Probiotic supplementation during pregnancy or infancy for the prevention of asthma and wheeze: systematic review and meta-analysis.

OBJECTIVE: To evaluate the association of probiotic supplementation during pregnancy or infancy with childhood asthma and wheeze.
DESIGN: Systematic review and meta-analysis of randomised controlled trials. DATA SOURCES: Medline, Embase, and Central (Cochrane Library) databases from inception to August 2013, plus the World Health Organization's international clinical trials registry platform and relevant conference proceedings for the preceding five years. Included trials and relevant reviews were forward searched in Web of Science. REVIEW
METHODS: Two reviewers independently identified randomised controlled trials evaluating probiotics administered to mothers during pregnancy or to infants during the first year of life. The primary outcome was doctor diagnosed asthma; secondary outcomes included wheeze and lower respiratory tract infection.
RESULTS: We identified 20 eligible trials including 4866 children. Trials were heterogeneous in the type and duration of probiotic supplementation, and duration of follow-up. Only five trials conducted follow-up beyond participants' age of 6 years (median 24 months), and none were powered to detect asthma as the primary outcome. The overall rate of doctor diagnosed asthma was 10.7%; overall rates of incident wheeze and lower respiratory tract infection were 33.3% and 13.9%, respectively. Among 3257 infants enrolled in nine trials contributing asthma data, the risk ratio of doctor diagnosed asthma in participants randomised to receive probiotics was 0.99 (95% confidence interval 0.81 to 1.21, I(2)=0%). The risk ratio of incident wheeze was 0.97 (0.87 to 1.09, I(2)=0%, 9 trials, 1949 infants). Among 1364 infants enrolled in six trials, the risk ratio of lower respiratory tract infection after probiotic supplementation was 1.26 (0.99 to 1.61, I(2)=0%). We adjudicated most trials to be of high (ten trials) or unclear (nine trials) risk of bias, mainly due to attrition.
CONCLUSIONS: We found no evidence to support a protective association between perinatal use of probiotics and doctor diagnosed asthma or childhood wheeze. Randomised controlled trials to date have not yielded sufficient evidence to recommend probiotics for the primary prevention of these disorders. Extended follow-up of existing trials, along with further clinical and basic research, are needed to accurately define the role of probiotics in the prevention of childhood asthma. SYSTEMATIC REVIEW REGISTRATION: PROSPERO (CRD42013004385).

Introduction

Over the past half century, there has been a sharp rise in the global prevalence of asthma, particularly in children.1 About 300 million people worldwide are estimated to have asthma, and the prevalence has been increasing by 50% every decade.1 As the most common chronic disease of childhood, asthma affects roughly one in five children in the United Kingdom and United States,2 and is the leading cause of school absenteeism.3 The total annual cost of asthma to society has been estimated at €19bn (£16.2bn; $26.2bn) in Europe4 and $56bn in the US.5 Recurrent wheeze frequently precedes the diagnosis of asthma, and is estimated to occur in more than 20% of infants.6 7

The microflora hypothesis of allergic disease has been proposed to explain the rising incidence of asthma and other allergic disorders.8 Commensal gut bacteria stimulate development of the neonatal immune system; therefore, disruption of the gut microbiota during early life may contribute to immune disorders later in childhood.9 Indeed, prospective studies have shown that perturbation of the infant gut microbiota precedes development of atopic dermatitis (allergic eczema),10 11 12 which is widely regarded as the first step in the progressive “atopic march” towards allergic rhinitis and asthma.13 Moreover, early life factors that disrupt the gut microbiota (such as caesarean delivery, lack of breastfeeding, and use of antibiotics) increase the risk of asthma.14 15 16

In the light of this evidence, probiotics—live micro-organisms that, when administered in adequate amounts, confer a health benefit on the host—have been proposed for the prevention and treatment of allergic disorders including asthma.17 18 Although naturally present in fermented foods, probiotics are increasingly being produced and administered as supplements in preventative and therapeutic medicine.19 A recent meta-analysis of 14 randomised controlled trials showed that probiotic supplementation during pregnancy or infancy decreased the incidence of atopic dermatitis by 21%.20 Less clinical evidence exists for probiotics in the prevention of wheeze or asthma,21 but animal studies have shown that perinatal use of probiotics can prevent airway inflammation and hyper-reactivity.22 23 A 2007 Cochrane review of early life probiotics for prevention of allergic diseases reported no benefit for asthma prevention after probiotic supplementation,24 based on findings from three trials25 26 27 enrolling a total of 617 infants. All three existing trials have since published extended follow-up results,28 29 30 31 and six additional trials enrolling 2308 infants have published new findings on probiotics for asthma prevention.21 32 33 34 35 36 Prevention of related outcomes (wheeze or lower respiratory infection) was not covered in the 2007 Cochrane review.

The purpose of this systematic review was to identify, critically appraise, and meta-analyse data from prospective randomised trials evaluating the use of probiotic supplements for the primary prevention of asthma or childhood wheeze.

Methods

Using an a priori published protocol,37 we conducted our systematic review using methodological approaches outlined in the Cochrane Handbook for Systematic Reviewers38 and reported according to the criteria of preferred reporting items for systematic reviews and meta-analyses (PRISMA).39 A technical panel of experts from multiple fields (nutrition, paediatric asthma, research methodology) formulated the review questions, reviewed the search strategies and review methods, and provided input throughout the review process.

Populations, interventions, comparators, outcome measures, settings, and study designs

We included only randomised controlled trials of pregnant women or healthy infants under 1 year of age (web appendix, table S1). Our primary research question was “In healthy infants under 1 year of age, are probiotics (administered prenatally or postnatally) safe and protective against the development of wheeze or asthma, compared to placebo, or to no intervention?” The main outcome measure was doctor diagnosed asthma. Secondary outcomes included wheeze (incident or recurrent), asthma drug use, hospital admission for asthma, and the asthma predictive index score.40 Lower respiratory tract infections were also included as a secondary outcome because these infections generally involve wheeze and could predispose for asthma.41 Safety outcomes included gastrointestinal disturbances, allergic reaction to probiotics, and withdrawal due to perceived side effects. Table S2 presents inclusion and exclusion criteria (web appendix).

Search strategy for identification of studies

We searched Medline, Embase, and Central (Cochrane Library) databases from inception to August 2013 for relevant citations of published trials, using individualised search strategies for each database. Table S3 presents the Medline strategy (web appendix). We searched the World Health Organization’s international clinical trials registry platform and relevant conference proceedings for the preceding five years to identify relevant planned, ongoing, or recently completed but unpublished trials (American Thoracic Society; American Academy of Allergy, Asthma, and Immunology; European Respiratory Society; European Academy of Allergy and Clinical Immunology; and the American Association for Parenteral and Enteral Nutrition). We performed forward searches of included trials and relevant reviews in the Web of Science to identify additional citations, and contacted study authors to request relevant unpublished data. Reference lists of narrative and systematic reviews and of the included trials were searched for additional citations. We performed reference management in EndNote X6 (Thomson Reuters).

Study selection

We used a two stage process for study screening and selection using standardised and piloted screening forms. Firstly, two reviewers (MBA and JGC) independently screened the titles and abstracts of search results to determine whether each citation met the inclusion criteria. Second, the full text versions of potentially relevant citations were reviewed independently with reference to the predetermined inclusion and exclusion criteria (web appendix, table S2). Discrepancies between the two reviewers were resolved through consensus by discussion with a third reviewer (AMA-S), as required.

Data abstraction and management

Two reviewers (MBA and JGC) independently extracted data from included trials, using standardised and piloted data extraction forms. Discrepancies between the two reviewers were resolved through consensus in discussion with a third reviewer (AMA-S), as required. Extracted data included funding sources, demographics of the enrolled mothers and infants (family history of allergic disease, mode of delivery, infant sex, gestational age or age at enrolment, and breastfeeding), details of probiotic intervention (organism, daily dose, timing, duration, and method of administration), and relevant outcomes as described above. Since the nature of asthma and wheeze can change over time,42 outcome data were extracted for predefined time periods (age <3 years, 3 to <6 years, and ≥6 years). When a trial reported results for more than one time period, results from the longest follow-up were included in the main meta-analysis; results from earlier time periods were included in subgroup analyses. Data management was performed using Microsoft Excel 2007.

Assessment of methodological quality and potential risk of bias

We evaluated internal validity using the Cochrane Collaboration’s risk of bias tool,43 which assesses randomisation and allocation of participants; blinding of participants, personnel, and outcome assessors; incomplete or selective reporting; and other relevant sources of bias. If trial methodology was unclear from the published report, authors were contacted for clarification.

Measures of treatment effect

We analysed data from the included trials using Review Manager (RevMan 5.2, the Cochrane Collaboration).44 A formal meta-analysis was conducted if the data were statistically and clinically homogeneous. Pooled dichotomous data were expressed as a risk ratio, or Peto odds ratio in the event of rare outcomes.45 A risk ratio or odds ratio less than one suggests a lower rate of the outcome (for example, asthma) among participants randomised to receive probiotics than among the control group. We used the random effects model for all analyses, with the exception of the Peto odds ratio (fixed effect model). Statistical heterogeneity was explored and quantified using the I2 statistic.46 All tests of statistical inference reflect a two sided α value of 0.05.

Subgroup analyses and meta-regression

We performed subgroup analyses to determine summary effect estimates in several prespecified groups, including: the participant receiving the intervention (mother or infant), duration and timing of intervention (prenatal or postnatal), probiotic organism and dose, duration of follow-up or age at assessment, asthma risk, caesarean delivery rate, geographical area, and source of funding.37 We conducted meta-regression to evaluate differences in effect according to duration of follow-up as a continuous variable, using Comprehensive Meta Analysis (Biostat).47

Results

Trial characteristics and study populations

Of 3011 citations identified from electronic and hand searches, we included 20 unique randomised trials enrolling a total of 4866 infants (fig 1, table 1). These trials were represented by 20 primary articles,21 25 26 27 32 33 34 35 36 48 49 50 51 52 53 54 55 56 57 58 four companion articles,59 60 61 62 10 extended follow-up publications28 29 30 31 63 64 65 66 67 68 plus one forthcoming report, and eight duplicate conference abstracts.69 70 71 72 73 74 75 76 All were double blind, placebo controlled trials published in peer reviewed journals between 2001 and 2013. Sixteen trials were conducted in Europe, while four trials were conducted in Australia,27 49 New Zealand,36 and Taiwan.34 Based on family history or existing allergic disease in the mother or infant (definitions provided in web appendix, table S4), 14 of 20 trials enrolled participants at high risk for asthma; the remaining six trials21 35 50 53 56 57 were conducted in unselected populations. The caesarean delivery rate in study populations ranged from 0% to 45%, and was not reported in six trials.21 26 34 48 51 52 Most trials (14 of 20) did not restrict infant feeding practices, although six trials50 53 54 56 57 58 required exclusive formula feeding for enrolment. Nearly all trials (19 of 20) reported some degree of industry support (funding, salary support, or supplied products), including seven trials involving authors employed by the industry sponsor.32 33 50 53 56 57 58

Fig 1 Study flow diagram, following PRISMA criteria with modifications39

Table 1

 Characteristics of included trials and study populations

Primary article	Companion articles*and unpublished data	Country, enrolment period	Age at lastfollow-up	Number of participants†	Infantasthma risk	FeedingRestrictions	Caesareandelivery rate (%)	Relevant outcomes included‡
								Asthma	Wheeze	LRTI
West 200935	West 2008,61 West 201367	Sweden, 2000-03	8 years	179 infants	Unselected	None	0	Yes	Yes (I)	—
Abrahamsson 200725	Abrahamsson 201328	Sweden, 2001-03	7 years	239 mothers (209 infants)	High (family history)	None	13	Yes	Yes (I)	—
Kalliomaki 200126	Kalliomaki 2003,63 Kalliomaki 200730	Finland, 1997-98	7 years	159 mothers (155 infants)	High (family history)	None	Not reported	Yes	—	—
Niers 200933	Gorissen (unpublished data),** data provided	Netherlands, 2004-05	6 years	156 mothers (123 infants)	High (family history)	None	10	Yes	Yes (I)	—
Wickens 200836	Wickens 2012,66 Wickens 201368	New Zealand, 2004-05	6 years	512 mothers (474 infants)	High (family history)	None	32	Yes	Yes (I)	—
Kukkonen 200732	Kuitunen 2009,64 data provided	Finland, 2000-03	5 years	1223 mothers (1018 infants)	High (family history)	None	17	Yes	—	—
Taylor 200727	Prescott 2008,31 Jensen 201229	Australia, 2002-05	5 years	226 mothers (218 infants)	High (family history)	None	44	Yes	Yes (I, R)	Yes
Gore 201251	None	UK, 2002-04	3 years	137 infants§	High (eczema)	FF required, BF permitted	Not reported	Yes¶	Yes (I)	—
Ou 201234	None	Taiwan, 2002-06	3 years	191 mothers (151 infants)	High (family history)	None	Not reported	Yes	—	—
Dotterud 201021	None	Norway, 2003-05	2 years	415 mothers (363 infants)	Unselected	BF required	Not reported	Yes	—	—
Kopp 200855	None	Germany, 2002-04	2 years	105 mothers (104 infants)	High (family history)	None	19	—	Yes (R)	—
van der Aa 201158	van der Aa 201060	Netherlands, 2005-07	18 months	90 infants	High (eczema)	Exclusive FF at enrolment	18	—	Yes (R)	—
Hol 200854	Data provided	Netherlands, 2004-07	16 months	119 infants	High (milk allergy)	Exclusive FF at enrolment	19	—	Yes (I)	—
Boyle 201149	None	Australia, 2006-08	12 months	250 mothers (250 infants)	High (family history)	None	27	—	Yes (I)	—
Maldonado 201256	None	Spain, 2008-09	12 months	215 infants	Unselected	Exclusive FF at enrolment	45	—	—	Yes
Chouraqui 200850	None	France, 2004-05	12 months	284 infants	Unselected	Exclusive FF at enrolment	17	—	—	Yes
Gruber 200752	None	Germany, not reported	10 months	106 infants	High (eczema)	None	Not reported	—	—	Yes
Allen 201048	None	UK, not reported (2004)	6 months	454 mothers (454 infants)	High (family history)	None	Not reported	—	—	Yes
Hascoet 201153	None	France, not reported (2006)	4 months	79 infants§	Unselected	Exclusive FF at enrolment	8	—	—	Yes
Puccio 200757	Data provided	Italy, not reported	4 months	138 infants	Unselected	Exclusive FF at enrolment	40	—	Yes (I)	—

Trials are listed in order of decreasing duration of follow-up.

BF=breastfeeding; FF=formula feeding; LRTI=lower respiratory tract infection.

*Excludes conference abstracts from which no unique data were extracted.

†Indicates mothers or infants randomised; where recruitment occurred prenatally, includes number of infants eligible at birth (in brackets).

‡I=incident wheeze; R=recurrent wheeze.

§Trial included additional groups (for example, observational) not considered in this review.

¶This trial reported asthma outcomes in the text only, and the authors declined to provide data for meta-analysis.

**Unpublished data from a forthcoming report provided by Gorissen DMW, Rutten NBMM, Oostermeijer CMJ, Niers LEM, Hoekstra MO, Rijkers GT, et al. Preventive effects of selected probiotic strains on the development of asthma and allergic rhinitis in childhood. The PandA study.

Overall, most trials were adjudicated to be of unclear (nine of 20) or high risk of bias (10 of 20); only one trial55 was considered to have a low risk of bias across all domains (web appendix, table S5; fig S1). Of 20 trials, most had adequate random sequence generation (n=18), allocation concealment (n=18), and blinding of outcome assessment (n=18). However, eight and nine trials were subject to unclear and high risk of attrition bias, respectively, owing to incomplete outcome data after substantial loss to follow-up. Three trials were at high risk of performance bias due to unblinding of study participants at extended follow-up assessments (including one trial with unpublished data).28 68

Probiotic interventions

Table 2 presents details of the probiotic interventions administered in each trial. All trials evaluated probiotic supplements, rather than consumer food products. Supplements were delivered orally by various methods, including capsules; oil droplets; and suspensions in water, milk, or infant formula. One trial49 exclusively evaluated prenatal maternal supplementation, 10 exclusively evaluated postnatal infant supplementation, while nine evaluated a combination of prenatal and postnatal supplementation. The total duration of intervention ranged from one to 25 months (median 6.3 months). Various probiotic organisms were tested in isolation or in combination, including four Bifidobacterium species (B bifidum, B longum, two strains of B breve, and four strains of B lactis), and six Lactobacillus species (L acidophilus, L casei, L lactis, L reuteri, two strains of L paracasei, and three strains of L rhamnosus). Six trials21 32 33 48 50 54 evaluated combinations of multiple probiotic organisms, and five32 50 56 57 58 evaluated probiotics in combination with prebiotics (selectively fermented compounds that facilitate changes in the composition or activity of the gut microbiota to confer benefits on host health).77 The daily dose of probiotics ranged from 108 to 1011 colony forming units, and was not quantifiable in six trials using supplemented infant formulas that were fed without restraint.50 53 54 56 57 58 Compliance was assessed by a variety of methods, including maternal interview or daily diaries, counting of unused supplements, and faecal analysis.

Table 2

 Characteristics of probiotic interventions in included trials

Primary article	Prenatal or postnatal intervention	Participant receiving probiotics	Duration of intervention (months)	Timing of intervention	Probiotic organism*	Added prebiotic	Total daily dose (CFU)
West 200935	Postnatal	Infant	10	Age 4-13 months (infant)	L paracasei F19	No	1×108 (minimum)
Abrahamsson 200725	Prenatal and postnatal	Mother and infant	13	36 WG to delivery (mother), birth to 12 months (infant)	L reuteri	No	1×108
Kalliomaki 200126	Prenatal and postnatal	Mother and infant (if not breastfed)	6.5	36-38 WG to 6 months postpartum if breastfeeding (mother), weaning to 6 months (infant)	L rhamnosus GG	No	2×1010
Niers 200933	Prenatal and postnatal	Mother and infant	14	24 WG to delivery (mother), birth to 3 months (infant)	B bifidum W23, B lactis W52, and L lactis W58	No	3×109
Wickens 200836	Prenatal and postnatal	Mother and infant	25	35 WG to 6 months postpartum if breastfeeding (mother), birth to 2 years (infant)	2 probiotic groups: L rhamnosus HN001 or B lactis HN019	No	6×109 (HN001) or9×109 (HN019)
Kukkonen 200732	Prenatal and postnatal	Mother and infant	7	36-38 WG to delivery (mother), birth to 6 months (infant)	L rhamnosus GG, L rhamnosus LC705, B breve Bb-99, and Propionibacterium freudenreichii JS	Yes	2.4×1010 (mother), 1.2×1010 (infant)
Taylor 200727	Postnatal	Infant	6	Birth to 6 months (infant)	L acidophilus LAVRI-A1	No	3×109
Gore 201251	Postnatal	Infant	3	Age 5-8 months (infant)	2 probiotic groups: L paracasei or B lactis	No	1×1010
Ou 201234	Prenatal and postnatal	Mother and infant (if not breastfed)	10	24 WG to 6 months postpartum if breastfeeding (mother), weaning to 6 months (infant)	L rhamnosus GG	No	1×1010
Dotterud 201021	Prenatal and postnatal	Mother	4	36 WG to 3 months postpartum (mother)	L rhamnosus GG, B lactis Bb-12, and L acidophilus La-5	No	1.05×1011
Kopp 200855	Prenatal and postnatal	Mother and infant	7	34-36 WG to 3 months postpartum, if breastfeeding (mother), weaning to 6 months, or 3-6 months (infant)	L rhamnosus GG	No	1×1010
van der Aa 201158	Postnatal	Infant	3	Age 5-8 months (infant)	B breve M-16V	Yes	1.3×106 per 100 mL†
Hol 200854	Postnatal	Infant	12	Age 4-16 months (infant)	L casei and B lactis Bb-12	No	2×107 per g†
Boyle 201149	Prenatal	Mother	1	36 WG to delivery (mother)	L rhamnosus GG	No	1.8×1010
Maldonado 201256	Postnatal	Infant	6	Age 6-12 months (infant)	L fermentum CECT5716	Yes	2×108 (average)
Chouraqui 200850	Postnatal	Infant	4	Age 2 weeks to 4 months (infant)	3 probiotic groups: B longum BL999 and L rhamnosus LPR; BL999 and LPR plus prebiotic; or BL999 and L paracasei ST11 plus prebiotic	Yes	5-8×108 per 100 mL†
Gruber 200752	Postnatal	Infant	3	Age 7-10 months (infant)	L rhamnosus GG	No	1×1010
Allen 201048	Prenatal and postnatal	Mother and infant	7	36 WG to delivery (mother), birth to 6 months (infant)	L salivarius CUL61, L paracasei CUL08, B lactis CUL34, and B bifidum CUL20	No	1×1010
Hascoet 201153	Postnatal	Infant	4	Age 4 days to 4 months (infant)	B longum BL999	No	2×107 per g†
Puccio 200757	Postnatal	Infant	3.7	Age 2-16 weeks (infant)	B longum BL999	Yes	2×107 per g†

CFU=colony forming units; WG=weeks’ gestation.

*B=Bifidobacterium; L=Lactobacillus.

†Fed without restraint.

Outcomes

Among the included trials, the duration of follow-up ranged from four months to eight years, and the median age at final assessment was 24 months (table 1). Nine trials reported clinical asthma diagnosis, 11 reported wheezing outcomes, and six reported lower respiratory tract infections; table S6 provides individual study definitions for these outcomes (web appendix). One trial reported the asthma predictive index score,49 two reported asthma drug use,54 58 and none reported admission to hospital for asthma. Adverse events were inconsistently reported.

Primary outcome: asthma

Nine trials including 3257 children contributed asthma data for meta-analysis (fig 2). Incidence of doctor diagnosed asthma at final assessment was 11.2% among patients randomised to receive probiotics and 10.2% among those receiving placebo (risk ratio 0.99, 95% confidence interval 0.81 to 1.21, I2=0%). Results were similar when expressed as a Peto odds ratio for rare events.

Fig 2 Probiotic supplementation during pregnancy or infancy and doctor diagnosed asthma in children. The longest available follow-up data (intention to treat) were extracted from each contributing trial. Trials are sorted in order of decreasing duration of follow-up. df=degrees of freedom; M-H=Mantel-Haenszel

Secondary outcomes: wheeze, lower respiratory tract infection, and adverse events

Nine trials including 1949 children contributed incident wheeze data for meta-analysis (fig 3). Incident wheeze at final assessment was similar after supplementation with probiotics or placebo (35.0% v 31.1%; risk ratio 0.97, 95% confidence interval 0.87 to 1.09, I2=0%). Three trials reported recurrent wheezing, and meta-analysis was not pursued owing to considerable statistical heterogeneity (I2=83%). Two of these trials29 55 reported an increased risk of recurrent wheeze after probiotic supplementation, whereas the third trial58 reported a decreased risk (web appendix, fig S2).

Fig 3 Probiotic supplementation during pregnancy or infancy and incident wheeze in children. The longest available follow-up data (intention to treat) were extracted from each contributing trial. Trials are sorted in order of decreasing duration of follow-up. df=degrees of freedom; M-H=Mantel-Haenszel

Six trials including 1364 children contributed data on lower respiratory tract infections. The incidence of lower respiratory tract infection was 14.5% among children randomised to receive probiotics, and 13.2% among those who received placebo (risk ratio 1.26, 95% confidence interval 0.99 to 1.61, I2=0%). Notably, four of six trials documented lower respiratory tract infections non-systematically as adverse events,48 50 52 53 rather than as primary or secondary outcomes. Excluding these four trials, the pooled risk ratio of lower respiratory tract infection associated with probiotics was 1.11 (0.70 to 1.76, I2=35%; fig 4).

Fig 4 Probiotic supplementation during pregnancy or infancy and lower respiratory tract infection in children. The longest available follow-up data (intention to treat) were extracted from each contributing trial. Trials are sorted in order of decreasing duration of follow-up, and subgrouped according to whether the incidence of lower respiratory tract infection was systematically reported as a primary or secondary outcome, or as an adverse event. AE=adverse event; df=degrees of freedom; LRTI=lower respiratory tract infection; M-H=Mantel-Haenszel

Most trials did not systematically screen for or report the incidence of relevant safety outcomes, including severe gastrointestinal disturbances or allergic reactions (web appendix, table S7). The Peto odds ratio associated with withdrawal due to perceived side effects was 1.45 (95% confidence interval 0.66 to 3.17, I2=0%; eight trials, 2732 children; web appendix, fig S3).

Subgroup analysis

We evaluated the efficacy of probiotics for prevention of asthma in children according to predefined subgroups (table 3). Subgroup analyses by participant type (mother, infant, or both), timing of intervention (prenatal, postnatal, or both), or duration of intervention (≤6 or >6 months) did not show significant differences. However, individual subgroups were subject to type II errors, owing to small sample sizes. Similarly, we observed no statistical differences according to baseline asthma risk, probiotic dose or organism, caesarean delivery rate, feeding restrictions, geographical area, risk of bias, or industry authorship. Differences were not observed according to duration of follow-up, whether classified according to predefined strata (table 3) or assessed as a continuous variable by meta-regression (web appendix, fig S4). We found no significant differences across subgroups for incident wheeze (table 3); subgroup analyses were not pursued for recurrent wheeze or lower respiratory tract infection because of the small number of trials reporting these outcomes.

Table 3

 Subgroup analyses for probiotic supplementation during pregnancy or infancy and asthma or wheeze in children

Subgroup	Diagnosed asthma				Incident wheeze
	No of Studies	Pooled risk ratio (95% CI)	I2 (%)		No of Studies	Pooled risk ratio (95% CI)	I2 (%)
Overall	9	0.99 (0.81 to 1.21)	0		9	0.97 (0.87 to 1.09)	0
Participant receiving intervention
Mother	1	0.64 (0.27 to 1.54)	—		1	0.93 (0.59 to 1.48)	—
Mother and infant	6	1.00 (0.80 to 1.23)	0		3	0.96 (0.83 to 1.11)	0
Infant	2	1.27 (0.61 to 2.66)	0		5	1.00 (0.78 to 1.28)	26
Timing of intervention
Prenatal	0	—	—		1	0.93 (0.59 to 1.48)	—
Prenatal and postnatal	7	0.97 (0.79 to 1.19)	0		3	0.96 (0.83 to 1.11)	0
Postnatal	2	1.27 (0.61 to 2.66)	0		5	1.00 (0.78 to 1.28)	26
Duration of intervention
≤median (6 months)	2	0.98 (0.39 to 2.45)	43		4	0.93 (0.73 to 1.19)	1
>median	7	0.99 (0.81 to 1.23)	0		5	0.98 (0.86 to 1.12)	2
Probiotic dose
<median (1010 CFU daily)	5	0.98 (0.74 to 1.31)	0		5	1.04 (0.87 to 1.24)	23
≥median	4	1.06 (0.68 to 1.66)	38		2	0.79 (0.56 to 1.11)	0
Fed without restraint	0	—	—		2	0.97 (0.76 to 1.25)	0
Probiotic organism*
Bifidobacterium species	1	1.04 (0.63 to 1.71)	—		3	0.96 (0.80 to 1.16)	3
Lactobacillus species	6	1.13 (0.82 to 1.55)	0		6	0.99 (0.80 to 1.23)	19
Combination	3	0.86 (0.63 to 1.16)	0		2	1.03 (0.78 to 1.37)	0
Follow-up duration†
<3 years	6	0.90 (0.62 to 1.31)	0		8	0.98 (0.86 to 1.11)	0
3 to <6 years	4	1.04 (0.77 to 1.39)	0		3	0.91 (0.76 to 1.10)	18
≥6 years	5	1.01 (0.76 to 1.34)	0		2	0.99 (0.79 to 1.25)	47
Asthma risk
High (infant atopic disease)	0	—	—		2	0.85 (0.65 to 1.12)	0
High (family history)	7	1.01 (0.82 to 1.25)	0		5	0.97 (0.85 to 1.12)	0
Unselected	2	0.78 (0.40 to 1.51)	0		2	1.19 (0.76 to 1.86)	40
Infant feeding
Unrestricted	9	—	—		7	0.98 (0.84 to 1.15)	15
Exclusive formula feeding	0	—	—		2	0.97 (0.76 to 1.25)	0
Caesarean delivery rate
≤median (19%)	4	0.92 (0.69 to 1.21)	0		4	1.11 (0.88 to 1.41)	0
>median	2	1.03 (0.73 to 1.45)	0		4	0.96 (0.84 to 1.09)	0
Not reported	3	1.25 (0.57 to 2.74)	53		1	0.64 (0.38 to 1.07)	0
Geographical area
Europe	6	0.94 (0.72 to 1.22)	0		6	1.01 (0.83 to 1.23)	7
Asia or Oceania	3	1.07 (0.78 to 1.46)	0		3	0.95 (0.82 to 1.10)	0
Risk of bias
Low or unclear	2	1.47 (0.45 to 4.77)	71		3	0.87 (0.69 to 1.10)	0
High	7	0.98 (0.76 to 1.26)	0		6	0.97 (0.87 to 1.09)	6
Industry authorship
No or unclear	7	1.05 (0.82 to 1.36)	0		7	0.96 (0.84 to 1.08)	0
Yes	2	0.89 (0.65 to 1.23)	0		2	1.09 (0.80 to 1.49)	0

CFU=colony forming units. Tables 1 and 2 summarise the studies contributing data for each outcome and subgroup.

*Two trials compared Lactobacillus with Bifidobacteria in separate groups.

†With regard to subgroup analysis of follow-up duration, based on the number and duration of follow-up assessments, some trials contributed data to more than one predefined subgroup. For all other subgroup analyses, the longest available follow-up data from each trial were used.

Discussion

In this systematic review and meta-analysis of randomised controlled trials, we found no evidence to support a protective association between probiotic supplementation during pregnancy or early life, and subsequent development of childhood asthma or wheeze. Although inadequately reported, probiotic supplementation could be associated with clinically relevant increases in lower respiratory tract infections.

Comparison of results with other studies

Our review provides a timely update to the 2007 Cochrane review of probiotics for prevention of allergic diseases.24 The Cochrane review reported no benefit for the prevention of asthma after probiotic supplementation, based on findings from three trials enrolling a total of 617 infants.25 26 27 Our review evaluates updated long term findings from these three original trials,28 29 30 31 and adds results from six new trials (2308 infants) reporting on probiotics for asthma prevention.21 32 33 34 35 36 Furthermore, we have evaluated 11 additional probiotic trials (1976 infants) reporting asthma related outcomes (wheeze or lower respiratory infection) that were not analysed in the 2007 Cochrane review. After systematic evaluation of these new and extended trial results, involving over 4000 additional children, we conclude that there is still insufficient evidence to recommend probiotics for the primary prevention of asthma. Our findings further identify several unanswered questions and highlight opportunities for future research.

Opportunities for future research

Despite widespread enthusiasm for evaluating probiotics to prevent allergic disease17 and recognition of asthma as a major allergic disease in childhood,1 78 relatively few randomised trials have formally tested the use of probiotics for asthma prevention. Only nine probiotic trials have reported asthma diagnosis, and none were powered for asthma detection as the primary outcome. Long term follow-up is essential for asthma prevention studies because diagnosis is challenging before age 6 years.79 80 However, among the 20 trials included in our review, the median age at last follow-up was 24 months, and only five trials reported outcomes at or beyond 6 years of age (including one trial with unpublished data).28 30 67 68 Moreover, long term follow-up was frequently subject to high or unclear risk of bias owing to attrition or the unblinding of participants. Extended follow-up data from other established trials are highly anticipated, including planned adolescent assessments by Kalliomaki26 and Kukkonen32 and colleagues. Because of the paucity of long term follow-up data among probiotic trials, we also evaluated wheeze as an early presentation of asthma. However, only a minority of wheezing infants will ultimately develop asthma later in childhood.81

Thus, extended follow-up of existing studies, combined with novel, respiratory focused trials,82 83 84 will be necessary to define the role of probiotics for asthma prevention. Owing to the dynamic nature of the gut microbiota, trials evaluating prolonged probiotic supplementation (beyond the first year of life) may also be needed. As West and colleagues have shown,67 probiotics are transient colonisers of the intestine, indicating that prolonged supplementation may be required to achieve durable benefit.

Our findings also highlight a need to further consider the effect of probiotics on the incidence and severity of recurrent wheeze and lower respiratory tract infections. Recurrent wheeze during infancy is considered a better predictor of asthma than incident wheeze,40 yet only three of 10 trials documenting wheeze reported variable measures of recurrence,27 55 58 and statistical heterogeneity precluded meta-analysis. Reporting of lower respiratory tract infections was similarly incomplete and variably defined. In six trials reporting this outcome, we observed a trend toward increased infections in children who received probiotics. Future trials should systematically define and capture recurrent wheeze, along with lower respiratory tract infections and other relevant safety outcomes.

Our subgroup analyses indicated that the effect of probiotics was similar regardless of the timing of intervention (prenatal v postnatal v both) or the participant receiving the intervention (mother v infant v both). The efficacy of specific probiotic organisms was difficult to assess because of the large variety of strains, combinations, and doses tested, and requires further investigation. Different probiotic organisms probably have distinct effects on the gut microbiota and host physiology. A recent animal study found that four Lactobacillus species had markedly different immunomodulatory effects,85 and at least one clinical study has shown strain specific anti-allergic effects.36 Further basic and clinical research is also needed to characterise the mechanisms by which probiotics influence asthma development, including how specific organisms colonise the gut, modify the resident microbiota, and ultimately affect host immunity and health. Such knowledge will help optimise the selection of probiotic organisms and the design of intervention regimens for future study.

Finally, identifying infant populations most likely to benefit from probiotics is highly desirable. For example, one trial has shown that probiotics were protective against IgE associated allergic disease in infants delivered by caesarean section (whose gut microbiota is disrupted86), but not in their vaginally delivered counterparts.64 With trial level data, we could not identify differential efficacy according to caesarean delivery rate, but this and other microbiota disrupting exposures (such as formula feeding and antibiotic treatment) warrant further study as possible indications for probiotic supplementation in the prevention of asthma and wheeze.

Strengths and weaknesses of the study

The strengths of this review included the completeness of the search strategy, which reviewed multiple citation databases, trial registries, and conference proceedings. By omitting outcome related search terms, we identified trials that were not primarily focused on asthma or allergic disease, but nevertheless reported relevant outcomes.50 53 56 57 We focused on patient centred outcomes and evaluated efficacy in the context of relevant safety outcomes and adverse events. Finally, we used an a priori published protocol and followed established methodological guidelines in the conduct and reporting of this review. Limitations include pooling data from trials conducted in distinct populations (for example, infants at high risk for asthma, or unselected populations) receiving different probiotic formulations (various organisms with a 1000 times range in daily dose) through varying regimens (prenatal or postnatal supplementation, for 1-25 months). Subgroup analyses were susceptible to type II errors owing to relatively small sample sizes.

Conclusion

We found no evidence to support a protective association between perinatal administration of probiotics, and doctor diagnosed asthma or childhood wheeze. There is currently insufficient evidence to recommend probiotics for the primary prevention of these disorders, and further research is warranted to explore the potential association between probiotic supplementation and increased risk of lower respiratory tract infection. Extended follow-up of existing trials, along with further clinical and basic research, are needed to accurately define the role of probiotics in the prevention of childhood asthma.

Asthma is the most common chronic disease of childhood, and is frequently preceded by wheeze

Increases in asthma prevalence have been partly attributed to disruption of the commensal gut microbiota in early life

Perinatal probiotics have been shown to prevent atopic dermatitis, but uncertainty remains regarding their effectiveness in asthma prevention

We found no evidence to support a protective association between perinatal probiotics and childhood asthma or wheeze. Although inadequately reported, probiotic supplementation could be associated with increases in lower respiratory tract infections

Additional basic research and adequately powered long term clinical trials are needed to fully define the role of probiotics in the prevention of asthma

Probiotics cannot be recommended for primary prevention of childhood asthma or wheeze at this time