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W H Oddy a TVW Telethon Institute
for Child Health Research, PO Box 855, West Perth, Western Australia,
Australia 6872, b Faculty of Medicine and
Dentistry, University of Western Australia, Nedlands, Western
Australia, Australia 6009, c TVW Telethon Institute for Child Health Research, d Division of Biostatistics and Genetic
Epidemiology, Department of Paediatrics, University of Western
Australia, Princess Margaret Hospital for Children, Subiaco, Western
Australia 6008, Australia
Correspondence to: W H Oddy Wendyo{at}ichr.uwa.edu.au
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Abstract |
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Objectives:
To investigate the association between the duration of exclusive breast feeding and the development of asthma related outcomes in children at age 6 years.
Design:
Prospective cohort study.
Setting:
Western Australia.
Subjects:
2187 children ascertained through antenatal clinics at the major tertiary obstetric hospital in Perth and followed
to age 6 years.
Main outcome measures:
Unconditional logistic
regression to model the association between duration of exclusive
breast feeding and outcomes related to asthma or atopy at 6 years of
age, allowing for several important confounders: sex, gestational age,
smoking in the household, and early childcare.
Results:
After adjustment for confounders, the
introduction of milk other than breast milk before 4 months of age was
a significant risk factor for all asthma and atopy related outcomes in
children aged 6 years: asthma diagnosed by a doctor (odds ratio 1.25, 95% confidence interval 1.02 to 1.52); wheeze three or more times since 1 year of age (1.41, 1.14 to 1.76); wheeze in the past year (1.31, 1.05 to 1.64); sleep disturbance due to wheeze within the past
year (1.42, 1.07 to 1.89); age when doctor diagnosed asthma (hazard
ratio 1.22, 1.03 to 1.43); age at first wheeze (1.36, 1.17 to 1.59);
and positive skin prick test reaction to at least one common
aeroallergen (1.30, 1.04 to 1.61).
Conclusion:
A significant reduction in the risk of
childhood asthma at age 6 years occurs if exclusive breast feeding is
continued for at least the 4 months after birth. These findings are
important for our understanding of the cause of childhood asthma and
suggest that public health interventions to optimise breast feeding may help to reduce the community burden of childhood asthma and its associated traits.
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Key messages
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Introduction |
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Asthma is the leading cause of admission to hospital in Australian children and its prevalence is increasing. 1 2 Susceptibility to asthma may be increased by factors present early in life.3 These include being male, low birth weight, preterm birth, young maternal age, maternal smoking and, possibly, early cessation of exclusive breast feeding.4 Environmental allergens including house dust mite, grasses, or pollens may also cause sensitisation. Conversely, early exposure to respiratory infections may be protective. 5 6
Environmental exposures in the early months of life are critical for the development of the immune system but have the potential to predispose to allergy or atopy. 7 8 Breast feeding may be an important determinant of the immune response, 9 10 but whether breast feeding protects against asthma or atopy, or both, is controversial.11
A prospective study of children from birth to 17 years concluded that exclusive breast feeding protects against atopic disease throughout childhood and into adolescence.4 Although this study was comparatively small, a larger study of children from birth to 7 years reported that the probability of respiratory symptoms (wheeze, breathlessness, or cough) occurring at or before age 7 was reduced in children exclusively breast fed for at least 15 weeks.12 Another study13 showed a protective effect of breast feeding on atopy only when the age of the infant when other milk was introduced was considered.14 Several studies have, however, failed to show an association between breast feeding and either asthma or atopy.15-17 Halpern et al17 reported childhood allergy to be equally common in children fed human, soy, or cows' milk. Breast feeding has also been reported to have no effect on the incidence of eczema,13 atopic disease, or raised IgE concentrations and may only be protective in non-atopic children.18 In one review, Kramer19 concluded that many studies, both negative and positive, exhibited significant flaws, and that future studies should improve both biological and methodological aspects of design and analysis.
If exclusive breast feeding is protective against childhood
asthma it requires investigation in a large childhood cohort followed prospectively from birth, with assessment of both exposures and outcomes. We report the results of a unique study in Western Australian of this type.
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Subjects and methods |
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Study population
The Western Australian pregnancy cohort study is a prospective
birth cohort initially established (1989-92) as a randomised trial,
which showed that pregnancy outcome was not improved in women who had
multiple ultrasonography.20 Recruitment to the study was
from antenatal clinics at King Edward Memorial Hospital and nearby
private practices. A total of 2979 children were enrolled by 18 weeks
of gestation. Of 2860 live births, 13 have since died and 121 have been
lost to follow up. The parents of 124 children declined follow up or
withdrew at a later date. Overall, 2602 (91.0%) of the liveborn
children remained available for follow up at 6 years of age.
Questionnaires
At enrolment, parents completed a questionnaire about the general
health of the study child. Data recorded at birth included sex,
gestational age, birth weight, and smoking within the household.
Parents kept a diary of their child's health in the first year. When
the children were 1 year old, the parents completed a standardised
questionnaire that included items about feeding: 2411 questionnaires
were returned (84.3% of the liveborn cohort, 92.7% of those
consenting to follow up), and 2365 children (82.7%, 90.9%) attended
for clinical assessment.
Outcomes
Childhood asthma is a complex phenotype, and several
phenotypic definitions were applied in children aged 6 years. These
included cumulative incidence measures
asthma diagnosed by a doctor
and wheeze three or more times since age 1 year
and point prevalence
measures
wheeze in the past year, sleep disturbance due to wheeze in
the past year,2 and objective atopy defined by the results
of a skin prick test. A child was categorised atopic if the wheal to
one or more aeroallergens (house dust mite, ryegrass, cat dander, and
aspergillus mould) was
2 mm and larger than a control 10 minutes
after testing. A 2 mm cut off point was used rather than 3 mm, which is
more usual in studies of adults, because wheal size increases with
age.21
Exposures
Exposure to breast feeding was measured in two ways: the duration
of breast feeding and the duration of exclusive breast feeding
(child's age when other milk was introduced). These were analysed both
as continuous and binary variables and were highly correlated
(P<0.001). Over a range of models, the duration of exclusive breast
feeding was found to be a better determinant of outcome than the
duration of breast feeding, and child's age at which other milk was
introduced became the key exposure variable. Given the statistical
power, we elected for conservatism and based the primary analysis on a
binary variable. Dichotomisation was at 4 months, the integer cut off
point closest to the median; this choice was statistically powerful and
bioclinically logical.12
1 cigarettes a day were smoked inside the house. Maternal
education and family income were modelled as categorical factors.
Maternal age and height were modelled as continuous covariates.
Childcare or attendance at a playgroup in the first 3 months of life
was used as a proxy for early exposure to respiratory infections. Our
conclusions pertaining to the effects of breast feeding were, however,
similar no matter how respiratory infections were modelled. Parental
history of asthma was used as a potential binary confounder, but it was
only used in secondary (exploratory) analysis because family history
may in part reflect the action of the causal pathway of interest, and
naive adjustment could be misleading.
Statistical analysis
Significance tests for contingency tables were on the basis of the
2 test for association (without continuity correction).
Unconditional logistic regression was used to investigate the
multivariate relation between binary response variables and explanatory
exposures of interest. Age at asthma diagnosis and onset of wheezing
were analysed using Kaplan Meier survival functions and the log rank
statistic. We used Cox regression for analysis of multivariate
survival. Regression models were subjected to standard tests of
goodness of fit including an investigation of the need for additional
polynomial or interaction terms, an analysis of Pearson and Martingale
residuals, and tests of regression leverage and
influence.22
Statistical power
Statistical significance was defined at the two sided P=0.05
level. The final data set generated more than 99% power to detect an
odds ratio of 2.0, and more than 95% power to detect an odds ratio of
1.5 for most analyses.
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Results |
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The key characteristics of the cohort are detailed in table 1. A strong association (P<0.001) was found between most measures of asthma and wheeze suggesting that the end point definitions were internally consistent with each other.
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Binary end points
Table 2 details the results of the logistic regression analyses.
Having adjusted for the potential confounding of other risk factors,
the introduction of milk other than breast milk before 4 months of age
was positively associated with all primary end points at age 6 years.
Being male, of gestational age less than 37 weeks, and smoking in the
household were also significant risk factors for the development of
asthma and wheeze. Early exposure to childcare was negatively
associated.
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Age at onset of asthma and wheeze
Figures 1 and 2 detail the Kaplan Meier survival functions
indicating age at diagnosis of asthma by a doctor, and age at onset of
wheezing, stratified by duration of exclusive breast feeding. The
cumulative incidence of both asthma (P=0.001) and wheeze (P<0.001) was
higher if other milk was introduced before 4 months. Cox regression
showed that age at asthma diagnosis (hazard ratio 1.22, 1.03 to 1.43;
P=0.02) and age at first wheeze (1.36, 1.17 to 1.59; P=0.0002) were
both earlier if other milk was introduced before 4 months, after
controlling for confounders.
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Discussion |
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Validity
The Western Australian pregnancy cohort study has a high response
rate. Nevertheless, random non-response may have reduced statistical
power. Our reported power calculations are on the basis of the
final sample size and take account of non-response, therefore our
positive conclusions remain valid. Any systematic non-response is most
likely to be determined by disease status and social class and this may
have biased estimated effects in either direction. But, the addition of
social class covariates to our models made little difference to point
estimates. Mothers were enrolled in midpregnancy, and selection bias
was unlikely in relation to key outcomes. Most dropout occurred early in the study and was unlikely to have been associated with the later
development of asthma or atopy. The study is often viewed as
representative of the general Western Australian
population.20 Nevertheless, recruitment was mainly through
a tertiary obstetric hospital and included a small excess of mothers
with preterm babies. Our models, however, include a covariate
reflecting preterm delivery, and confounding due to pregnancies at risk
should not have distorted our conclusions.
Protective effect of exclusive breast feeding
Our study provides evidence consistent with others
4 12 14
of a protective effect of exclusive breast feeding (
4 months) against a range of end points reflecting asthma and atopy. This protective effect may operate through several mechanisms. These include the exclusion of milk other than breast milk
(and its potentially allergenic components) from the infant's diet;
and the provision of immunomodulatory, anti-inflammatory, nutritional,
or other components in human milk.
9 23 24
Like others,14 we found that it was the age that other milk was
introduced rather than the duration of breast feeding that was more
closely associated with asthma or atopy at age 6 years. This favours
"exclusion" mechanisms. The two variables are, however, strongly
correlated, and we cannot definitively reject the possibility that it
is breast feeding itself that is of prime importance.
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Conclusions |
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Delaying the introduction of milk other than breast milk until at
least 4 months of age may protect against asthma and atopy later in
childhood. These findings are relevant to our understanding of the
cause of childhood asthma and also to public health. Although further
studies and analyses are required to confirm these benefits and to
understand better the mechanisms concerned, public health interventions
promoting an increased duration of exclusive breast feeding may help to
reduce the morbidity and prevalence of childhood asthma.
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Acknowledgments |
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Contributors: WHO developed the hypothesis, undertook statistical analyses, and wrote the main drafts of the paper; she will act as guarantor for the paper. PGH, PDS, AWR, and GEK were involved in the design and conduct of the key follow ups and assisted with the interpretation of the data. FJS and LIL were involved in the initial study design and coordinating the study follow ups. PRB was involved in the design and conduct of the key follow ups and supervised the statistical analyses and write up of the paper.
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Footnotes |
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Funding: WHO was supported by a research award from the Western Australian Health Promotion Foundation. The Western Australian pregnancy cohort study is funded by project and programme grants from the National Health and Medical Research Council of Australia, and GlaxoWellcome.
Competing interests: None declared.
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References |
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| 1. | Peat JK, Li J. Reversing the trend: reducing the prevalence of asthma. J Allergy Clin Immunol 1999; 103: 1-10[Medline]. |
| 2. | Robertson CF, Dalton MF, Peat JK, Haby MM, Bauman A, Kennedy JD, et al. Asthma and other atopic diseases in Australian children: Australian arm of the international study of asthma and allergy in childhood. Med J Australia 1998; 168: 434-438[Medline]. |
| 3. |
Martinez FD, Wright AL, Taussig LM, Holberg CJ, Halonen M, Morgan WJ.
Asthma and wheezing in the first six years of life.
New Engl J Med
1995;
332:
133-138 |
| 4. | Saarinen UM, Kajosaari M. Breastfeeding as prophylaxis against atopic disease: prospective follow-up study until 17 years old. Lancet 1995; 346: 1065-1069[Medline]. |
| 5. | Strachan DP. Hay fever, hygiene, and household size. BMJ 1989; 299: 1259-1260. |
| 6. | Martinez FD. Role of viral infections in the inception of asthma and allergies during childhood: could they be protective? Thorax 1994; 49: 1189-1191[Medline]. |
| 7. | Wold AE, Adlerberth I. Does breastfeeding affect the infant's immune responsiveness? Acta Paediatr 1998; 87: 19-22[Medline]. |
| 8. | Holt PG, Macaubas C. Development of long term tolerance versus sensitisation to environmental allergens during the perinatal period. Curr Opin Immunol 1997; 9: 782-787[Medline]. |
| 9. | Hanson LA. Breastfeeding provides passive and likely longlasting active immunity. [Review.] Ann Allergy, Asthma Immunol 1998; 81: 523-537[Medline]. |
| 10. | Pabst HF. Immunomodulation by breast-feeding. Pediatr Infect Dis J 1997; 16: 991-995[Medline]. |
| 11. | Golding J, Emmett PM, Rogers IS. Eczema, asthma and allergy. Early Hum Dev 1997; 49: 121-30S. |
| 12. |
Wilson AC, Stewart Forsyth J, Greene SA, Irvine L, Hau C, Howie PW.
Relation of infant diet to childhood health: seven year follow up of cohort of children in Dundee infant feeding study.
BMJ
1998;
316:
21-25 |
| 13. | Hide DW. The clinical expression of allergy in breastfed infants. Adv Exp Med Biol 1991; 310: 475-480[Medline]. |
| 14. | Tariq SM, Matthews SM, Hakim EA, Stevens M, Arshad SH, Hide DW. The prevalence of and risk factors for atopy in early childhood: a whole population birth cohort study. J Allergy Clin Immunol 1998; 101: 587-593[Medline]. |
| 15. | Juvonen P, Månsson M, Andersson C, Jakobsson I. Allergy development and macromolecular absorption in infants with different feeding regimens during the first three days of life. A three-year prospective follow-up. Acta Paediatr 1996; 85: 1047-1052[Medline]. |
| 16. | Strachan DP, Anderson HR, Johnston IDA. Breastfeeding as prophylaxis against atopic disease. [Letter.] Lancet 1995; 346: 1714[Medline]. |
| 17. | Halpern SR, Sellars WA, Johnson RB, Anderson DW, Saperstein S, Reisch JS. Development of childhood allergy in infants fed breast, soy, or cow milk. J Allergy Clin Immunol 1973; 51: 139-151[Medline]. |
| 18. | Wright AL, Holberg CJ, Taussig LM, Martinez FD. Relationship of infant feeding to recurrent wheezing at age 6 years. Arch Pediatr Adolesc Med 1995; 149: 758-763[Abstract]. |
| 19. | Kramer MS. Does breastfeeding help protect against atopic disease? Biology, methodology, and a golden jubilee of controversy. J Pediatr 1988; 112: 181-190[Medline]. |
| 20. | Newnham JP, Evans SF, Michael CA, Stanley FJ, Landau LI. Effects of frequent ultrasound during pregnancy: a randomised controlled trial. Lancet 1993; 342: 887-891[Medline]. |
| 21. | Skassa-Brociek W, Manderscheid JC, Michel FB, Bousquet J. Skin test reactivity to histamine from infancy to old age. J Allergy Clin Immunol 1987; 80: 711-716[Medline]. |
| 22. | McCullagh P, Nelder JA. Generalized linear models, 2nd ed. Oxford: Chapman and Hall, 1989. |
| 23. | Hamosh M. Protective functions of proteins and lipids in human milk. [Review.] Biol Neonate 1998; 74: 163-176[Medline]. |
| 24. | Xanthou M. Immune protection of human milk. Biol Neonate 1998; 74: 121-133[Medline]. |
(Accepted 13 July 1999)
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