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BRIEF REPORT
A pilot feasibility study exploring the effects of a moderate time-restricted
feeding intervention on energy intake, adiposity and metabolic physiology
in free-living human subjects
Rona Antoni*, Tracey M. Robertson, M. Denise Robertson and Jonathan D. Johnston
Faculty of Health and Medical Sciences, University of Surrey, Guildford GU2 7XH, UK
(Received 26 June 2018 –Accepted 6 July 2018)
Journal of Nutritional Science (2018), vol. 7, e22, page 1 of 6 doi:10.1017/jns.2018.13
Abstract
This pilot study explored the feasibility of a moderate time-restricted feeding (TRF) intervention and its effects on adiposity and metabolism. For 10 weeks,
a free-living TRF group delayed breakfast and advanced dinner by 1·5 h each. Changes in dietary intake, adiposity and fasting biochemistry (glucose, insulin,
lipids) were compared with controls who maintained habitual feeding patterns. Thirteen participants (29 (SEM 2) kg/m
2
) completed the study. The average
daily feeding interval was successfully reduced in the TRF group (743 (SEM 32) to 517 (SEM 22) min/d; P<0·001; n7), although questionnaire responses
indicated that social eating/drinking opportunities were negatively impacted. TRF participants reduced total daily energy intake (P=0·019) despite ad libitum
food access, with accompanying reductions in adiposity (P=0·047). There were significant between-group differences in fasting glucose (P=0·008), albeit
driven primarily by an increase among controls. Larger studies can now be designed/powered, based on these novel preliminary qualitative and quantitative
data, to ascertain and maximise the long-term sustainability of TRF.
Key words: Chrononutrition: Circadian rhythms: Intermittent fasting: Metabolism: Food intake
Many aspects of mammalian metabolism exhibit daily rhythms,
driven by an integrated network of circadian clocks throughout
the body
(1)
. One consequence of metabolic rhythms is the
interaction between time of day and food intake (‘chrononutri-
tion’)
(2)
. An emerging area of chrononutrition is time-restricted
feeding (TRF), in which the daily duration of food intake is
shortened
(3)
. TRF reduces animals’body weight and improves
markers of metabolic health without altered energy consump-
tion
(4,5)
.Beneficial effects of TRF schedules on murine body
weight can occur within 8 weeks; lower fat mass and serum
cholesterol, together with improved glucose tolerance, occur
by the end of a 9-week protocol using a daily 15 h window
of food availability
(6)
.
In humans 24 h rhythms of glucose homeostasis and post-
prandial responses are well known
(7–9)
, but TRF data are
extremely limited. Most human TRF-related studies are limited
by short study duration, use of extreme temporal restriction
that is unrealistic for a long-term intervention, or change to
nocturnal energy intake as occurs during Ramadan
(10,11)
.
There is a clear need to develop human TRF research, using
protocols that reflect realistic interventions for free-living indi-
viduals. The present 10-week study therefore aimed to assess
the feasibility of a TRF protocol in reducing the food intake
window, in addition to the effect and variability in changes
in several secondary outcomes (attrition rates, changes in diet-
ary intake, body weight, adiposity and fasting cardiometabolic
risk markers). Due to the lack of comparable TRF experiments
in human subjects, this work was conducted in the first
instance as a pilot study using a controlled study design com-
paring control v. treatment groups.
Abbreviation: TRF, time-restricted feeding.
*Corresponding author: Dr Rona Antoni, email r.antoni@surrey.ac.uk
© The Author(s) 2018. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creative-
commons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is
properly cited.
JNS
JOURNAL OF NUTRITIONAL SCIENCE
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Methods
Participants
Sixteen healthy participants (BMI 20–39 kg/m
2
) aged 29–57
years were recruited. No a priori sample size was selected
with the intention that data obtained from this pilot study
would be used to inform power calculations for future work.
Participants were weight-stable (±2 kg) over the preceding 6
months and had no significant medical history. Participants
were excluded if they had travelled across more than two
time zones within the month preceding the study or if they
had participated in rotating or night shift work for more
than 6 months. The study received a favourable ethical opinion
from the University of Surrey Ethics Committee. Written,
informed consent was obtained from all participants.
Study design
The study protocol is provided in Fig. 1(a). All participants
undertook a 2-week baseline period, during which they fol-
lowed habitual sleep–wake and feed–fast cycles. The timing
and composition of energy intake were recorded in diet diaries
over the final 4 d. At the end of the baseline period, partici-
pants made an initial morning laboratory visit. Participants
abstained from alcohol and strenuous exercise for 24 h before
the visit, consumed their preceding evening meal before 20.00
hours and therefore arrived following at least a 12 h fasting per-
iod. Body weight and composition were measured by bioimpe-
dance (Tanita MC180A; Tanita Corp.). A fasting venous blood
sample was taken into K-EDTA tubes (TAG, cholesterol, insu-
lin analysis) and sodium oxalate tubes (glucose analysis).
After the initial laboratory visit, participants were assigned to
the control (n7) or TRF (n9) group, ensuring no statistical dif-
ferences in average age, BMI and body fat between groups.
Both groups undertook a 10-week intervention period at
home. The TRF group delayed first energy intake of the day
and advanced last energy intake of the day, each by 1·5h,com-
pared with their dietary patterns calculated from the baseline
diaries. The symmetrical compression of energy intake in the
TRF group was chosen to minimise possible effects of morning
v. evening differences in metabolism and thus increase
likelihood that physiological changes were due to feeding dur-
ation per se, rather than time-of-day effects. Control participants
maintained the dietary patterns reported in their baseline diaries.
Both groups were asked to maintain habitual sleep–wake pat-
terns. The timing and composition of energy intake were
recorded in diet diaries over four consecutive days on two occa-
sions during the intervention: mid-way through the intervention
period (week 5) and in the final week (week 10). At the end of
the 10th intervention week, participants made a repeat labora-
tory visit and were asked to consume the same evening meal
as they did prior to the first laboratory visit. Completing parti-
cipants were also given a questionnaire to assess their subjective
experience of following the dietary pattern (Table 1).
Dietary analysis
Participants recorded food intake in validated diet diaries
(12)
,
which included pictorial guides to aid portion size estimations
Table 1. Baseline characteristics for study completers in time-restricted
feeding (TRF) and control groups
(Mean values with their standard errors; numbers of participants)
TRF (n7) Control (n6)
TRF v. controlMean SEM Mean SEM
Age (years) 47 3 45 4 NS
Sex (n)NS
Male 1 0
Female 6 6
Weight (kg) 86·25·277·87·6NS
BMI (kg/m
2
)29·01·728·62·8
Body fat (%)* NS
All 36·02·934·63·5NS
Males 21·9 N/A N/A N/A
Females 38·41·934·63·5NS
N/A, not applicable.
* Bioimpedance.
Fig. 1. Study design and effect of time-restricted feeding (TRF) on food intake.
(a) Participants had 2-week baseline on habitual meal times and were then
split into one of two groups for a 10-week intervention period; a control
group maintained habitual meal times, whereas a TRF group restricted their
daily feeding duration by 3 h. Diet, body weight, adiposity and fasting blood
markers were assessed in the final week of the baseline and intervention per-
iods. Dietary assessment was also made during week 5 of the intervention per-
iod. (b) Average daily energy intake in both groups at baseline and 10 weeks of
the interventions. (c) and (d) Distribution of daily energy intake in each group
during assessments at (c) baseline and (d) week 10 of the intervention period.
Data are presented as means, with standard errors represented by vertical
bars. –––, Control group (n5); ---, TRF group (n7). Pvalues represent the
group x time interactions.
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when exact weights could not be provided. Diet diary analysis
was performed with Diet Plan 7 (Forestfield Software), using
the McCance and Widdowson’s composition of foods inte-
grated dataset. Generic foods in the nutritional analysis pro-
gram were used unless specific food brands were provided,
in which case nutritional composition information was manu-
ally inputted as a user added food. Recorded time of first and
last energy-containing food was used to calculate the daily eat-
ing window. To assess for changes in daily energy intake dis-
tribution, each individual participant’s daily eating window was
divided into three time periods (thirds): early, mid, and late.
Analysis of fasting blood samples
Blood plasma aliquots were stored at −20°C. Insulin was ana-
lysed using ELISA (Millipore); glucose, TAG, total cholesterol
and HDL-cholesterol were analysed using commercial kits for
the ILAB650 (Instrumentation Laboratory). LDL-cholesterol
was calculated using the Friedewald equation
(13)
. Intra-assay
CV were <5 %.
Questionnaire
An exit questionnaire was devised for the study to allow par-
ticipants to provide a subjective assessment of the intervention
and to suggest modifications to the TRF protocol which could
be used by future studies to improve compliance. Questions
included: (1) ‘How difficult did you find the intervention?’;
(2) ‘Do you think you could maintain this protocol for longer
than 10 weeks?’; (3) ‘What were your main reasons for non-
compliance?’; (4) ‘Do you think the plan made you eat differ-
ently?’; (5) ‘Do you think you might carry on with the plan in
any form?’.
Statistics
Statistical analyses were performed on data from participants
who completed the study. Data were tested for normality using
the Shapiro–Wilk test. Differences between intervention groups
at baseline were assessed using independent ttests for continuous
variables or the χ
2
test for categorical variables, with any signifi-
cant differences reported in the text. Paired ttests were used to
assess for within-group changes over the intervention period.
Other data were analysed using a two-way ANOVA, with the
period of daily eating window or laboratory visit as a repeated
measure. Where data were not normally distributed, non-
parametric tests were used to assess between-group differences
(Mann–Whitney U) and within-group changes (Wilcoxon
signed-rank test). Moreover, due to the small sample size, data
were also inspected for outliers; where outliers were observed
(adiposity) but exhibited normality using the Shapiro–Wilk test,
non-parametric tests were also used owing to their greater resili-
ence against outliers. Due to lower completion rates, intervention
week 5 dietary intake data are presented as Supplementary mater-
ial (Supplementary Fig. S1 and Table S2), but are not included in
the primary statistical analyses. We tested the hypothesis that the
TRF intervention would result in a significant group × time inter-
action. Data are presented as mean values with their standard
errors unless otherwise stated.
Results and discussion
Recruitment, attrition and feasibility of time-restricted feeding
intervention
A total of sixteen healthy and overweight individuals were ini-
tially recruited into the study. Overall, attrition rates were low.
One control subject dropped out due to faintness during
blood collection at the first clinical visit so did not commence
the study. Within the TRF group, seven of the nine partici-
pants successfully completed the 10-week intervention. One
TRF participant was lost to follow-up and so the reason for
drop out is unknown but may relate to the TRF intervention.
The second TRF participant was excluded as they had partici-
pated in another research project (resulting in changes to their
habitual diet), and therefore the reason for drop out was not
due to difficulties with adhering to the TRF intervention.
Participants in the TRF group were asked to delay and
advance the timing of their first and last energy intakes, respect-
ively, by 1·5 h, with no restrictions placed on meal frequency or
overall energy intake. This differs from recent studies in which
male participants ate three prescribed meals per d
(14,15)
.
Moreover, the symmetrical change in feeding duration differs
from asymmetrical reductions in feeding duration in which
physiological changes could result from time-of-day effects in
addition to any consequence of TRF. In our study, the total
eating window was reduced by an average of about 4·5h
(from 743 (SEM 32) and 517 (SEM 22) min/d) based on compar-
isons between 4 d dietary records kept at baseline and the final
Fig. 1. (Continued)
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week of the intervention; participants achieved their target eating
window (≥3hreductionv. baseline) on average about 2·5/4 d in
week 10. By comparison, there was minimal change in the feed-
ing window of the control group (652 (SEM 50) to 677 (SEM 41)
min/d), resulting in a significant group × time interaction
(P<0·001; two-way repeated-measures ANOVA).
Therefore, these data suggest that the TRF intervention was
achievable over a 10-week time-frame. However, many partici-
pants found it difficult to stick to the regimen every day, and
questionnaire data revealed an average difficulty score of 7/10
(1: easy; 10: extremely difficult) among TRF participants.
Some common themes emerged from the questionnaires,
with most participants reporting TRF protocol deviations
occurring due to social eating/drinking events. Other reasons
reported by one participant included illness and a late running
work meeting. Of TRF participants, 57 % (n4) felt they could
not have maintained the TRF protocol beyond 10 weeks. This
was mainly due to incompatibility with family/social life, with
one participant noting that they found sticking to the TRF
regimen relatively easy as they lived alone. Of the participants,
43 % (n3) felt they would consider continuing the protocol if
it had demonstrable health benefits or would consider con-
tinuing a more flexible protocol, e.g. Monday–Friday, earlier
dinners or later breakfasts/dinners. An important consider-
ation for future work is therefore the positioning of the
TRF window within the 24 h day, and whether the TRF win-
dow should remain constant or can vary.
Dietary intake
Consistent with other human studies
(16–18)
, the TRF interven-
tion also caused a decrease in daily energy intake despite ad libi-
tum food access, indicated by a significant group × time
interaction (Fig. 1(b)). This was corroborated by questionnaire
responses, with 57 % (n4) of participants noting a reduction in
food intake either due to reduced appetite, reduced duration of
eating opportunities and/or reduced snacking (particularly in
the evening). Overall, daily energy intake distribution was
unaffected, as there was no significant group × time interaction
at baseline (Fig. 1(c)) or intervention week 10 (Fig. 1(d)). Some
TRF participants reported eating ‘less healthily’via increased
consumption of convenience foods due to time restrictions
with food preparation (n3), whilst others consumed less alcohol
(n5), presumably due to reduced evening social opportunities,
although this did not translate into significant between-group dif-
ferences in the changes in macronutrient intakes (Supplementary
Table S1). Analysis of diet diaries from participants who com-
pleted them at baseline, intervention week 5 and intervention
week 10 indicates that changes in dietary intake were similar
throughout the intervention period (Supplementary Fig. S1 and
Table S2).
Body weight
Despite the observed changes in energy intake, body weight
was not significantly changed after either the control (from
77·8(
SEM 7·5) to 77·3(SEM 7·7) kg; P=0·114) or TRF
(from 86·2(
SEM 5·2) to 85·5(SEM 5·2) kg; P=0·374)
interventions, and this was comparable between groups (P=
0·748; two-way repeated-measures ANOVA). Similarly, there
wasnosignificant interaction between assessment group × time
for BMI (P=0·788; two-way repeated-measures ANOVA).
The control group had BMI of 28·6(
SEM 2·8) and 28·4
(SEM 2·9) kg/m
2
, whereas the TRF group had BMI of 29·0
(SEM 1·7) and 28·7(SEM 1·8) kg/m
2
(baseline v. post-intervention,
respectively).
Adiposity
In contrast to the body weight data, there was a significant
(P=0·047; Mann–Whitney Utest) effect of dietary interven-
tion on percentage body fat (Fig. 2(a)). Indeed, all members
of the TRF group exhibited lower body fat by the end of
the intervention period (Fig. 2(c)), with an average reduction
of 1·9(
SEM 0·3) percentage points after TRF. Lean body
mass was not measured, and this may explain the discrepancy
in the body weight data, whereby net body weight remained
unchanged despite reductions in adiposity and food intake.
Despite this, the consistency of the observed reduction in adi-
posity among all TRF participants suggests that these data
represent a true treatment effect; nonetheless, replication is
required given the type 1 error risk.
One purported mechanism for the health benefits of TRF is
that a higher percentage of energy is consumed during a restricted
phase of the endogenous circadian cycle. Additionally, TRF may
exert benefits by increasing the length of the daily fast
(19)
.
However, given our participants consumed significantly less
energy per d as a result of a TRF intervention (despite ad libitum
food access), this is likely to be a key driver of the observed
changes in adiposity
(16–18)
.
Fasting plasma biochemistry
Although there was no significant difference in fasting plasma
glucose between the two groups during baseline conditions, a
significant diet × group interaction was observed for the change
in fasting plasma glucose (Fig. 2(d)). This was largely driven by
elevated concentrations consistently observed among all partici-
pants in the control group at the end of the intervention period
but the reason for this change is unknown; given there were no
reported significant changes in dietary intake in the control
group, this is suggestive of changes in other unknown factor(s).
There were modest changes in other metabolic disease
risk markers, including trends in favour of a reduction in
LDL-cholesterol; however these were not significantly dif-
ferent from the control group (Fig. 2(e)–(i)). This is per-
haps unsurprising given the small, healthy cohort studied.
Nonetheless, data provide valuable pilot information that
can be used to design appropriately powered future studies
that include assessment of metabolic physiology.
Strengths, weaknesses and impact on future experimental
design
The particular strengths of the present study include its controlled
study design and modest but achievable TRF protocol. By
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contrast, many published TRF-related studies lasted for a max-
imum of 4 weeks, involved extreme temporal restriction, or
included alternate TRF/non-TRF days; others are observational
Ramadan studies in which participants changed not only their
feeding duration but timing from a diurnal to nocturnal feeding
pattern
(10,11)
. Our exit questionnaire also provides insights into
the factors affecting the acceptability of TRF and how this
could be improved, for instance by reducing the number of
TRF days per week. TRF for 5 d per week has proved efficacious
in rodents in terms of reducing adiposityand improving metabolic
markers
(6)
but the efficacy in humans remains to be established.
This pilot study did have limitations. The study was con-
ducted in a small group of both healthy and overweight
well-motivated, predominantly female, participants within a
high-income region in Surrey recruited as part of a television
documentary. This limits generalisability of the findings to
other population groups and increases the risk of type 1 and
2 errors. Other limitations include the reliance upon self-
reported energy intakes and the use of bioimpedance to assess
changes in body composition. Whilst bioimpedance is validated
against more robust anthropometrical techniques such as
dual-energy X-ray absorptiometry, it does systematically under-
estimate body fat with increasing adiposity and can be influ-
enced by hydration and hormonal status
(20)
. Lastly, whilst
participants were asked to maintain habitual activity levels, no
formal monitoring of physical activity was conducted. It is
therefore unknown whether TRF led to compensatory changes
in physical activity, which would also influence overall energy
balance. Although assessment of metabolic markers revealed
non-significant changes in plasma TAG/cholesterol concentra-
tion, our data provide valuable insight into the magnitude and
variation of expected responses, which will inform experimen-
tal design and power calculations for future experiments.
Conclusion
In conclusion, data from this 10-week pilot study provide ini-
tial evidence that a modest contraction of the eating window is
achievable within a free-living human population. Moreover,
the TRF intervention elicited favourable changes in dietary
intakes, accompanied by a reduction in adiposity. The import-
ance of this ‘unintentional’dietary modification is important in
the context of our obesogenic environment. However, partici-
pation in the study did affect social eating/drinking opportun-
ities in the evening. Larger studies are now required and, based
on our preliminary findings, should also carefully consider per-
sonal/social considerations of participants undertaking TRF
protocols to maximise compliance.
40(a) (b) (c)
P = 0·047 P = 0·001 P = 0·305
P = 0·729P = 0·904P = 0·008
P = 0·068 P = 0·104 P = 0·702
35
30
5·5
5·0
4·5
4·0
3·5
10
8
6
4
2
0
4·5
4·0
3·5
3·0
2·5
6·0
5·5
5·0
4·5
4·0
3·5
2·0
1·6
1·4
1·2
1·0
0·8
1·5
1·0
0·5
0
30
40
50
(d) (e) (f)
(g) (h) (i)
20
30
40
50
20
% Body fat
% Body fat
% Body fat
Baseline Intervention Baseline Intervention Baseline Intervention
Baseline Intervention Baseline Intervention Baseline Intervention
Baseline Intervention Baseline Intervention Baseline Intervention
Glucose (mmol/l)Total cholesterol
(mmol/l)
LDL-cholesterol
(mmol/l)
HDL-cholesterol
(mmol/l)
TAG (mmol/l)
Insulin (µU/ml)
Fig. 2. Effect of time-restricted feeding (TRF) on body fat and fasting plasma markers. After a 2-week baseline period, participants maintained habitual feeding pat-
terns or restricted their daily feeding duration by 3 h for 10 weeks, with data collected at the end of the baseline and 10-week intervention periods. (a) Average per-
centage body fat in both groups at the end of the baseline and intervention periods. (b) and (c) Percentage body fat in each individual in the (b) TRF and (c) control
groups at the end of the baseline and intervention periods. (d)–(i) Fasting plasma markers in both groups at the end of the baseline and intervention periods. –––,
Control group (n6); ---, TRF group (n7). In panels (b) and (c), data are individual values, Pvalues are within-group changes; in other panels, data are means, with
standard errors represented by vertical bars. Pvalues represent the group x time interactions.
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Supplementary material
The supplementary material for this article can be found at
https://doi.org/10.1017/jns.2018.13
Acknowledgements
The authors thank Leila Finikarides for recruitment and help
with study management and Dr Jo Sier for analysis of plasma
samples.
Financial support was received from the University of
Surrey and the British Broadcasting Corporation.
All authors conducted the experiment. R. A. analysed data
and wrote the manuscript; T. M. R. and M. D. R. revised the
manuscript; J. D. J. analysed data and wrote the manuscript.
J. D. J. has performed consultancy work for Kellogg
Marketing and Sales Company (UK) Limited, and collaborates
with the Nestlé Institute of Health Sciences.
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