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BRIEF REPORT
To swim or not to swim after eating: a randomised controlled
crossover feasibility trial [version 1; peer review: awaiting
peer review]
Sehriban Harmankaya1, Stina Öberg1, Jesper Ryg2, Marianne Vogsen3,
Jacob Rosenberg 1
1Department of Surgery, Herlev and Gentofte Hospital, University of Copenhagen, Herlev, 2730, Denmark
2Department of Geriatric Medicine, Odense University Hospital, Odense, Denmark
3Department of Oncology, Odense University Hospital, Odense, Denmark
First published: 14 Dec 2023, 12:1593
https://doi.org/10.12688/f1000research.142691.1
Latest published: 14 Dec 2023, 12:1593
https://doi.org/10.12688/f1000research.142691.1
v1
Abstract
Background
The aim was to investigate and challenge the belief that swimming
immediately after eating is dangerous and also to investigate what
potentially could happen when swimming immediately after a meal.
We wanted to explore feasibility and get background data to perform
sample size calculations and choose outcome parameters for a full-
scale randomized trial.
Methods
The trial was performed during lunch breaks at a medical writing
course in Turkey in June 2022. Participants were randomised on the
first trial day to swim 14 meters breaststroke immediately after lunch
or wait 30 minutes after eating, with crossover on the second trial day.
Main outcomes measures were mortality, and mood, discomfort, and
adverse events were assessed in participant-reported questionnaires.
Participants completed a Profile of Mood States-Adolescents
questionnaire, a visual analogue scale for discomfort, and a
questionnaire of adverse events after each swim.
Results
A total of 26 participants completed the trial. No cases of mortality,
drowning, resuscitation, side stitch, or muscle cramps were reported.
Open Peer Review
Approval Status AWAITING PEER REVIEW
Any reports and responses or comments on the
article can be found at the end of the article.
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F1000Research 2023, 12:1593 Last updated: 14 DEC 2023
Corresponding author: Jacob Rosenberg (jacob.rosenberg@regionh.dk)
Author roles: Harmankaya S: Conceptualization, Data Curation, Formal Analysis, Investigation, Methodology, Project Administration,
Writing – Original Draft Preparation; Öberg S: Conceptualization, Data Curation, Investigation, Methodology, Project Administration,
Writing – Review & Editing; Ryg J: Conceptualization, Data Curation, Investigation, Methodology, Writing – Review & Editing; Vogsen M:
Conceptualization, Data Curation, Investigation, Methodology, Writing – Review & Editing; Rosenberg J: Conceptualization, Data
Curation, Investigation, Methodology, Supervision, Writing – Review & Editing
Competing interests: No competing interests were disclosed.
Grant information: The author(s) declared that no grants were involved in supporting this work.
Copyright: © 2023 Harmankaya S et al. This is an open access article distributed under the terms of the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
How to cite this article: Harmankaya S, Öberg S, Ryg J et al. To swim or not to swim after eating: a randomised controlled crossover
feasibility trial [version 1; peer review: awaiting peer review] F1000Research 2023, 12:1593
https://doi.org/10.12688/f1000research.142691.1
First published: 14 Dec 2023, 12:1593 https://doi.org/10.12688/f1000research.142691.1
The participants reported no significant difference in their mood
states after each swim and no significant difference was found
regarding the participants’ swimming time (P = 0.53). However, more
discomfort was reported when swimming immediately instead of 30
minutes after eating (P = 0.05).
Conclusions
It seems that swimming after eating is safe since no mortality or
cramps were registered. However, a significant level of discomfort
was found when swimming immediately after eating lunch. This
feasibility trial provides background data for a future full-scale
randomised trial.
Clinicaltrials.gov registration
NCT05401396 (25/05/2022).
Keywords
Eating, swimming, cramps, crossover trial
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F1000Research 2023, 12:1593 Last updated: 14 DEC 2023
Introduction
There is a common belief that swimming immediately after having a meal is dangerous and might cause cramps that
potentially could lead to drowning.
1
Interestingly, the World Health Organization does not mention food intake before
swimming as a risk factor for drowning.
2
This narrative about the danger of swimming or playing in water immediately
after eating is regarded by many as a myth.
3
Nevertheless, many still believe it to be true. It is doubtful how long this belief
has flourished, however, in a manual for boy scouts from 1908, the author instructed the scouts to avoid bathing in deep
water right after a meal as this could cause cramps resulting in drowning.
4
According to the narrative, one should usually
wait half an hour to two hours after having a meal before swimming.
5
After a meal, postprandial hyperemia occurs with
increased blood flow to the gut resulting in less peripheral circulation.
6
This is probably one explanation to why there is a
fear of less peripheral blood circulation resulting in cramps. In contrast, synchronous swimmers with an intense exercise
program are encouraged to consume a well-balanced diet both before, during, and after the exercise.
7
A review of relevant research in the field found no direct association between food intake before swimming and
fatal outcomes such as drowning.
1
The review found a few observational studies conducted in the 1960s that assessed
the impact of eating and swimming after different time intervals. No effect on swimming performance or any case of
discomfort with nausea or cramping were reported.
1
Nonetheless, to our knowledge, no randomised study has been
conducted to assess the danger of swimming after eating. In order to provide background data for a future full scale
randomised trial enabling sample size calculations based on proper choice of outcome parameters, we chose to perform a
feasibility study. With the current feasibility study and a future full scale randomised trial, we wanted to challenge the
myth that it may be dangerous to swim immediately after eating.
Materials and methods
The trial was conducted over two days as a randomised controlled trial with a crossover design. We have reported the
study according to the Consolidated Standards of Reporting Trials (CONSORT) statement extension to randomised
crossover trials,
8,12
and the trial was registered at www.clinicaltrials.gov (NCT05401396) before initiation on 25
th
May
2022.
The study and collection of data were conducted during lunch-breaks at a research retreat at a hotel in Turkey in June 2022
with participants from Denmark.
Participants
Participants were researchers attending the writing course. They were eligible for inclusion if they were adults, could
swim, and complied with local pool regulations.
Setting
The lunch restaurant and the pool area were in close proximity. Participants received written information about the
study by e-mail one week before departure and were informed orally on the day of departure at the airport. The water
temperature and pH were constant on the study days, being 29 °C and 7.6, respectively, whereas the weather temperature
ranged from 29–32 °C. The pool was 1.4 meters deep and 7 meters wide, making the total swimming distance 14 meters.
Intervention
On the first trial day, participants were randomised to either swim immediately after lunch or 30 minutes after. On the
second day, each participant crossed over and performed the opposite of either swimming immediately or 30 minutes after
lunch. The trial course is illustrated in Figure 1. The trial intervention was swimming immediately after eating while the
control was waiting 30 minutes, hence every participant served as their own control. The participants were instructed to
have lunch as they normally would to reflect their usual behaviors. Participants who swam 30 minutes after lunch jumped
in the water 30 minutes after their last bite. Participants who swam immediately after eating were observed when walking
from the dining table to the pool to ensure that they ate until they arrived at the pool area.
Baseline vital signs of blood pressure and oxygen saturation were measured during lunch on the first trial day. Blood
pressures were measured with a blood pressure monitor (Seagull-Healthcare, model HL888HA, Taipei, Taiwan) and
oxygen saturation with a fingertip pulse oximeter (CONTEC LED CMS50M Pulse Oximeter, China). Participants were
instructed to walk and slowly enter the pool by the ladder and swim breaststroke briskly with their head over the water.
Up to three participants swam simultaneously with the presence of lifeguards and observers in case of any incidents. The
time to complete the swimming distance was noted on all participants. Immediately after swimming, the participants
filled in three questionnaires: the validated Profile of Mood States –Adolescents (POMS-A) questionnaire,
9,10
a visual
analogue scale (VAS) about discomfort, and a self-reported questionnaire on adverse advents.
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User involvement
The trial was designed and conducted in collaboration with a professional lifeguard, an experienced swimmer, and a
restaurant chef to reflect user perspectives. Each user reviewed the study protocol and provided valuable insight and
feedback to the research team. The lifeguard provided information on the impact of swimming distance and mentioned
that during swimming competitions, he regularly experienced parents forcing their children not to swim immediately after
eating. The experienced swimmer focused on the importance of swimming style, which had to be identical on both days.
He also stressed the importance of swimming at the same timepoint as performance might vary throughout the day. The
previous Michelin restaurant chef expressed no concerns about the safety of the trial, but he experienced many restaurant
guests being tired and inflated after eating a meal. Therefore, he worried that the participants might lack amusement and
attention when swimming immediately after eating.
Outcomes
As this was a feasibility trial, we arbitrarily chose the primary outcome to be mortality after swimming. Secondary
outcomes included differences in POMS-A, discomfort, adverse events, pulse rate, and swimming time. The adjectives in
the POMS-A questionnaire were translated to Danish using back- and forward translation by two authors and face-
validation was performed on two different laymen. The POMS-A questionnaire evaluates mood by focusing on different
categories involving tension, depression, anger, fatigue, confusion, and vigour with adjectives in each category on a five-
point scale from one to five. The scores for each category were calculated by adding the raw scores from the adjectives
for the specific category. A high vigour score and low scores on the other five categories indicate a person with a
more stable and positive mood profile. The VAS was face-validated and measured current level of any discomfort,
ranging from no discomfort to the worse imaginable discomfort (0–100 mm). Furthermore, a questionnaire of adverse
events was developed within the author group for the participants to self-report (see Extended data
12
). Face-validation
was performed to ensure the correct interpretation of the questions. The questionnaire assessed adverse events such as
drowning, resuscitation, experience of any gastrointestinal symptoms, side stitch, or muscle cramps both during and
immediately after swimming. Since all participants served as their own control, secondary outcomes were assessed as the
difference between outcomes on the two study days, i.e., swimming immediately versus 30 minutes after eating.
Randomisation
Randomisation of each participant was performed by the same investigator by throwing dice at the dining table after each
participant had selected their lunch. Participants with an even dice number were randomised to swim immediately after
lunch the first day, while participants with an uneven dice number were randomised to wait 30 minutes.
Statistical methods
Convenience sampling was used, hence, participants were included based on availability, time, and interest. The
statistical analyses were performed in SPSS Statistics 28 (IBM). Continuous data were assessed for normal distribution
by visual inspection of histograms and QQ-plots. Since data were not normally distributed, continuous data are presented
as median, interquartile range (IQR), and range and were analysed with the Wilcoxon signed-rank test. Categorical data
are presented as numbers and percentages and were analyzed with the Fisher’s exact test since expected values were <5 in
at least one cell. POMS-A is an ordinal categorical scale, however, it was handled as continuous data in our statistical
analyses. We considered statistical significance when P ≤0.05.
Figure 1. Illustration of each step of the first and second day of the trial.
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Ethical approval
According to Danish legislation, the Regional Ethics Committee in Copenhagen waived ethical approval (journal number
22020290). Written consent was obtained from every participant before participation, and data were stored in a secure
database.
Results
A total of 31 medical researchers attended the writing course, and 27 participants were eligible for inclusion. A flowchart
of the inclusion process, participation, and randomisation is illustrated in Figure 2. Unfortunately, one participant
dropped out since she was dropped in the pool by some of her colleagues after being randomised and having lunch on the
first day. Consequently, 26 participants were included and completed the trial as intended with 100% complete data. The
baseline characteristics of the study participants are illustrated in Table 1.
Figure 2. A flow diagram depicting the number of participants who were randomly assigned to 0 minutes or
30 minutes the first trial day, who received the allocated sequence the second trial day, and who were
analysed for the outcomes. n: number.
Table 1. Participant characteristics and vital signs at baseline.
Participant characteristics Participant, n = 26
Sex, n (%)
Male 10 (38)
Female 16 (62)
Age in years, median (IQR) 31 (27–38)
Vital signs at baseline Day 1 Participant, n = 27
Saturation in %, median (IQR) 98 (98–98)
Blood pressure in mmHg, median (IQR)
Diastolic 84 (79–91)
Systolic 126 (116–123)
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Adverse events
No cases of mortality were registered. Similarly, no adverse events of drowning, resuscitation, and experience of any side
stitch or muscle cramps were found. Two participants reported gastrointestinal symptoms while swimming 30 minutes
after lunch: one participant experienced flatulence and the other participant reported a bloated feeling. However, there
was no statistically significant difference in gastrointestinal symptoms when comparing the intervention and control
(P = 0.41).
Visual Analogue Scale of discomfort
Participants reported their level of discomfort with a median VAS of 2 (scale 0–100) both after swimming immediately
(range 0–34, IQR: 0–4) and after 30 minutes (range 0–4, IQR: 0–3) after lunch (Table 2). However, when swimming
30 minutes after lunch, the participants reported significantly less discomfort compared with swimming immediately
after lunch (P = 0.05).
Profile of Mood States –Adolescents
The participants’mood profile was assessed for the six mood categories of tension, depression, anger, fatigue, confusion,
and vigour. After calculating the participants’mood score when swimming immediately and 30 minutes after lunch for
each category, we found no significant difference (Table 2). Overall, the participants’mood weighted to be more positive
than negative with a high vigour score and low scores in the negative mood categories. The reported median vigour score
was 6 when swimming immediately and 7 when swimming 30 minutes after lunch (P = 0.23). Similarly, no significant
difference was found among the negative mood categories of tension, depression, anger, fatigue, and confusion (Table 2).
Swimming time and pulse rate
We found no significant difference (P = 0.53) when comparing the participants’performance in swimming time when
they swam immediately or 30 minutes after lunch (Table 2). The observed median swimming time was 17 seconds (IQR:
15–20) and 16 seconds (IQR: 14–20) when swimming immediately and half an hour after eating, respectively (P = 0.53).
Furthermore, the participants’pulse rate was measured after each swim and no significant difference was found (P = 0.99)
with a median of 82 beats per minute on both days.
Discussion
This trial showed that swimming 14 meters in a pool with limited depth immediately or half an hour after lunch did not
result in any deaths or other serious adverse events. Furthermore, the overall result of the trial did not find differences in
the mood categories after the two swimming regimens. The only disadvantage when swimming immediately after lunch
was a higher level of discomfort compared with swimming half an hour after.
There are some important limitations to this trial. Since the participants were all from Denmark where the population
rarely swim in outdoor pools, some level of adaptation might have occurred on the second trial day. Nevertheless,
an equal number of participants were randomly distributed to undergo the intervention or comparison swim during day 1
Table 2. Overall reported results of swimming immediately and 30 minutes after having lunch. Swimming time
is reported in seconds, pulse is reported per minute, and level of discomfort is reported in millimeters based on a
visual analogue scale. IQR: interquartile range; VAS: visual analogue scale; P: p-value.
Outcomes Intervention
Swimming immediately
after lunch
Control
Swimming 30 minutes
after lunch
Difference
Intervention-Control
P
Reported with median (IQR, range)
Pulse rate 82 (71–89, 61–150) 82 (72–92, 51–129) 0 (-7–7, -29–64) 0.99
Discomfort (VAS) 2 (0–4, 0–34) 2 (0–3, 0–4) 0 (0–3, -4–32) 0.05
Swimming time 17 (15–20, 10–31) 16 (14–20, 13–24) 0 (-1–2, -4–9) 0.53
Profile of Mood States–Adolescents, median (range)
Anger 0 (0–5) 0 (0–8) 0 (-3–2) 0.85
Confusion 0 (0–5) 0 (0–7) 0 (-3–5) 0.30
Depression 0 (0–3) 0 (0–8) 0 (-5–1) 0.70
Fatique 1 (0–7) 1 (0–5) 0 (-2–3) 0.09
Tension 0 (0–1) 0 (0–8) 0 (-8–1) 0.68
Vigour 6 (0–15) 7 (2–16) -1 (-7–5) 0.23
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and 2. A further limitation is that it remains unknown whether the participants had lunch as they usually would since they
were in a hotel with plenty of food, however, no participants informed the investigators that they overate or were on a diet.
A strength of the trial was that we required the participants to follow the local pool regulations. Another strength is that the
design of the trial minimised recall bias since the participants filled the questionnaires immediately after each swim.
Furthermore, the POMS-A, VAS, and questionnaire of adverse events were face-validated thoroughly, which avoided
any potential misinterpretations and thereby optimised the assessment. Another strength is the user involvement in the
design phase of the trial, thus reflecting user perspectives of swimming after eating.
A recent study found that the time-of-day (morning or evening) can influence the swimming performance in an
adolescent population with swimming experience, however, with variations depending on the swimming distance and
sex.
11
The importance of varying performance depending on the time-of-day was also stressed by the experienced
swimmer involved in the design phase and we minimised this time-of-day variance in our study since the participants
swam during the lunch breaks on both days. The same study also demonstrated the importance of sleep quality as a factor
influencing participants’level of anxiety and depression, consequently affecting their performance in swimming.
11
Since
our study investigated an adult population during a course with an intense scientific program, it is possible that the
participants worked from early mornings til late nights, which could have affected their swimming performance because
of less sleep.
Since this study found that swimming directly after a meal was not dangerous, we judge it to be safe to perform a full-scale
randomised trial with swimming directly after eating as long as routine safety instructions are followed. However,
swimmers may expect a higher level of discomfort when swimming directly after a meal, and this parameter seems to be
the obvious primary outcome for a future randomised trial. Our data can be used as background data for sample size
calculations for such a full-scale trial, which may employ different swimming distances and/or swimming in sea water
rather than in a pool.
Conclusions
In conclusion, we found that swimming immediately after eating seems to be unhazardous and safe but may in some be
associated with discomfort. The present feasibility trial, being the first randomised trial on the subject, will form basis for
future large scale conclusive trials.
Data availability
Underlying data
Zenodo: To swim or not to swim after eating: a randomized controlled crossover feasibility trial. https://doi.org/10.5281/
zenodo.10074765.
12
This project contains the following underlying data:
-Data overview.csv
-Description of data overview.pdf
-Adverse events questionnaire.pdf
Reporting guidelines
Zenodo: CONSORT checklist for ‘To swim or not to swim after eating: a randomized controlled crossover feasibility
trial’.https://doi.org/10.5281/zenodo.10074765.
12
Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).
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