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Breastfeeding and Asthma in Young Children
Findings From a Population-Based Study
Sharon Dell, MD;
Teresa To, PhD
Arch Pediatr Adolesc Med. 2001;155:1261-1265.
ABSTRACT
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Objective To evaluate the association between breastfeeding and asthma in young
Canadian children.
Methods Baseline data from the National Longitudinal Survey of Children and
Youth (a population-based study of child health and well-being) were used.
A weighted sample of 331 100 (unweighted n = 2184) children between the
ages of 12 and 24 months, whose biological mother reported data on breastfeeding
and asthma, were included. Outcomes included parental report of physician-diagnosed
asthma and wheeze in the previous year. Breastfeeding was categorized by duration
as follows: less than 2 months, 2 to 6 months, 7 to 9 months, and longer than
9 months. Logistic regression analyses were conducted with breastfeeding duration
dichotomized at various cutoffs. Important potential confounders were considered
in the adjusted analyses. Published statistical methods appropriate for the
sampling strategy were used.
Results The prevalence of asthma was 6.3%; and wheeze, 23.9%. Almost half of
the children (44.0%) were breastfed for less than 2 months. After adjustment
for smoking, low birth weight, low maternal education, and sex, a duration
of breastfeeding for 9 months or less was found to be a risk factor for asthma
(odds ratio, 2.39; 99% confidence interval, 0.95-6.03) and wheeze (odds ratio,
1.54; 99% confidence interval, 1.04-2.29). A dose-response effect was observed
with breastfeeding duration.
Conclusions A longer duration of breastfeeding appears to be protective against
the development of asthma and wheeze in young children. More public health
efforts should be directed toward increasing the initiation and duration of
breastfeeding.
INTRODUCTION
THE MORBIDITY of childhood asthma, when measured by health care use,
is highest in the young, with children aged 1 to 4 years accounting for up
to 50% of asthma emergency visits.1 Studies2 looking at hospital discharge data in Ontario, Canada,
have shown that recent hospitalization rates for asthma have decreased in
all age groups except for young children (4 years of age and younger). Although
much progress has occurred in developing effective treatment regimens for
the control of asthma,3 there is still no cure
for asthma and little is known about how we can prevent this disease. In recent
years, it has become evident that factors in early life may increase a child's
susceptibility to developing asthma.4
Human breast milk is recognized to have many beneficial health effects.5 Because the etiology of asthma is multifactorial,
with genetic predisposition, immunologic profile, and allergic sensitization
playing major roles,6 it is biologically plausible
that breastfeeding may offer some protection against the occurrence of asthma
by decreasing allergic sensitization and/or modulating the infant's immune
system.7 From a public health point of view,
even a small protective effect would be important to detect, because asthma
is a highly prevalent chronic disease in children6
and breastfeeding rates are modifiable by postnatal support programs.8
Several recent birth cohort studies,9, 10, 11, 12, 13
evaluating the association between breastfeeding and asthma and/or other respiratory
illnesses in children, have shown consistently protective effects, particularly
with exclusive breastfeeding for more than 4 months. There has, however, been
much controversy about the protective effect of breastfeeding against atopic
diseases, particularly with asthma.14 This
study evaluates the association between breastfeeding and asthma in young
children, who experience significant morbidity with asthma,2
in a Canadian population-based sample.
PARTICIPANTS AND METHODS
Baseline data from the Canadian National Longitudinal Survey of Children
and Youth cycle 1, collected in 1994 and 1995, were used in this study.15 The National Longitudinal Survey of Children and
Youth is a prospective questionnaire designed to measure child development,
health, and well-being. The total cycle 1 sample size consists of 22 831
children from birth to the age of 11 years. A complex clustered sampling scheme
was used to be representative of the Canadian childhood population. The clusters
were designed to have sufficient sample sizes within large geographic areas
and within 7 key age groupings, with an overemphasis on the youngest age groups
(<2 years). Trained Statistics Canada, Ottawa, Ontario, surveyors went
to households and administered standardized questionnaires to the person most
knowledgeable about the child. The person most knowledgeable was the biological
mother in 90% of the cases. The overall response rate to the survey was 86%,
while response rates for health outcomes of children were 91% or more. Informed
consent was obtained from the legal guardians and/or the child as appropriate.
All children between the ages of 12 and 24 months, whose biological mother
was the person most knowledgeable, were eligible for our study. Subjects missing
breastfeeding or asthma data were excluded.
Asthma-related outcomes included parental report of wheeze within the
previous year and physician diagnosis of asthma. The exposure variable of
interest—duration of breastfeeding—was reported in 7 categories.
Because a duration of less than 2 months is too short to expect any biological
protective effect,14 the variable was recategorized
into 4 levels for analysis: none or less than 2 months, 2 to 6 months, 7 to
9 months, and longer than 9 months. Breastfeeding was grouped as a binary
variable, evaluating the influence of less than 2 months, 6 months or less,
and 9 months or less of breastfeeding exposure. The exclusivity of breastfeeding
and the introduction of solid foods were not measured in the survey.
Other exposures that were explored for possible confounding effects
included prematurity (gestational age  258 days), low birth weight (LBW)
(<2500 g), postnatal household smoking (at least 1 spouse living in the
child's home currently smoking), prenatal smoking (smoking at any time during
the pregnancy), low maternal education (less than a high school graduate),
low income (family income variable derived by Statistics Canada that accounts
for household income and the number of people dependent on that income15; low income corresponds closely with poverty levels
in 199516), day care (child is in a day care
center—data were only available if the mother was in school or working,
and missing data were coded as no day care), siblings (any residing in the
household, including full, half, and step siblings), and parental asthma history
(either the biological mother or the biological father had asthma ever diagnosed
by a health professional). Data on parental history of asthma for the biological
father were missing for 16% of the sample. These missing cases were recoded
as negative for father's history of asthma in the combined parental asthma
history variable. Recoding did not have a significant effect on any of the
logistic modeling.
Statistics Canada's data publication guides were followed throughout
all analyses15 and, thus, data were weighted
up to the population level. Corrections for the effect of sampling design
on variable estimates were produced using coefficients of variation derived
by Statistics Canada.15 Demographic and other
characteristics of the study population were tabulated to obtain a description
of the population under study. Spearman rank correlations between covariates
were examined. The Pearson  2 and unadjusted logistic regression
were used to test the statistical significance of risk factors for asthma
or wheeze and breastfeeding. All variables used in logistic regression modeling
had coefficients of variation meeting Statistics Canada's quality standards.
A sensitivity analysis was conducted to evaluate the influence of dichotomizing
the breastfeeding duration exposure variable at different cutoff points. The
effects of a breastfeeding duration of less than 2 months, 6 months or less,
and 9 months or less were examined. Factors associated with asthma or wheeze
and breastfeeding were considered confounders and, thus, put into multiple
regression models to adjust for potential effects on the association between
asthma or wheeze and breastfeeding exposure. Subjects missing data (<1%
of the sample) were excluded from the multivariate analysis. Where covariates
were highly correlated, only one was put into the adjusted model. Interactions
with sex and parental asthma history were also explored.
The effect of a clustered sampling design cannot be fully adjusted for
in our logistic regression analyses, as the clustering sample weights are
not released by Statistics Canada for public use. We have used standardized
individual sample weights (normalized with a mean of 1) to derive statistical
estimates that preserve the original sample size of each group, and reduce
the variance estimate bias effect.17 In addition,
tests of significance for logistic regression were considered at P<.01 (instead of P = .05), to reduce variance
estimate bias and partially correct for lack of sample design information.17
RESULTS
The 2184 subjects included in the study resulted in a sample size of
331 100 when weighted up to the population level. Restricting the analysis
to those children whose biological mother completed the survey excluded 10%
of the original sample. Another 1.7% of the subjects were excluded because
of missing breastfeeding information. The characteristics of the children
are shown in Table 1.
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Table 1. Characteristics of Canadian Children Aged 12 to 23 Months
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The prevalence of asthma was 6.3%; and wheeze, 23.9%. Almost half of
the children (44.0%) were not breastfed or were breastfed for less than 2
months. An unadjusted logistic regression analysis (Table 2) revealed that the duration of breastfeeding, male sex,
parental asthma history, prenatal and postnatal smoking, prematurity, and
LBW were all risk factors for asthma, and less strongly so for wheeze. The
breastfeeding duration exhibited a dose-response effect in the unadjusted
analysis. Low income and low maternal education were risk factors for asthma
but not wheeze, and day care was a risk factor for wheeze but not asthma.
Risk factors for not breastfeeding included prenatal and postnatal smoking,
prematurity, LBW, low maternal education, and low income. Therefore, these
potentially confounding variables were considered for inclusion in the adjusted
model.
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Table 2. Unadjusted OR Estimates for Asthma and Wheeze in Children
Aged 12 to 23 Months*
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Based on an adjusted sensitivity analysis comparing results with less
than 2 months vs 6 months or less vs 9 months or less of breastfeeding duration
(Table 3), it appeared that breastfeeding
only conferred a statistically significant (based on a level of P<.01, decided a priori) protective effect if done so for at least
9 months. Table 3 is also suggestive
of a dose-response effect for breastfeeding duration. Future analyses were
completed for breastfeeding exposure at the 9-month cutoff.
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Table 3. Adjusted OR Estimates for Asthma and Wheeze by Various Thresholds
of Breastfeeding Exposure*
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The final adjusted model for breastfeeding less than a full 9 months
and asthma or wheeze is shown in Table 4. The adjusted odds ratio (OR) for breastfeeding was higher than
the risk associated with smoking and asthma.
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Table 4. Adjusted OR Estimates for Asthma and Wheeze in Children Aged
12 to 23 Months*
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The inclusion of covariates was based in part on patterns of correlation.
Prenatal and postnatal household smoking were strongly correlated (r = 0.53, P<.001). Because prenatal smoking
had a stronger association with asthma or wheeze and breastfeeding, it was
used in the adjusted model. Prematurity and LBW were also strongly correlated
(r = 0.51, P<.001). Because
LBW is inherently a more precise measure and had a stronger association with
asthma or wheeze and breastfeeding, it was used in the final model. Low income
and low maternal education were moderately correlated (r = 0.36, P<.001). Low maternal education
had stronger associations with asthma and breastfeeding, and because adding
low income did not change the breastfeeding effect, it was not included in
the final model.
Day care, siblings, parental asthma history, and sex were not associated
with breastfeeding. Because the main purpose of this study was to examine
the association between breastfeeding and asthma, only those variables for
which there was evidence of the potential for confounding were left in the
final model, except sex. Although some investigators would point out that
it is not necessary to include sex in the model because it is not a true confounder,
sex has traditionally always been adjusted for in logistic models with health
outcomes and is a known correlate of asthma; therefore, we kept it in the
model. Exploratory analyses were performed to examine the influence of these
nonconfounding variables on the adjusted model. Adding parental asthma history,
day care, and sibling variables into the adjusted model did not change the
OR estimates for breastfeeding significantly; thus, they were left out of
the final model. No significant interactions between sex or parental asthma
history and breastfeeding were found.
COMMENT
This study suggests that infant feeding practices may have an effect
on the development of asthma or wheeze in children younger than 2 years. The
effect of breastfeeding appears to be stronger with a longer duration (suggesting
a dose-response effect). Although the association between the duration of
breastfeeding and asthma does not quite reach our a priori conservatively
defined statistical significance level of P<.01,
it does show a clearly increasing protective effect with an increasing duration
of breastfeeding. The association between breastfeeding for 9 months or less
and wheeze does reach statistical significance at the P<.01 level, although it is weaker (OR, 2.39 vs 1.54 for asthma vs
wheeze). This is not surprising, because the wheezing group will contain many
children who have asthma or may develop asthma4
and has a larger group size to obtain statistical significance.
The relations found in this study are consistent with 5 other recent
birth cohort studies that have looked at the association between asthma and/or
wheeze in children and breastfeeding and also controlled for confounding influences.
Oddy et al9 prospectively studied 2187 children
from Western Australia, and after controlling for sex, gestational age, household
smoking, and day care, they found that the introduction of milk other than
breast milk before the age of 4 months was a risk factor at the age of 6 years
for an asthma diagnosis by a physician (OR, 1.25; 95% confidence interval,
1.02-1.52) and for wheeze in the previous year (OR, 1.31; 95% confidence interval,
1.05-1.64). They did not demonstrate a dose-response effect. Wilson et al10 examined 674 children from Dundee, Scotland, and
after controlling for parental asthma, sex, and social class, they found that
exclusive breastfeeding for at least 15 weeks resulted in decreased respiratory
illness (defined as persistent cough, wheeze, or breathlessness) throughout
childhood up to the age of 10 years. The association between breastfeeding
and an asthma diagnosis was not statistically significant, but the influence
was in the same direction as in our study. Tariq et al11
also prospectively observed a birth cohort of 1218 children from the Isle
of Wight, England, and found that formula feeding before the age of 3 months
predisposed the child to asthma at the age of 4 years. Saarinen and Kajosaar12 observed a smaller sample of 236 children from Finland
and found persistence of a protective effect of breastfeeding against atopic
outcomes, extending into adolescence. Finally, Wright et al13
prospectively observed 1246 healthy infants and found that breastfeeding was
associated with lower rates of recurrent wheeze at the age of 6 years.
The magnitude of the breastfeeding effect at 6 months in our study (OR,
1.62; 99% confidence interval, 0.86-3.08) was consistent with that found in
other studies9 and was as would be expected
biologically (ie, an OR between 1.2 and 2.0).18
The magnitude at 9 months was surprisingly high (OR, 2.39; 99% confidence
interval, 0.95-6.03); however, to our knowledge, other studies have not looked
at the effect of breastfeeding this long. The protective effect of breastfeeding
longer than 6 and 9 months was stronger than the effect of smoking on asthma
(prenatally and postnatally) in the unadjusted and adjusted analyses. This
was also shown in 2 previous studies.9, 10
Several studies have suggested that the influence of breastfeeding on
atopic outcomes is modified in children of atopic mothers19
or in atopic children.13, 20 We
were not able to test these hypotheses in our study. A subanalysis in the
group of children with parental asthma history could not be performed because
of an inadequate sample size.
The high overall response rate for the survey (86%) and the random sampling
procedure with clustering within geographic areas suggest that our sample
is highly representative of the population and, thus, generalizable to young
Canadian children. The prevalences of asthma and wheeze4
and breastfeeding21 are consistent with those
reported in other North American studies, further validating the data. The
risk factors for asthma and wheeze identified in this population are consistent
with those reported previously for early childhood wheeze.4
One limitation of this study is that it is a secondary data analysis
of a cross-sectional design using disease prevalence as the outcome. Mild
or developing cases of asthma will be missed. There is also measurement error
in the classification of disease and exposure because these are based on parental
report. Wheezing is a heterogeneous disorder in childhood, with about half
of the children who wheeze before the age of 3 years being only transient
wheezers and half having persistent wheezing or asthma.4
With our state of knowledge, we are not able to predict which wheezers have
asthma and which will outgrow their wheezing. Some of the children who have
never wheezed by the age of 2 years may still develop asthma. Therefore, even
if asthma were strictly and prospectively defined, there would still be misclassification
in this young age group. Recall bias on breastfeeding duration has been minimized
by using only the biological mother's reporting of this variable and by limiting
the reporting to children younger than 2 years.
The greatest limitation of this study is the lack of finer assessments
of breastfeeding duration (ie, by months) and information on breastfeeding
exclusivity. It is possible that breastfeeding exclusivity is more important
than the duration of breastfeeding; however, because the 2 are obviously highly
correlated,9 it may be possible that we found
an effect with the duration simply because of the correlation with exclusivity.
We cannot exclude the possibility that a shorter duration of exclusive breastfeeding
is more important than simply a longer duration of breastfeeding.
This study examines asthma and wheeze only in children younger than
2 years. It is possible that the protective effect of breastfeeding may disappear
with age, ie, it may only delay the onset of asthma. Even if this is the case,
it is still an important protective effect, because most of the morbidity
and health care costs associated with asthma are in preschool-aged children.2 Also, delaying the onset of disease may potentially
result in decreased asthma severity.22
CONCLUSIONS
To our knowledge, this is the first national population-based study
to support that breastfeeding confers a protective effect against asthma and
wheeze in young children. This protective effect increases with a longer duration
of breastfeeding. The exact cutoff point for protection from breastfeeding
cannot be established from this study, but when considering all of the available
evidence, it is likely a continuous effect that becomes meaningful at a population
level after at least 4 to 6 months of breastfeeding. The effect of exclusivity
of breastfeeding warrants further investigation. Regardless, this study supports
the saying that "breast is best." Strengthening public health efforts to increase
the rates and duration of breastfeeding may help decrease asthma and wheeze
prevalence and related morbidity in young children. Further population-based
studies examining the effects of breastfeeding on asthma outcomes in older
children should be conducted.
AUTHOR INFORMATION
Accepted for publication May 16, 2001.
This study was supported by the Research Institute, The Hospital for
Sick Children, Toronto, Ontario.
We thank David Ip, BSc, for his help with the statistical analysis;
and Suzanne M. Cadarette, MSc, for her help in multiple steps of the study.
This analysis is based on Statistics Canada microdata tape National
Longitudinal Survey of Children and Youth, which contains anonymous
data collected in the 1994-1995 special survey. All computations on these
microdata were prepared by the Research Institute, The Hospital for Sick Children,
and the responsibility for the use and interpretation of these data is entirely
that of the authors.
What This Study Adds
There has been increasing recognition that factors in early life influence
the development of asthma. The relationship between breastfeeding and asthma
has been controversial. To date, to our knowledge, no population-based studies
have been conducted in this area; therefore, this study was designed to examine
the association between breastfeeding and asthma in a North American population-based
sample of young children.
This study shows that breastfeeding confers a protective effect against
asthma and wheeze in the first 2 years of life, when the alveolar stage of
lung development is ongoing. Furthermore, the association appears to be dose
dependent, meaning that the longer a mother breastfeeds, the more protection
is offered to the child. This study highlights the importance of promoting
breastfeeding as a public health intervention, as increasing population-based
breastfeeding rates may modify the prevalence of asthma in young children.
From the Division of Respiratory Medicine (Dr Dell) and Population
Health Sciences, Research Institute (Drs Dell and To), The Hospital for Sick
Children, Toronto, Ontario; and the Department of Public Health Sciences,
University of Toronto, and the Institute for Clinical Evaluative Sciences,
Toronto (Dr To).
Corresponding author and reprints: Teresa To, PhD, Population Health
Sciences, The Hospital for Sick Children, 555 University Ave, Toronto, Ontario,
Canada M5G 1X8 (e-mail: teresa.to{at}sickkids.ca).
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