HIIE modalities need to be established.
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