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Systematic Review

Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies

[version 1; peer review: 2 approved, 1 approved with reservations]
PUBLISHED 16 Jul 2020
Author details Author details
OPEN PEER REVIEW
REVIEWER STATUS

Abstract

Background: This study aimed to systematically evaluate the available evidence on prenatal and early infancy antibiotic exposure and the association with overweight and obesity in later childhood.
Methods: We conducted a comprehensive search of Embase, MEDLINE, and Web of Science for observational studies assessing prenatal and early antibiotic exposure on the risk of overweight and obesity. We independently assessed the risk of bias using the ROBINS instrument and the overall quality of evidence using the GRADE approach.
Results: Our search identified thirteen observational studies including 554,983 participants; most studies were at moderate risk of bias. We found a statistically significant impact of early antibiotic exposure and the risk of being overweight later in childhood (OR 1.18; 95% CI 1.05 to 1.34) (very low quality evidence). We also found that early childhood antibiotic exposure was associated with the risk for childhood obesity (OR 1.14; 95% CI 1.04 to 1.24) (very low quality evidence).
Conclusions: Very low quality evidence suggests that exposure to antibiotics early in life may be associated with an increased risk of being overweight and obese in later childhood.  However, very low quality evidence raises serious questions about the plausibility of prenatal and early infancy antibiotic exposure being causally related to weight in children.
PROSPERO registration: CRD42016050011 (14/12/2016)

Keywords

Antibiotics, Early Life, Obesity, Overweight, Prenatal

Introduction

Infants and children are commonly and frequently prescribed antibiotics and up to 40% of infants are exposed either directly or through maternal intra-partum antibiotic prophylaxis1,2. In the United States it is estimated that a child receives approximately three courses of antibiotics by the age of two and 10 courses by the age of 10 years3 and, often, these courses are prescribed for viral infections thus offering no therapeutic benefit4. The concerns with significant overuse of antibiotics are increased antibiotic resistance, increased rates of adverse drug reactions, such as rashes, fungal infections, and antibiotic-associated diarrhea. The intestinal microbiota begins development in utero and resembles adult microbiota by 2.5 years of age4. Thus, external exposures to antibiotics during this phase of microbiota development may potentially impact the normal colonization pattern, and the composition of the gut microbiota, both of which may play a role in health outcomes later in life, including weight gain and obesity2,3,5.

While the paradigm has been that the infant gut is sterile at birth, increasing evidence suggests that colonization may begin in utero with bacterial colonies detected in the placenta and meconium6. Thus, it is possible that prenatal or intrapartum antibiotic exposure may potentially affect these bacterial colonies in utero. Most notably, antibiotic exposure can affect the gut microbiota composition at any age; however, infants may especially be vulnerable, as the gut microbiota is highly unstable and dynamic during this period with greater interindividual variability, compared to that of an adult79. Cho et al. and Cox et al. have shown increased fat accumulation in mice who were treated with antibiotics early in age regardless of antibiotic class1012. This corresponds with an absence of certain populations of prominent microbes, such as Lactobacillus, Allobaculum, Rikenellaceae, and Candidatus Arthromitus due to antibiotic exposure. This suggests a potential protective role of these bacteria against patient-important outcomes, such as weight gain and obesity11,12. As such, this paper aims to evaluate and summarize the available evidence on the potential impact of prenatal, intrapartum, and early childhood antibiotic exposure on the risk of overweight and obesity later in life.

Methods

This review was registered with PROSPERO - International Prospective Register of Systematic Reviews on the 14th December 2016 under the number CRD4201605001113.

Search strategy

An experienced clinical librarian (TAW) conducted a literature search in the following electronic databases from inception to June 2018: Embase (Ovid), MEDLINE (Ovid), MEDLINE In-Process & Other Non-Indexed Citations (Ovid), MEDLINE® Epub Ahead of Print (Ovid), and Web of Science. The search terms included database-controlled vocabulary and keywords for the concepts of “antibacterial agents” AND “children” AND “microbiome” OR “microbiota” AND “health outcomes” (e.g. weight, obesity, diabetes). We also supplemented the search by reviewing bibliographies of review articles and other eligible clinical studies to ensure that studies that were not identified by the search strategy were included. Clinicaltrials.gov was searched for unpublished and ongoing trials. No language or date restrictions were applied. See extended data for full search strategies14.

Study selection

Inclusion criteria. Observational and experimental study designs were eligible, including cohort, cross-sectional, and case-control studies, data modelling studies, and randomized controlled trials. Further, to be eligible, studies had to examine maternal prenatal, intrapartum, and child (birth to 18 years) exposure to antibiotics and at least one of our target health outcomes of interest: overweight (body mass index [BMI] ≥ 25 kg/m2), or obesity (BMI ≥ 30 kg/m2). The comparison group included children that were not exposed to antibiotics. Studies with both fixed and variable follow-up periods were eligible. Studies or subgroups within studies that included premature infants, infants born with low birth weight or comorbidities, such as cystic fibrosis and Crohn’s disease were excluded.

Screening, data extraction and quality assessment. Blinded to the journal of publication and results, two teams of independent reviewers (AS with either LL or HK) screened titles and abstracts of the studies to determine eligibility. Full-text articles were retrieved and assessed for further eligibility assessment. Discrepancies were resolved by discussion and, when necessary, additional input from a third reviewer (BCJ, KC).

Two reviewers (AS with either KC or HK) extracted data independently using a standardized data extraction form. The following data was extracted: study design, study setting, demographic information, antibiotic regimen (frequency and duration of exposures, type of antibiotic/class), and outcome data for each of our target outcomes including reported time points and duration of follow-up.

As no randomized trials were identified, the Risk of Bias In Non-randomized Studies (ROBINS) instrument for different types of observational studies15 was used to assess study validity. Two reviewers (AS and KC) independently evaluated each observational study included for risk of bias (RoB). The ROBINS instrument consists of seven total RoB questions, with two questions under the pre-intervention domain, one question on the intervention domain and four questions under the post-intervention domain15. Response options for each question included ‘low RoB’, ‘moderate RoB’, ‘serious RoB’, and ‘critical RoB’. We modified the instrument; wherein low and moderate risk of bias was classified as ‘low risk of bias’ and serious and critical risk of bias was classified as ‘high risk of bias’. For each study, if one or more questions was judged to be high or critical risk of bias, the overall study was deemed to be at high risk of bias15. Any disagreements regarding data extraction or risk of bias items was resolved through discussion, and, when necessary, with an experienced methodologist (BCJ).

We applied the GRADE (Grading of Recommendations, Assessment, Development, and Evaluations) approach16 for rating the overall quality of evidence for the outcomes of interest. In particular, observational studies were considered low quality evidence, but may be rated up for three reasons: (1) when a large magnitude of effects exists (e.g. OR <0.5 or >2.0), (2) when there is a dose-response gradient or (3) when all plausible confounding or other biases may be working against the observed effect16. Observational studies without these characteristics were considered low quality evidence. In addition, if studies were limited by risk of bias, inconsistency, indirectness, imprecision or publication bias the overall quality of evidence may to rated down to very low quality17. The quality of evidence for each main outcome was determined after considering each of these elements, and categorized as either high (highly confident that the true effect lies close to that of the estimate of the effect); moderate (moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different); low (confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect); very low (very little confidence in the effect estimate: the true effect is like to be substantially different from the estimate of effect)17.

Data analysis

Meta-analysis was conducted using Stata 14 (Stata Corp., College Station, TX, USA). Effect estimates and corresponding confidence intervals were extracted from eligible articles and we calculated the adjusted odds ratio (OR) with corresponding 95% confidence interval with the generic inverse variance method was used to determine relative effects18.

We conducted two primary meta-analyses, one for overweight and one for obesity with obesity including overweight. Random effects models were used due to the anticipated heterogeneity between studies. We used the DerSimonian and Laird method for estimating tau-squared and subsequent adjustment for effect size19.

Heterogeneity was assessed with the I-squared measure and the corresponding Q statistical test18. To further explore heterogeneity, we conducted a priori subgroup and sensitivity analyses20. Our three subgroups of interest included (1) the timing of exposure (prenatal versus early exposure), (2) number of antibiotic exposures (1 to 2 versus 3 or more), and (3) time point of outcome assessment (less than 7 years versus 7 years of age or more). Subgroups on timing and number of antibiotic exposures were stated a priori13, while the time point of assessment was a post-hoc subgroup chosen based on the distribution of outcome assessments among eligible studies. We also conducted a sensitivity analysis on study design limitations (risk of bias). Low and moderate risk of bias was classified as low risk of bias and serious and critical risk of bias was classified as high risk of bias.

Results

Among 12,091 articles identified, 13 studies were deemed eligible for this review (Figure 1), including 11 cohort studies2,2125,2630, one cross-sectional study31, and one nested case-control study1. Five studies analyzed the effect of prenatal exposure to antibiotics23,24,26,30,31 while the remaining eight studies evaluated the impact of early childhood (birth to 2 years) antibiotic exposure and the risk of overweight and obesity1,2,21,22,25,2729. Included studies ranged in size from 9729 to 312,702 participants25 with a total sample size among all included studies of 554,983. The median age for weight assessment was 7 years (range 2 years to 18 years). A detailed summary of all 13 study characteristics is shown in Table 1.

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure1.gif

Figure 1. PRISMA flow diagram.

Table 1. Characteristics of studies involving weight-related outcomes.

Study;
Country
Study DesignPopulationTotal
Participants
Analyzed (%
incomplete
outcome
data)
Population DescriptionDefinition
of one
antibiotic
exposure
Mother,
Infant
or Both
Exposed to
Antibiotics
Time-
point of
Antibiotic
Exposure
Weight
Outcomes
Reported
Time-point
of Outcome
Assessment
Adjusted Risk
Factors
Ajslev
et al.,
2011;
Denmark
Prospective
cohort
40,64028,354
(30.3%)
Danish
National Birth Cohort
established between
1997–2002
NoneInfants≤6 months
old
Overweight:
25≤BMI<30
7 years oldMaternal age,
SES, pre-
pregnancy BMI,
gestational weight
gain, smoking,
paternal BMI,
parity, birth
weight, sex,
breastfeeding,
and age
Azad
et al.,
2014;
Canada
Nested, case-
control
723616 (14.8%)1995 birth cohort study
(Study of Asthma, Genes
and the Environment, or
SAGE cohort)
Number of
prescriptions
Infants≤1 year oldOverweight:
BMI≥85
percentile
9 and 12
years old
Birth weight,
breastfeeding,
smoke exposure
at birth, family
income, siblings,
diet, physical
activity at age 9,
current asthma,
maternal asthma,
and maternal
weight
Bailey
et al.,
2014;
USA
Retrospective
cohort
89,05765,480
(26.5%)
Infants in primary care
practice network affiliated
with the Children’s
Hospital of Philadelphia
14-day
antibiotic
interval
InfantsAges 0–23
months
Obesity:
BMI ≥95
percentile
Ages 24–59
months
Demographic,
medical practice,
and clinical
covariates
(parental obesity,
SES, early diet
and lifestyle)
Bridgman
et al.,
2018,
Canada
Prospective
cohort
988891 (9.9%)Sub-cohort of the
Canadian Healthy Infant
Longitudinal
Development (CHILD)
study
NoneMothers2nd and 3rd
trimester of
pregnancy
Overweight:
Weight for
length z
score>97
percentile
1 yearmode of delivery,
breastfeeding
status
and infant
antibiotic
exposure
Cassidy-
Bushrow
et al.,
2018; USA
Prospective
cohort
659527 (20%)Pregnant women
who delivered from
September 2003
through December
2007, and part of
Wayne County Health,
Environment, Allergy, and
Asthma cohort (WHEALS)
NoneMothers1st, 2nd
and 3rd
trimester of
pregnancy
Overweight:
BMI ≥85
percentile
2 years oldInfant gender,
delivery mode,
ever breastfed,
maternal age,
maternal
race, maternal
education,
birthweight Z-
score, child age
at 2-year visit
and BMI at first
prenatal care visit
Li et al.,
2017, USA
Prospective
cohort
312, 702260,556
(16.7%)
Infants delivered
between 1997 to 2013
who were a part of Kaiser
Permanente Northern
California (KPNC)
member
population
Number of
prescriptions
Infants≤24 months
old
Obesity:
BMI ≥95
percentile
18 years oldInfection type
and severity
(ie, number of
infection episodes),
maternal age,
race or ethnic origin, pre-
pregnancy BMI,
preterm delivery,
low birthweight,
maternal antibiotic
use, and infection
during pregnancy
Mor et al.,
2015;
Denmark
Cross-
sectional
9,8869,886 (0%)Infants born between
1994 and 1998 in
Aalborg municipality
who underwent routine
school anthropometric
evaluation
Number of
prescriptions
Mothers1st, 2nd
and 3rd
trimester of
pregnancy
Overweight:
25≤BMI<30
Obesity:
BMI ≥30
7–16 years
old
Maternal age at
delivery, marital
status, smoking
during pregnancy
and multiple
gestations
Mueller
et al.,
2015;
USA
Prospective
cohort
727436
(40%)
Mothers enrolled in the
Northern Manhattan
Mothers and
Children Study
NoneMothers2nd and 3rd
trimester of
pregnancy
Obesity:
BMI ≥95
percentile
7 years oldOffspring sex,
ethnicity, birth
weight, maternal
age, maternal
pre-gravid BMI,
receipt of public
assistance during
pregnancy,
delivery mode and
breastfeeding
Saari
et al.,
2015;
Finland
Retrospective
cohort
14,76412,062
(18.3%)
Infants of the Finnish
growth
reference study
population born
between 2003 and
2007 who attended child
welfare
clinics in the city of
Espoo, Finland
Number of
episodes
Infants≤24 months
old
Overweight:
25≤BMI<30
2 years oldMaternal
smoking after first
trimester, parental
relationship, mode
of delivery, birth
weight, and birth
length
Scott
et al.,
2016;
United
Kingdom
Retrospective
cohort
26,86721,714
(19.2%)
Infants born between
1995
to 2013 who are part of
The Health Improvement
Network (THIN).
NoneInfants<24 months
old
Obesity:
BMI z-score
≥2.37 for
males and
≥2.25 for
females
4 years oldYear of birth,
maternal and
sibling obesity,
maternal diabetes,
mode of delivery,
country of
origin, urban
environment, and
Townsend score
Trasande
et al.,
2013;
United
Kingdom
Prospective
cohort
14,54111,532
(20.7%)
Infants part of Avon
Longitudinal Study of
Parents and Children
(ALSPAC) born between
1991–1992
NoneInfants<6 months,
6–14
months,
15–23
months
Obesity:
BMI ≥95
percentile,
BMI z score
(38mo, 7y)
Overweight:
BMI: 85 to 94
percentile
(38mo, 7y)
Overweight,
Obesity: 38
months and
years
Birth weight,
maternal parity,
race, social
class, education,
parental BMI, first
trimester smoking,
breastfeeding,
timing of
introduction of
complementary
foods, time spent
per day watching
television, in car
on weekdays
/weekends, and
dietary pattern
classifications
Ville
et al.,
2017;
USA
Prospective
cohort
9774 (23.7%)Pregnant, self-identified
Latina women seen
in prenatal clinics at
San Francisco General
Hospital (SFGH) in 2012
and 2013
NoneInfants6 monthsObesity:
BMI ≥95
percentile
2 yearsMaternal BMI,
weight for length
z-score at birth,
breastfeeding at
6 months, rapid
infant weight gain,
infant sex
Wang
et al.,
2018; USA
Prospective
cohort
43,33239,615Mothers part of Perinatal
Collaborative Project
who gave birth between
1959–1965
Antibiotic
courses
Mothers1st, 2nd
and 3rd
trimester of
pregnancy
Overweight:
BMI ≥95
percentile
Obesity:
BMI ≥95
percentile
4 years old, 7
years old
Maternal age,
race, SES,
smoking, pre-
pregnancy BMI

SES = socioeconomic status; BMI = body mass index

Risk of bias assessment

Using ROBINS instrument to independently assess risk of bias for weight-related outcomes, 11 studies were rated as ‘moderate’ for risk of bias1,2125,2730,31 and two studies were rated as ‘serious’ for risk of bias2,26. The studies rated as serious risk of bias were missing significant participant outcome data, with exclusion of participants introducing bias in the study. Despite all studies having adjusted for potential confounders, each included study was rated as moderate risk of bias for confounding. The choice and number of variables adjusted for differed considerably between studies. In some instances, the adjustment factors were theoretically not potential confounders (e.g. ethnicity), while in other instances, potential confounders such as socioeconomic status were not adjusted for. The complete risk of bias assessment for each individual study is provided in Table 2.

Table 2. ROBINS-I Risk of Bias Assessment.

StudyPre-intervention (baseline)At
intervention
Post-interventionOverall RoB
Judgement
Bias due to
Confoundinga
Bias in
Selection of
Participantsb
Bias in
Measurement
of
Interventionsc
Bias Due to
Departures
from Intended
Interventionsd
Bias
Due to
Missing
Datae
Bias in
Measurement
of Outcomesf
Bias in
Selection
of
Reported
Resultsg
Ajslev,
2011
ModerateModerateModerateLowModerateModerateModerateModerate
Azad, 2014ModerateModerateModerateModerateModerateLowModerateModerate
Bailey,
2014
ModerateLowModerateLowModerateModerateLowModerate
Bridgman,
2018
ModerateLowLowModerateLowLowLow Moderate
Cassidy-
Bushrow,
2018
ModerateModerateLowModerateLowLowLow Moderate
Li, 2017ModerateModerateLowModerateLowLowModerate Moderate
Mor, 2015ModerateModerateLowModerateModerateLow ModerateModerate
Mueller,
2015
ModerateModerateModerateModerateSeriousModerateModerateSerious
Saari, 2015ModerateModerateLowModerateModerateModerateLowModerate
Scott, 2016ModerateModerateModerateModerateModerateModerateLowModerate
Trasande,
2013
ModerateModerateModerate ModerateSeriousModerateModerateSerious
Ville, 2017ModerateModerateModerateModerateModerateLowModerateModerate
Wang,
2018
ModerateModerateLowModerateModerateModerateModerateModerate

a Confounding: One or more prognostic variables also predicts the intervention received at baseline.

b Selection Bias: When exclusion of some eligible participants is related to both intervention and outcome, there will be an association between interventions and outcome even if the effect of interest is truly null.

c Information Bias: Bias introduced by either differential or non-differential misclassification of intervention status.

d Confounding: Bias that arises when there are systematic differences between experimental intervention and comparator groups in the care provided, which represent a deviation from the intended intervention.

e Selection Bias: Bias that arises when later follow-up is missing for individuals initially included and followed (e.g. differential loss to follow-up that is affected by prognostic factors)

f Information Bias: Bias introduced by either differential or non-differential errors in measurement of outcome data. Such bias can arise when outcome assessors are aware of intervention status, if different methods are used to assess outcomes in different intervention groups, or if measurement errors are related to intervention status or effects.

g Reporting Bias: Selective reporting of results from among multiple measurements of the outcome, analyses or subgroups in a way that depends on the findings.

Overweight

We found statistically significant effect of antibiotics on the risk of being overweight (OR 1.18; 95% CI 1.05 to 1.34, 8 studies, 125,533 participants, I2=58.5%) (Figure 2). Using the estimated population risk of being overweight32, the risk difference between exposed and non-exposed was equivalent to 26 more overweight cases per 1000 children (95%CI 7 more to 47 more cases). Our GRADE assessment indicates that the overall quality of evidence for the risk of being overweight was very low due to serious indirectness (Table 3).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure2.gif

Figure 2. Overweight outcome.

Table 3. GRADE summary of findings.

Early and frequent exposure to antibiotics in children and the risk of weight gain and obesity
Patient or population: Children with early and frequent exposure to antibiotics
Settings: Children followed in observational studies, primarily prospective cohorts
Exposure: Antibiotics
Non-exposure:No antibiotics
OutcomesIllustrative comparative risks
* (95% CI)
Relative
effect
(95%
CI)
No of Participants
(studies)
Quality of the
evidence (GRADE)
Comments
Baseline
risk
Corresponding risk
Risk with
Control
Risk with
early
exposure
to
antibiotics
Risk
Difference
(95% CI)
Overweight180 per
10001
206 per
1000
(187
to 227)
26 more
overweight
or obese
cases per
1000 (7
more to 47
more)
OR 1.18
(1.05 to
1.34)
n = 125, 533 (8 studies)Very Low28
Obesity70 per
10009
79 per
1000 (73 to
85)
9 more
obese
cases per
1000 (3
more to 15
more)
OR 1.14
(1.04 to
1.24)
n = 497, 209 (8 studies)Very Low58.1012
*The basis for the assumed risk (e.g. the median control group risk across studies) is provided in footnotes. The
corresponding risk (and its 95% CI) is based on the assumed risk in the comparison group and the relative effect of the
intervention (and its 95% CI).
CI: Confidence interval; RD: Risk difference; OR: Odds ratio
GRADE Working Group grades of evidence
High quality: Further research is very unlikely to change our confidence in the estimate of effect.
Moderate quality: Further research is likely to have an important impact on our confidence in the estimate of effect and may
change the estimate.
Low quality: Further research is very likely to have an important impact on our confidence in the estimate of effect and is
likely to change the estimate.
Very low quality: We are very uncertain about the estimate.

1 This control group estimate is the risk of being overweight or obese in children 5 to 19 years old. The risk estimate comes from the WHO website (https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight).

2 Seven studies were judged to be moderate risk of bias, while one was judged to be serious risk of bias and we did not downgrade for study limitations

3 Heterogeneity among eight studies (P = 0.018, I2 = 58.5%) was considered to be moderate according to the Cochrane Handbook (2011). All studies however had over lapping 95%CIs and demonstrated the same direction of effect. We therefor chose to not downgrade.

4 With respect to indirectness issues, participants exposed were different, wherein 4 studies included infants (post-natal), while the other 4 studies included mothers (pre-natal) as the population exposed to antibiotics. In our subgroup assessment of post-natal versus pre-natal, we found no significant difference between groups indicating limited evidence to suggest that timing of exposure impacts weight (P = 0.518). With respect to timing of assessment, there were different time points among 8 studies, ranging from 1 year age to 16 years of age. Our subgroup analysis on the timing of assessment, although dichotomized due to lack of power (<7 years versus ≥ 7 years), showed no significant difference (P = 0.699). With respect to measuring weight, there were 5 different definitions (e.g. BMI from 25 to <30; BMI > 85 percentile) of documenting weight among 8 studies. We decided to downgrade for serious indirectness related to the measurement of weight, particularly because we could not conduct an appropriate subgroup analysis (i.e. conduct meta-regression or dichotomize).

5 The results are precise and we did not downgrade for imprecision.

6 Given there were fewer than 10 studies we could not assess for publication bias.

7 With respect to the size of the effect related to antibiotic exposure, all studies consistently demonstrating an odds ratio of < 2 and we did not upgrade.

8 In our subgroup assessment of those receiving 1 to 2 antibiotic exposures versus those receiving 3 or more, we found no evidence of an increased risk with an increased dose and we therefor did not rate up for dose response.

9The risk estimates come from WHO website (https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight) and the Non-Communicable Diseases (NCD) Risk Factor Collaboration (Lancet 2017 Dec 16;390(10113):2627-2642).

10 Six of eight studies were judged to be at moderate risk of bias overall risk of bias and we did not downgrade for study limitations.

11 Substantial heterogeneity among studies (P < 0.001, I2 = 76.8%). Although all studies have same direction of effect, the 95%CIs do not fully overlap. Further, our subgroup analysis did not explain the observed heterogeneity and hence we downgraded.

12 With respect to indirectness issue, participants exposed were different, wherein 5 studies included infants (post-natal), while the other 3 studies included mothers (pre-natal) as the population exposed to antibiotics. In our subgroup assessment of post-natal versus pre-natal, we found no significant difference between groups indicating limited evidence to suggest that timing of exposure impacts weight (P = 0.353). With respect to timing of assessment, there were different time points among 8 studies, ranging from 2 years age to 16 years of age. Our subgroup analysis on the timing of assessment, although dichotomized due to lack of power (< 7 years versus ≥ 7 years), showed no significant difference (P = 0.853). With respect to measuring the potential impact of antibiotics on weight, there were 3 different definitions (e.g. BMI ≥30; BMI > 95 percentile) of documenting weight among 8 studies and the reference control group was both normal weight, normal weight plus overweight, and we rated down for indirectness issues.

Obesity

We found an overall significant effect of antibiotics on the risk of obesity (OR 1.14; 95% CI 1.04 to 1.24, 7 studies, 497,209 participants, I2=76.8%) (Figure 3). Based on the estimated population risk of obesity in children14, the risk difference between those exposed and non-exposed to antibiotics was equivalent to 9 more obesity cases per 1000 (95%CI 3 more to 15 more cases). Again, our GRADE assessment indicated that the overall quality of evidence for the obesity was very low due to serious inconsistency between studies (Table 3).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure3.gif

Figure 3. Obesity outcome.

Subgroup and sensitivity analysis for overweight outcome

Prenatal vs. early infancy antibiotic exposure. For prenatal timing of exposure, the effect of antibiotics on the risk of being overweight was not statistically significant, random effects (OR = 1.13, 95% CI 0.97 to 1.32, 4 studies, 54865 participants, I2=57.5%) (Figure 4). For early infancy exposure, we found a significant effect of antibiotics on the risk of being overweight (OR = 1.26, 95% CI 1.0 to 1.57, 4 studies, 70668 participants, I2=54.8%). Our meta-regression analysis showed no significant difference between subgroups with regard to the effect of antibiotics on the risk of being overweight outcome (p =0.518).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure4.gif

Figure 4. Overweight outcome, subgroup analysis by timing of exposure.

Outcome assessment based on follow-up time point. When the risk was assessed among those less than 7 years, we found a statistically significant effect of antibiotics on this risk of being overweight (OR = 1.22, 95% CI 1.05 to 1.42, 4 studies, 30952 participants, I2=0%). When the risk was assessed at 7 years or more, the effect of antibiotics on the risk of being overweight was not statistically significant, random effects OR = 1.18, 95% CI 0.98 to 1.43, 4 studies, 94581participants, I2=78.1%) (Figure 5). Meta-regression analysis showed no significant difference between subgroups with regards to the effect of antibiotics on the risk of being overweight (p = 0.699).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure5.gif

Figure 5. Overweight outcome, subgroup analysis by time-point of outcome assessment.

Number of antibiotic exposures. For 1 to 2 antibiotic exposures, the effect of antibiotics of antimicrobials on the risk of being overweight was not statistically significant (OR = 1.30, 95% CI 0.94 to 1.81, 3 studies, 53941 participants, I2=84.1%) (Figure 6). For 3 or more antibiotic exposures, the effect of antimicrobials on the risk of being overweight was not statistically significant, random effects (OR = 1.29, 95% CI 0.92 to 1.83, 3 studies, 53941participants, I2=67.7%). Similarly, our meta-regression analysis showed no significant difference between subgroups for antibiotic exposure and overweight risk (p =0.937).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure6.gif

Figure 6. Overweight outcome, subgroup analysis by number of antibiotic exposures.

Sensitivity analysis: Risk of bias assessment. For low risk of bias studies, we found a statistically significant effect of antibiotics on the risk of being overweight (OR = 1.18, 95% CI 1.02 to 1.37, 7 studies, 110992 participants, I2=61.6%) (Figure 7).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure7.gif

Figure 7. Overweight outcome, sensitivity analysis by risk of bias.

Subgroup and sensitivity analysis for obese outcome

Prenatal vs. early antibiotic exposure. For prenatal timing of exposure, we found a significant effect of antibiotics on obesity (OR 1.32; 95% CI 1.01 to 1.73, 3 studies, 53,945 participants, I2=80.2%) and a non-significant effect for early infancy exposure (OR 1.07; 95% CI 0.99 to 1.16, 5 studies, 443,264 participants, I2=68.4%) (Figure 8). However, subgroup analysis showed no significant difference between prenatal and early infancy subgroups on the risk of obesity (p = 0.353).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure8.gif

Figure 8. Obesity outcome, subgroup analysis by timing of exposure.

Timepoint of outcome assessment. For the time point of outcome assessment (less than 7 years), the effect of antibiotics on obesity risk was not statistically significant (OR 1.14, 95% CI 0.97 to 1.35, 4 studies, 130,562 participants, I2=68.9%), while for 7 years or more, we found a significant effect (OR 1.18, 95% CI 1.01 to 1.38, 4 studies, 366,647 participants, I2=84.1%) (Figure 9). Subgroup analysis showed no significant difference between subgroups (p = 0.853).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure9.gif

Figure 9. Obesity outcome, subgroup analysis by time-point of outcome assessment.

Number of antibiotic exposures. For those with 1 to 2 antibiotic exposures, the risk of obesity was not statistically significant (OR 1.03, 95% CI 0.98 to 1.09, 4 studies, 70,199 participants, I2=0%), while for those with 3 or more exposures, we found no statistically significant effect (OR 1.29, 95% CI 0.92 to 1.83, 4 studies, 70,199 participants, I2= 76.1%) (Figure 10). Again, subgroup analysis showed no significant difference between 1 to 2 exposures and 3 or more exposures (p = 0.085). The subgroup results, however, reached statistical significance with 3 or more antibiotics having a larger associated OR for obesity later in life.

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure10.gif

Figure 10. Obesity outcome, subgroup analysis by number of antibiotic exposures.

Sensitivity analysis: Risk of Bias assessment. For low (6 studies, 48,1941 participants) versus high risk of bias (2 studies, 15268 participants) studies, we found no significant difference between subgroups (p = 0.118) (Figure 11).

2aa43a67-6c4d-4d22-8458-46d9af025e3c_figure11.gif

Figure 11. Obesity outcome, sensitivity analysis by risk of bias.

Discussion

In total, 13 observational studies with over 554,983 participants were identified that examined the association between prenatal or early childhood antibiotic exposure and the risk of weight-related outcomes. Overall, based on very low quality evidence, antibiotic exposure may be associated with overweight and obesity. The absolute risk increase between those exposed and non-exposed to antibiotics was equivalent to 26 more overweight cases per 1,000 children followed, and 9 more obesity cases per 1,000 children followed.

We specified a priori subgroup hypotheses to assess the stability of association across population and intervention variables thought to potentially modify our outcomes of interest. We found a statistically significant association between prenatal antibiotic exposure and obesity, between late antibiotic exposure (>7 years) and obesity and between higher frequency of exposures to antibiotics (3 or more scripts) and the risk of obesity. When we conducted a test of interaction for each of our subgroups, although none were statistically significant, the frequency of exposure was almost significant (p = 0.085), suggesting that higher antibiotic frequency may increase the risk of obesity later in life. This finding corresponds to our a priori hypothesis13 that with higher exposure comes a higher risk of obesity, potentially attributable to a more frequent disruption of the composition of the gut microbiota79.

The quality of evidence for each outcome was determined using the GRADE criteria17. For the primary outcome, overweight, the quality of evidence was categorized as very low because of serious indirectness related to the measurement of weight, there were 5 different definitions (e.g. BMI from 25 to <30; BMI > 85 percentile). For the obesity health outcome, the quality of evidence was categorized as very low due to indirectness (the reference control group was both normal weight and normal weight plus overweight) and inconsistency between studies (P < 0.001, I2 = 76.8%), and our subgroup analyses did not explain the observed heterogeneity. Despite authors’ adjustment for a variety of known or suspected confounders, observational studies are prone to residual confounding and after considering each of the GRADE criteria, including the possibility of rating up for magnitude of effect and for dose-response, the evidence provides only very low quality evidence.

The association between antibiotic exposure in early life and increased risk of being overweight or obese is also supported by observations from animal studies. A study conducted by Angelakis et al. (2013)33 reported an increase in fat accumulation in mice when the mice were exposed to antibiotics at the weaning stage. The underlying mechanisms may be due to an upregulation of genes involved in lipogenesis, as well as antibiotic-induced changes in gut microbiota composition resulting in increased production of short-chain fatty acid, an additional energy source which results in imbalance of energy regulation, contributing to obesity34,35. In other studies, it has been suggested that microbial colonization may begin in utero through exchange of placental bacteria from mother to fetus; thus, placental transfer of antibiotics consumed during pregnancy may enter fetal circulation resulting in microbial changes similar to that of early life antibiotic exposure10,36.

Our paper has several strengths. First, our review included a systematic literature search of three primary databases and gray literature sources and is the most comprehensive review to date including 13 studies on weight-related outcomes. Second, an a priori design was published as a written protocol and registered with PROSPERO13. We closely followed our a priori protocol, however, we did make some post-hoc analysis and presentation decisions including the addition of a subgroup on follow-up time (up to 6 years vs 7 or more years of age). Third, we independently assessed the quality of the evidence for each outcome using the GRADE approach, allowing us to document the uncertainty we have in attributing antibiotic exposure as a causal risk factor. Fourth, we quantitatively analyzed our a priori subgroups of interest including prenatal versus early infancy antibiotic exposure, and frequency of exposure to explore hypothesized effect modifiers1,22,26, suggesting based on the available data, that frequency of exposure deserves further study.

This systematic review also has two major limitations. First, the search for included articles in this review was completed in June 2018. However, the discussion (below) has been updated to include recent evidence from studies published since spring 2018. Second, there was significant heterogeneity across the included studies with respect to the number and types of antibiotics used, the timeframe between antibiotic exposure and weight assessment, and the method of weight assessment. Although we minimized confounding by using the most adjusted analyses from each study in our meta-analyses, spurious results due to residual confounding remains a plausible explanation for all associations. That is, given that all the included studies were observational in nature, there is a risk of uncontrolled confounding factors despite multivariable adjustment37.

The potential relation between early infancy antibiotic exposure and weight issues for children is a burgeoning field of investigation, with new SRMAs and a new cross-sectional study including twins published between 2018–2020. Below, we summarized the consistency and methodological aspects of recent studies including the 2 highest quality SRMAs38,39. In a SRMA conducted by Rasmussen et al. (2018)38, 13 studies were included with 8 studies included in the meta-analysis. The reasons for excluding 5 studies from the quantitative synthesis were: high risk of bias and incomplete data for use in the meta-analysis. The outcomes of interest were childhood weight, obesity and body mass index (BMI). The study concluded that antibiotic exposure in early infancy is associated with slightly higher risk of combined overweight or obesity in childhood (OR 1.11, 95% CI 1.02 to 1.20). The study also conducted a subgroup analysis involving number of exposures to antibiotics and time point of exposure. More than 1 antibiotic treatment among participants was associated with an OR of 1.24 (95% CI 1.09 to 1.43), and exposure within the first 6 months of life was associated with an OR of 1.20 (95% CI 1.04 to1.37). Similarly, Aghaali et al. (2019)39 conducted a systematic review and meta-analysis of 19 total studies. The outcomes of interest were childhood overweight or obesity, difference in childhood body mass index (BMI) and weight between the exposed group and the non-exposed group. The study found a significant association between early childhood antibiotic exposure (<2 years) and risk of childhood weight gain and obesity (OR 1.05, 95% CI 1.04 to 1.06). The study also conducted a subgroup analysis involving time point of exposure. Among infants exposed to antibiotics before 6 months of age, the odds of more weight gain was 11%, while for infants exposed to antibiotics after 6 months of age, the odds of more weight gain was 7%. However, unlike our review, Rasmussen et al. (2018)37 and Aghaali et al. (2019)39 did not conduct a test of interaction for their subgroups based on the number of exposures and timing of exposure, nor was there a study protocol available. In addition to having a publicly available protocol, our study is the only systematic review and meta-analysis to rate the quality of the evidence for each outcome using the GRADE approach, and to present data as an absolute risk difference, an approach to presenting results that has been shown to be more intuitive and helpful for decision-makers40,41.

Our findings must be interpreted in the light of a recent cross-sectional study42 of 284,211 participants that included siblings and twins in New Zealand and that analyzed prenatal and early infancy antibiotic exposure (first two years) and its association with obesity at 4 years. The study found that both prenatal and early infancy exposure to antibiotics were independently associated with obesity at 4 years in a dose dependent manner. For the child’s exposure, the OR for the association between antibiotic exposure and obesity was 1.04 (95%CI 1.03 to 1.05) among siblings and 1.05 (95%CI 1.02 to 1.09) among twins. However, fixed effect analysis of siblings (6249) and twins (522) with discordant outcomes showed no association between antibiotic exposure and obesity, with ORs of 0.95 (95%CI 0.90 to 1.00) for maternal exposure, 1.02 (95%CI 0.99 to 1.04) for early infancy exposure among all children, and 0.91 (95%CI 0.81 to 1.02) for twins’ exposure. The findings by Leong et al. (2020) indicates unmeasured confounding factors in previous studies, and supports our findings, indicating very low quality evidence for a trivial to very small increased risk of weight issues later in life.

Overall, our systematic review suggests that prenatal and early childhood antibiotic exposure is associated with an increased associated risk of overweight and obesity among children and adolescents, potentially independent of more established early determinants of obesity. Our findings suggest that the first few years of life may represent a critical window of development, where external exposures, such as antibiotics, may program metabolic pathways and obesity risk through mechanisms involving the gut microbiota; however, experimental studies are needed to establish the impact of antibiotics, particularly frequent antibiotic use, on the gut microbiota and how this may impact weight-gain in humans. Until registered protocol-driven higher quality cohort studies with explicit plans for adjustment and statistical analysis that better demonstrate an antibiotic dose-response curve or controlled clinical trials (e.g. watchful waiting for otitis media) with long-term follow-up are conducted to confirm or refute these findings, very low quality evidence raises serious questions about the plausibility of prenatal and early antibiotic exposure being causally related to weight in children.

Data availability

Underlying data

All data underlying the results are available as part of the article and no additional source data are required.

Extended data

Harvard Dataverse: Antibiotics and Weight Outcomes, Diabetes and Microbiome Literature Search. https://doi.org/10.7910/DVN/BOGPJL14

This project contains the following extended data:

  • - antibiotics_health_outcomes_search_strategy.docx (Search Strategy)

Reporting guidelines

Harvard Dataverse: PRISMA checklist for ‘Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies’ https://doi.org/10.7910/DVN/BOGPJL14

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Srivastava A, Chau K, Kwon H et al. Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies [version 1; peer review: 2 approved, 1 approved with reservations]. F1000Research 2020, 9:711 (https://doi.org/10.12688/f1000research.24553.1)
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Reviewer Report 13 Aug 2020
Xiuxia Li, School of Public Health, Lanzhou University, Lanzhou, China 
Approved
VIEWS 9
The authors conducted a systematic review and meta-analysis to evaluate evidence on the impact of prenatal, intrapartum, and early childhood antibiotic exposure and the association with the risk of overweight and obesity later in childhood. They concluded that very low-quality ... Continue reading
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Li X. Reviewer Report For: Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies [version 1; peer review: 2 approved, 1 approved with reservations]. F1000Research 2020, 9:711 (https://doi.org/10.5256/f1000research.27087.r68273)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 10 Aug 2020
Bartłomiej M. Zalewski, Department of Paediatrics, Medical University of Warsaw, Warsaw, Poland 
Maciej Kołodziej, Department of Paediatrics, Medical University of Warsaw, Warsaw, Poland 
Approved with Reservations
VIEWS 15
The reviewers' objective was to evaluate evidence on the impact of prenatal, intrapartum, and early childhood antibiotic exposure and the association with the risk of overweight and obesity later in childhood.

To answer this question, the researchers ... Continue reading
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Zalewski BM and Kołodziej M. Reviewer Report For: Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies [version 1; peer review: 2 approved, 1 approved with reservations]. F1000Research 2020, 9:711 (https://doi.org/10.5256/f1000research.27087.r67985)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 03 Aug 2020
Sepideh Soltani, Department of Nutrition, Faculty of Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran 
Approved
VIEWS 14
The manuscript by Srivastava et al. examines the association between Early and frequent exposure to antibiotics in children and the risk of obesity. This is a well-conducted thorough review and meta-analysis. The literature search and the meta-analysis appear to have ... Continue reading
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Soltani S. Reviewer Report For: Early and frequent exposure to antibiotics in children and the risk of obesity: systematic review and meta-analysis of observational studies [version 1; peer review: 2 approved, 1 approved with reservations]. F1000Research 2020, 9:711 (https://doi.org/10.5256/f1000research.27087.r68274)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.

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