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Journal of
Functional Morphology
and Kinesiology
Review
Effect of Overnight Fasted Exercise on Weight Loss
and Body Composition: A Systematic Review
and Meta-Analysis
Daniel Hackett 1, *ID and Amanda D. Hagstrom 2
1Discipline of Exercise and Sport Science, The University of Sydney, Sydney, NSW 2141, Australia
2School of Science and Technology, University New England, Armidale, NSW 2351, Australia;
ahagstro@une.edu.au
*Correspondence: daniel.hackett@sydney.edu.au
Received: 31 October 2017; Accepted: 22 November 2017; Published: 25 November 2017
Abstract:
It remains unclear whether training in fasted compared to fed states leads to greater weight
loss and whether this practice results in beneficial or detrimental changes in body composition.
We conducted a systematic review to examine the effect of overnight-fasted versus fed exercise on
weight loss and body composition. Seven electronic databases were searched using terms related to
fasting and exercise. Inclusion criteria were: randomised and non-randomised comparative studies;
published in English; included healthy adults; compared exercise following an overnight fast to
exercise in a fed state; used a standardized pre-exercise meal for the fed condition; and measured
body mass and/or body composition. A total of five studies were included involving 96 participants.
Intra-group analysis for the effect of fasted and fed aerobic exercise revealed trivial to small effect
sizes on body mass. The inter-group effect for the interventions on body mass was trivial. Intra-group
effects were small for % body fat and trivial for lean mass in females, with trivial effects also found for
the inter-groups analyses. Whilst this is the first systematic review and meta-analysis to investigate
this topic, caution is warranted when interpreting the findings due to the limited number of studies
and hence insufficient data.
Keywords: weight loss; obesity; caloric restriction; exercise training
1. Introduction
It is well acknowledged that body mass and composition are factors that can influence athletic
performance. Diet and exercise play an important role in weight loss and promoting positive changes
in body composition [
1
]. Based on the first law of thermodynamics, an imbalance between energy
intake and energy expenditure as a result of diet and/or exercise is accounted for by a gain or loss
of body mass [
2
,
3
]. However, it is generally recognised that the energy balance principle is over
simplistic for individuals wanting to lose fat mass or gain lean mass. This was demonstrated by
Longland et al. [
4
] who found higher compared to lower protein intake while following a hypocaloric
diet, combined with intense exercise, led to greater increases in lean mass and loss of fat mass.
An interesting strategy that is claimed to aid weight loss and enhance the loss of fat mass is to perform
aerobic exercise in a fasted state [
5
]. There are two types of fasting: the first type involves abstaining
from food and fluids with exception of water (i.e., water fasting), and the second type involves
abstinence from all food and fluids (i.e., dry fasting) [
6
,
7
]. Daily fasting is commonly performed and
is referred to as the ‘overnight fast’ which for most people lasts between 8 to 12 h [
8
]. Furthermore,
exercise in a fasted state may be more conveniently implemented when performed in the morning prior
to breakfast (i.e., overnight-fasted). A recent review and meta-analysis examined the effect of aerobic
exercise performed during fasted versus fed states in a total of 273 adult participants. The results of
J. Funct. Morphol. Kinesiol. 2017,2, 43; doi:10.3390/jfmk2040043 www.mdpi.com/journal/jfmk
J. Funct. Morphol. Kinesiol. 2017,2, 43 2 of 11
this previous meta-analysis indicated that aerobic exercise performed in a fasted compared to fed
state induces higher fat oxidation [
9
]. Fat oxidation refers to catabolic processes that generate energy
for bodily functions (e.g., muscle contraction and repair of body tissue) [
10
]. In non-fasted states,
aerobic exercise acutely increases fat oxidation compared to resting conditions, and following aerobic
training there is an increased capacity to oxidize fat during aerobic exercise [
11
]. Supposedly, to induce
a reduction in fat mass requires a negative fat balance which can be achieved through altering energy
intake and/or expenditure such that fat oxidation exceeds fat intake. Therefore it seems plausible that
the increased fat oxidation from performing aerobic exercise in a fasted compared to a fed state may
lead to greater weight loss via the creation of a larger negative net fat balance.
Therefore, the purpose of the current review was to employ a systematic review format to examine
the effects of overnight-fasted versus fed exercise on weight loss and body composition. Information
gathered from this review may be useful to coaches, athletes, and personal trainers when devising
exercise training programs targeting weight loss.
2. Methods
This review was conducted in accordance with the recommendations outlined in the Preferred
Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [
12
]. A search from
the earliest record (shown in parentheses for each respective database) up to and including October
2017 was carried out using the following electronic databases: Medline (1946–), SPORTDiscus (1961–),
Web of Science (1900–), Cinahl (1982–), AMED (1985v), Science Direct (1823–), and PubMed (1950–).
The search strategy employed combination of the terms “fasting” or “intermittent fasting” or “water
fasting” or “food deprivation” or “caloric restriction” or “food restriction” and “strength training”
or “weight training” or “resistance training” or “progressive training” or “progressive resistance” or
“resistance exercise” or “weight lifting” or “power training” or “power lifting” or “aerobic exercise” or
“aerobic interval training” or “aerobic interval exercise” or “endurance exercise” or “aerobic training”
or “endurance training” or “cardio training” or “high interval intensity training” or “HIIT” or “high
intensity training” or “HIT” or “high intensity exercise” or “interval training” or “interval exercise” or
“intermittent training” or “intermittent exercise”.
3. Evaluation of Articles
Titles and abstracts of retrieved articles were individually evaluated by the two authors to
assess their eligibility for review and meta-analysis according to the eligibility criteria detailed below.
Any disagreements were settled by consensus with peers in the Department of Exercise and Sport
Science, University of Sydney. Evaluation of full-text articles was required when abstracts did not
provide sufficient information to assess eligibility for inclusion. In the event that an article had missing
data or clarification of data was required, the corresponding author was contacted and the original
data was requested. In the event that data was not able to be provided, the manuscript was excluded
from review. Articles were eligible for inclusion if they met the following criteria: (1) randomized
and non-randomized comparative studies; (2) published in English; (3) included healthy adults;
(4) compared exercise following an overnight fast to exercise in a fed state; (5) used a standardized
pre-exercise meal for the fed condition; and (6) measured body mass and/or body composition.
The two authors separately and independently evaluated full-text articles and conducted data
extraction, using a standardized, predefined form. The data for the following variables were extracted:
participant characteristics (sex, age, height, body mass, and training experience), fasting/nutritional
intervention, exercise prescribed (training mode, training frequency, exercise intensity, sets, repetitions,
rest between sets), and intervention length. Shortly after extractions were performed, the authors
crosschecked the data to confirm their accuracy. Any discrepancies were resolved by mutual consent.
Risk of bias in individual studies was assessed using the Cochrane risk of bias tool [
13
]. Studies were
independently rated by the two authors and checked for selection bias, performance bias, detection
bias, attrition bias, and reporting bias. Internal (intra-rater) consistency across items was checked
J. Funct. Morphol. Kinesiol. 2017,2, 43 3 of 11
before the results were combined into a spreadsheet for discussion. Any discrepancies between ratings
were resolved by mutual consent.
4. Statistical Analysis
Data is presented as mean
±
standard deviation (SD) or 95% confidence interval (CI). All analyses
were conducted using Comprehensive Meta-Analysis version 2 software (Biostat Inc., Englewood, NJ,
USA). The level of significance was set at
α
< 0.05 and trends were declared at p= 0.05–0.10). Effect
size (ES) values were calculated as standardized mean differences (difference between mean post-test
scores divided by pooled SD) and expressed as Hedges’ gwhich corrects for parameter bias due to
small sample size [
14
]. ESs were calculated from the pooled data for both intra- and inter-groups
using a conservative random-effects model. An ES of 0.2 was considered a small effect, 0.5 a moderate
effect, and 0.8 a large effect [
15
]. Between-study variability was examined for heterogeneity, using
the I
2
statistic for quantifying inconsistency [
16
]. The heterogeneity thresholds were set at I
2
= 25%
(low), I
2
= 50% (moderate), and I
2
= 75% (high) [
16
]. A funnel plot and rank correlations between
effect estimates and their standard errors (SE), using Kendall’s
τ
statistic [
17
], were used to examine
publication bias only when a significant result (p< 0.05) was found.
5. Results
5.1. Description of Studies
The database search yielded 8135 potential studies with the addition of five studies identified from
reference lists and external sources (Figure 1). Five studies met the eligibility criteria and were included
in the systematic review and meta-analysis [
18
–
22
]. There were a total of 96 participants (60 males
and 36 females) aged 21–27 years (Table 1). Three studies included only male participants [
18
,
21
,
22
]
while the other two studies had only female participants [
19
,
20
]. The majority of participants had an
exercise background such as track and field [
20
] or regularly played sports [
18
,
21
,
22
]. Participants
for one study were described as being previously sedentary [
19
]. The exercise interventions involved
3–4 supervised sessions performed over 4–6 weeks. High intensity interval training (cycling) was
performed in one study [
19
], continuous cycling in three studies [
18
,
21
,
22
], and continuous treadmill
exercise in one study [20].
J. Funct. Morphol. Kinesiol. 2017, 2, 43 3 of 10
was checked before the results were combined into a spreadsheet for discussion. Any discrepancies
between ratings were resolved by mutual consent.
4. Statistical Analysis
Data is presented as mean ± standard deviation (SD) or 95% confidence interval (CI). All analyses
were conducted using Comprehensive Meta-Analysis version 2 software (Biostat Inc., Englewood,
NJ, USA). The level of significance was set at α < 0.05 and trends were declared at p = 0.05–0.10). Effect
size (ES) values were calculated as standardized mean differences (difference between mean post-
test scores divided by pooled SD) and expressed as Hedges’ g which corrects for parameter bias due
to small sample size [14]. ESs were calculated from the pooled data for both intra- and inter-groups
using a conservative random-effects model. An ES of 0.2 was considered a small effect, 0.5 a moderate
effect, and 0.8 a large effect [15]. Between-study variability was examined for heterogeneity, using
the I2 statistic for quantifying inconsistency [16]. The heterogeneity thresholds were set at I2 = 25%
(low), I2 = 50% (moderate), and I2 = 75% (high) [16]. A funnel plot and rank correlations between effect
estimates and their standard errors (SE), using Kendall’s τ statistic [17], were used to examine
publication bias only when a significant result (p < 0.05) was found.
5. Results
5.1. Description of Studies
The database search yielded 8135 potential studies with the addition of five studies identified
from reference lists and external sources (Figure 1). Five studies met the eligibility criteria and were
included in the systematic review and meta-analysis [18–22]. There were a total of 96 participants (60
males and 36 females) aged 21–27 years (Table 1). Three studies included only male participants [18,21,22]
while the other two studies had only female participants [19,20]. The majority of participants had an
exercise background such as track and field [20] or regularly played sports [18,21,22]. Participants for
one study were described as being previously sedentary [19]. The exercise interventions involved 3–4
supervised sessions performed over 4–6 weeks. High intensity interval training (cycling) was
performed in one study [19], continuous cycling in three studies [18,21,22], and continuous treadmill
exercise in one study [20].
Figure 1. Flow chart of study retrieval process.
Figure 1. Flow chart of study retrieval process.
J. Funct. Morphol. Kinesiol. 2017,2, 43 4 of 11
All five studies assessed changes in body mass [
18
–
22
], two studies assessed changes in body
fat [
19
,
20
], one study assessed changes in lean body mass [
19
], and one study assessed changes in
fat-free mass [
20
]. Body fat percentage was assessed via a BodPod in one study [
20
] and dual-energy
X-ray absorptiometry (DXA) in another study [
19
]. For the meta-analysis, data from lean body mass
and fat-free mass was combined as it has previously been shown not to impact results [23].
5.2. Effect on Body Mass and Composition
Table 2details the effects of fasted versus fed exercise on weight loss and body composition.
The intra-group effect sizes (ES) for fasted and fed exercise on body mass were found to be trivial to
small for the combined, male, and female analyses (ES = 0.01 to
−
0.12) and were not significant(Table 2).
The inter-group ES for the interventions on body mass for the combined, male, and female analyses
were trivial (ES = 0.02 to 0.05) with no significant difference between interventions.
Analyses on % body fat and lean mass could only be performed on females because the studies
that included males only did not include these outcome measures. The intra-group ES for fasted and
fed exercise on % body fat for females were small (ES =
−
0.10 to
−
0.12) and were not significant
(Table 2). The inter-group ES of the interventions on % body fat were trivial (ES = 0.05) and not
significant. The intra-group ES of the intervention on lean mass for females were trivial (ES = 0.01) and
were not significant (Table 2). The inter-group ES of the interventions on lean mass was also trivial
(ES = 0.04) and not significant. For all the analyses (body mass, % body fat, and lean mass) there was
no heterogeneity between studies (I2= 0%).
5.3. Risk of Bias
All included studies randomised participants into intervention groups, however it was not
identified whether they used an acceptable method of random sequence generation (Table 3). Therefore,
these five studies were rated as having an unclear risk for random sequence generation. As per the
previous item, the same ratings were given to all studies for allocation concealment. All studies were
rated as high risk for blinding of participants (performance bias) and blinding of outcome assessment
(detection bias). All studies were rated as low risk for incomplete outcome data due to no drop outs
and no missing data, and all studies were rated as low risk for selective reporting.
J. Funct. Morphol. Kinesiol. 2017,2, 43 5 of 11
Table 1. Participant and training characteristics of included studies.
Study Group Sex: M (%) Age (Year) Training Status Exercise Prescription Duration
(wk)
Frequency
(d/wk)
De Bock et al. [18]Fasted (n= 10) 100 21.2 ±0.4 Trained Cycling: 60–120 min @ 75% 70-VO2peak (supervised) 6 3
Fed (n= 10) 100 21.2 ±0.4 Trained
Gillen et al. [19]Fasted (n= 8) 0 27.0 ±9.0 Untrained Cycling: 10 ×60 s efforts @ 90% HRmax with 60 s
recovery (supervised) 6 3
Fed (n= 8) 0 27.0 ±7.0 Untrained
Schoenfeld et al. [20]Fasted (n= 10) 0 23.8 ±3.0 Trained Treadmill: 60 min @ 70% MHR (supervised) 43
Fed (n= 10) 0 21.0 ±1.7 Trained
Van Proeyen et al. [21]Fasted (n= 10) 100 21.2 ±1.0 Trained
Cycling: 2/wk = 60 min & 2/wk = 90 min. Cycling performed @
70–75% VO2max and running @ 85% VO2max (supervised) 64
Fed (n= 10) 100 21.2 ±1.0 Trained
Van Proeyen et al. [22]Fasted (n= 10) 100 23.0 ±1.1 Trained
Cycling: 2/wk = 60 min & 2/wk = 90 min performed @ ~70-VO
2
max (supervised) 64
Fed (n= 10) 100 22.1 ±0.9 Trained
Data is reported as mean
±
SD or as a range. d = days; HRmax = heart rate maximum; M = males; MHR = maximal heart rate; min = minutes; s = seconds; VO
2
max = maximal oxygen
consumption; wk = weeks.
Table 2. The effects of fasted versus fed exercise on weight loss and composition.
Study Fasted Exercise Fed Exercise Between Groups
nPre-Training Post-Training Hedges’ g95% CI nPre-Training Post-Training Hedges’ g95% CI Hedges’ g95% CI p
Body Mass (Males)
De Bock et al. [18]
10
74.3 ±2.8 74.2 ±3.0 −0.03 (0.29) −0.60 to 0.54 10 75.3 ±3.0 75.5 ±2.9 0.06 (0.29) −0.51 to 0.63 0.10 (0.43) −0.74 to 0.94 0.82
Van Proeyen et al. [21]
10
73.3 ±9.8 74.1 ±8.8 0.08 (0.29) −0.49 to 0.65 10 70.2 ±11.4 71.6 ±10.7 0.12 (0.29) −0.45 to 0.69 0.06 (0.43) −0.78 to 0.90 0.89
Van Proeyen et al. [22]
10
76.0 ±4.6 75.8 ±4.3 −0.04 (0.33) −0.61 to 0.53 10 77.6 ±3.7 76.9 ±3.4 −0.18 (0.29) −0.75 to 0.39 −0.12 (0.43) −0.96 to 0.72 0.77
Mean Effect - - - 0.01 (0.17) −0.33 to 0.33 - - 0.01 (0.17) −0.33 to 0.33 0.02 (0.20) −0.36 to 0.41 0.90
Body Mass (Females)
Gillen et al. [19] 8 79.0 ±15.0 79.0 ±15.0 0.01 (0.31) −0.62 to 0.62 8 77.0 ±12.0 77.0 ±13.0 0.01 (0.31) −0.62 to 0.62 0 (0.47) −0.93 to 0.93 1.00
Schoenfeld et al. [20]
10
62.4 ±7.8 60.8 ±7.8 −0.19 (0.29) −0.76 to 0.39 10 62.0 ±5.5 61.0 ±5.7 −0.16 (0.30) −0.73 to 0.41 0.08 (0.43) −0.76 to 0.92 0.84
Mean Effect - - - −0.10 (0.21) −0.52 to 0.32 - - −0.09 (0.21) −0.51 to 0.33 0.05 (0.32) −0.58 to 0.67 0.88
Body Mass (Combined)
Mean Effect - - - −0.04 (0.13) −0.30 to 0.22 - - −0.03 (0.13) −0.29 to 0.23 0.02 (0.20) −0.36 to 0.41 0.90
% Body Fat (Females)
Gillen et al. [19] 8 42.3 ±8.1 41.6 ±7.8 −0.08 (0.32) −0.70 to 0.54 8 40.9 ±5.8 40.1 ±5.4 −0.13 (0.32) −0.75 to 0.49 0.01 (0.47) −0.91 to 0.94 0.98
Schoenfeld et al. [20]
10
26.3 ±7.9 25.0 ±7.7 −0.15 (0.29) −0.72 to 0.42 10 24.8 ±8.4 24.1 ±8.5 −0.08 (0.29) −0.64 to 0.49 0.07 (0.43) −0.77 to 0.91 0.87
J. Funct. Morphol. Kinesiol. 2017,2, 43 6 of 11
Table 2. Cont.
Study Fasted Exercise Fed Exercise Between Groups
nPre-Training Post-Training Hedges’ g95% CI nPre-Training Post-Training Hedges’ g95% CI Hedges’ g95% CI p
Mean Effect - - - −0.12 (0.21) −0.54 to 0.30 - - −0.10 (0.21) −0.52 to 0.32 0.05 (0.32) −0.58 to 0.67 0.89
Lean Mass (Females)
Gillen et al. [19] 8 42.8 ±5.5 43.3 ±5.5 0.08 (0.32) −0.54 to 0.70 8 43.5 ±8.2 44.1 ±7.8 0.07 (0.32) −0.55 to 0.68 0.01 (0.47) −0.91 to 0.94 0.98
Schoenfeld et al. [20]
10
45.9 ±6.7 45.4 ±6.1 −0.07 (0.29) −0.64 to 0.50 10 46.3 ±3.8 46.1 ±4.3 −0.05 (0.29) −0.61 to 0.52 0.05 (0.43) −0.79 to 0.89 0.90
Mean Effect - - - 0.01 (0.21) −0.42 to 0.42 - - - 0.01 (0.21) −0.41 to 0.42 0.04 (0.32) −0.59 to 0.66 0.91
Statistical significance accepted as p≤0.05; % = Percentage; CI = Confidence interval. Data are mean ±SD. Statistical significance accepted as p≤0.05.
Table 3. Study risk of bias *.
Study
Random Sequence
Generation
(Selection Bias)
Allocation
Concealment
(Selection Bias)
Blinding Participants
and Personnel
(Performance Bias)
Blinding of Outcome
Assessment
(Detection Bias)
Incomplete
Outcome Data
(Attrition Bias)
Selective
Reporting
(Reporting Bias)
De Bock et al. [18] Unclear risk Unclear risk High risk High risk Low risk Low risk
Gillen et al. [19] Unclear risk Unclear risk High risk High risk Low risk Low risk
Schoenfeld et al. [20] Unclear risk Unclear risk High risk High risk Low risk Low risk
Van Proeyen et al. [21] Unclear risk Unclear risk High risk High risk Low risk Low risk
Van Proeyen et al. [22] Unclear risk Unclear risk High risk High risk Low risk Low risk
* Risk of bias in individual studies was assessed using the Cochrane risk of bias tool [13].
J. Funct. Morphol. Kinesiol. 2017,2, 43 7 of 11
6. Discussion
To the best of the authors’ knowledge, this is the first systematic review and meta-analysis
to investigate the effects of overnight-fasted exercise versus fed exercise on weight loss and body
composition. The data shows minimal changes in body mass and composition following aerobic
exercise interventions in both fasted and fed states. Furthermore, performing exercise in a fasted state
did not influence weight loss or changes in lean and fat mass. These findings support the notion that
weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful
caloric deficit over a period of time, rather than exercising in fasted or fed states. However, caution is
warranted when interpreting the findings due to the limited number of studies and hence insufficient
data. Hence, future well-controlled longitudinal studies are required in cohorts of healthy adults to
confirm and extend our findings.
A common rationale for undertaking fasted exercise is to increase the oxidation of fatty acids as a
source of fuel during an exercise bout, thus creating a larger negative net fat balance compared fed
exercise, translating to greater losses of body fat. However, although acute exercise in the fasted state
has been shown to result in greater fat oxidation than exercise performed in a fed state [
9
], the research
is currently equivocal as to whether or not this influences 24 h energy expenditure [
24
,
25
]. Also, there
is evidence of a differential sex effect on fat oxidation in a fasted state [
26
]. Based on findings from the
present review it seems that fasted compared to fed exercise does not increase the amount of weight
loss and fat mass loss. An explanation for the disparity between the acute studies showing increased fat
oxidation following fasted exercise and the review findings could be due to a compensatory decrease
in fat oxidation in the post-exercise period once a meal is consumed [27].
Based on the minimal weight loss found for the studies included in this review, it can be argued
that the interventions were not adequate for achieving significant weight loss. Previously it was
thought that lower intensity exercise, conducted in the “fat-burning” zone, was superior for weight
loss when compared to high intensity exercise. This theory was based on the fact that higher intensity
exercise elicits an acutely lower level of fat oxidation [
28
]. In the present review one of five studies
involved high intensity interval training (HIIT) which may have affected weight loss [
19
], whereas
the other studies involved moderate intensity continuous training (MICT). However, similar energy
expenditure over 24 h has been observed following HIIT and MICT [
29
]. Furthermore, recent systematic
reviews and meta-analyses have shown that HIIT and MICT can induce similar improvements in
body adiposity, with HIIT possibly being a more “time-efficient” exercise strategy [
30
,
31
]. Although,
as seen in the present review, when HIIT or MICT are performed on their own without any dietary
intervention, it is unlikely that clinically meaningful weight loss (>5% reduction [
32
]) in body mass
and body fat can be achieved unless performed at very high volumes [33].
Studies have shown that consumption of food prior to exercise increases the thermic effect of
the bout, thus leading to greater energy expenditure post-exercise compared to exercise in a fasted
state [
34
–
36
], therefore suggesting that fed compared to fasted exercise may be more efficacious for
weight loss. Also, an acute bout of fasted compared to fed exercise has been shown to result in a
significantly greater loss in muscle protein [
37
], which may lead to a significant loss of lean mass if this
practice is performed over week or months. However, even in the absence of exercise which is shown
to improve the net muscle–protein balance [
38
,
39
], lean mass can be preserved during short duration
fasting (<24 h) over short periods of time (
≤
8 weeks) [
40
]. This is in agreement with the findings from
this review of no differences in lean mass for females between the fasted and fed exercise conditions.
An explanation for the preservation of lean mass during short duration fasts may be due to increases in
daily protein intake so that net muscle–protein balance is maintained [
4
]. Other possible mechanisms
include increases in anabolic hormones such as growth hormone to stimulate greater muscle protein
synthesis [
41
] and increased utilization of ketone bodies for fuel, thus suppressing skeletal muscle
breakdown [42].
There are several limitations that should be taken into account when interpreting the results of
this review. Firstly, there were only five studies that met the inclusion criteria for this review. Of these
J. Funct. Morphol. Kinesiol. 2017,2, 43 8 of 11
studies, all involved body mass analysis; however only two studies (involving females) included %
body fat and lean mass analyses. Therefore, this will impact on the ability to generalize the precise
effects of overnight-fasted exercise versus fed exercise on weight loss and body composition. Secondly,
the majority of participants in the included studies were trained, therefore it is possible that their
response to the fasted versus fed exercise interventions may have been different to the untrained
participants. However, trained and untrained participants have similar metabolic responses to acute
exercise in both the fasted and fed state, with the exception of low intensity exercise [
43
]. While the
authors of this review are unaware of any research directly comparing training status in a chronic or
long-term fasting and fed model, if the response is similar to the acute response, an effect of training
status may not be apparent. Also, based on evidence of a differential sex effect on fat oxidation in a
fasted state [
26
], it did not seem prudent to combine males and females for the body mass analysis.
However, we are confident this did not confound the combined analysis as there were similar negligible
differences between the interventions for males (ES = 0.02) and females (ES = 0.05).
Findings from this review are also limited due to the heterogeneity between dietary interventions,
such as the macronutrient composition, quantity, and timing of meals between participants and
groups may have influenced the findings of this review. Three studies provided supervised meals or
take-home meal packages [
18
,
21
,
22
], however one of these diets was a hypercaloric high-fat diet [
21
]
which would, in theory, alter the body composition responses to the exercise intervention. Conversely,
one study provided a customized diet plan, tracked via a daily online food diary, aimed at eliciting
weight loss [
20
]. Three studies utilized food diaries, implemented three days per week, with no
other nutritional controls. One study provided a standardized breakfast on training days, with the
recommendation to continue with regular dietary habits for the duration of the study [
19
]. As such,
given that some studies intended to implement either a caloric surplus or deficit, and that others
intended for diets to be isocaloric and did not have strict nutritional controls in place, it is impossible
to be sure that diet has not had an influence on the outcome of this analysis. However, based on the
similar intra-group effect sizes (trivial to small) for all studies, it seems unlikely that heterogeneity
between dietary interventions influenced the findings of this review.
7. Practical Applications
Our review of a small number of studies does not support the use of fasted exercise for weight loss
and positive changes in body composition. Futhermore, our findings also suggest there is no detrimental
effect on body mass and body composition with utilizing this practice. Future research studies on this
topic should use interventions of larger exercise volumes and durations to allow significant changes in
weight loss and body composition. Also, for future-fasted versus fed exercise studies, the dietary habits
of participants need to be well controlled. This could be achieved through prescribing participants
specific diets and monitoring their compliance at regular time points throughout the intervention
(e.g., three-day weighed food record or iPhone app). Acutely, fasted exercise has been shown to
increase fat oxidation and the subsequent use of fatty acids as a fuel source, potentially inducing
improvements in insulin sensitivity which may have important implications for type 2 diabetes and
insulin-resistant patients [
44
]. Until further research on this topic is performed, it appears individuals
can participate in whichever form of exercise that they prefer in either fasted or fed states when
targeting improvements in body composition.
Author Contributions:
Daniel Hackett was involved in search strategy, study selection, data extraction, risk of
bias scoring, statistical analysis, and manuscript write up. Amanda D. Hagstrom was involved in study selection,
data extraction, risk of bias scoring, and manuscript write up.
Conflicts of Interest: The authors declare no conflict of interest.
J. Funct. Morphol. Kinesiol. 2017,2, 43 9 of 11
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