Association between breast feeding and asthma in 6 year old children: findings of a prospective birth cohort study
BMJ 1999; 319 doi: https://doi.org/10.1136/bmj.319.7213.815 (Published 25 September 1999) Cite this as: BMJ 1999;319:815
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Editor - In their study of the association between breastfeeding and
childhood asthma and atopy, Oddy et al referred to several factors as
confounders.1 We believe that two of them warrant further attention - male
sex and parental history of asthma.
When sex of the child was introduced in the study model as a
confounding variable, a positive association was found between male sex
and asthma or atopy. However, no evidence was provided by the authors
regarding the association of child's sex and his or her chances of being
breast-fed. Our examination of the literature revealed that sex has
little2 or no3 influence on the duration of breastfeeding. A confounding
variable is defined as "a variable that can cause or prevent the outcome
of interest, is not an intermediate variable, and is associated with the
factor under investigation".4 Since child's sex is apparently unrelated to
breastfeeding, this variable should not have been considered a possible
confounder in the analysis.
Parental history of asthma or atopy has been described as a risk factor
for childhood asthma or atopy through a combination of genetic and
environmental influences.5 Although not a confounder per se (as no
association has been described between parental atopy and breastfeeding
practices), the substantially higher risk of atopy among children of
atopic parents warrants a separate assessment of the protective effect of
breastfeeding.
1. Oddy WH, Holt PG, Sly PD, Read AW, Landau LI, Stanley FJ, Kendall
GE, Burton PR. Association between breast feeding and asthma in 6 year old
children: findings of a prospective birth cohort study. BMJ 1999;319:815-
819.
2. Pande H, Unwin C, Haheim LL. Factors associated with the duration of
breastfeeding: analysis of the primary and secondary responders to a self-
completed questionnaire. Acta Paediatr 1997;86:173-177.
3. Hornell A, Aarts C, Kylberg E, Hofvander Y, Gebre-Medhin M.
Breastfeeding patterns in exclusively breastfed infants: a longitudinal
prospective study in Uppsala, Sweden. Acta Paediatr 1999;88:203-211.
4. Last JM. A dictionary of epidemiology. 3d ed. Oxford University Press
1995. p.35.
5. Borish L. Genetics of allergy and asthma. Ann Allergy Asthma Immunol
1999;82:413-426.
Michael Gdalevich, MD, MPH
Epidemiologist
Schneider Children's Medical Center of Israel
Daniel Mimouni, MD
Physician
Rabin Medical Center,
Petah Tikva,
Israel
Marc Mimouni, MD
Professor
Schneider Children's Medical Center of Israel
Competing interests: No competing interests
The results of the study by W H Oddy and colleagues confirm that
factors present in the early months of life are critical regarding the
risk of asthma in childhood (1). Besides the duration of exclusive
breastfeeding and the age that solids were introduced, the authors looked
at maternal age, older silblings, the percentage of expected birth weight,
smoking in pregnancy, maternal education and family income.
Further epidemiological studies of asthma in childhood should
investigate the immunization status. We analysed criteria of health in a
population of 446 children (mean age 8 years) that was homogeneous in
terms of infant feeding. All the children had been breastfed more than a
year and had received only breast milk during the first 6 months. None of
them had BCG in infancy. In this particular population pertussis
vaccination appeared as a significant risk factor for asthma in childhood
(2).
References:
1- Oddy W H, Holt P G, Sly P D, et al. Association between
breastfeeding and asthma in 6 year old children: findings of a prospective
birth cohort study. BMJ 1999; 319: 815-9.
2- Odent M R, Culpin E E, Kimmel T. Pertussis Vaccination and Asthma:
Is there a link? JAMA 1994; 272: 592-3.
Competing interest: none.
Competing interests: No competing interests
Definition of exclusive breast feeding is important
EDITOR - Oddy et al found a significant reduction in the risk of
childhood asthma at age 6 years when exclusive breastfeeding (EBF) is
continued for at least 4 months after birth (1). The authors defined the
duration of EBF as "child's age when other milk was introduced". In 1991,
the WHO proposed standard definitions of breastfeeding to allow
comparisons among studies (2). In the WHO's definition EBF requires that
the infant receive breastmilk only and nothing else (no other food or
fluid, except for drops or syrups of vitamins, minerals, and drugs). The
definition used by Oddy et al. excludes other milk, but does not exclude
the possibility that their infants received other food or fluids, and
among them some may be allergenic. This bias undermines the accuracy of
the data, and makes the results more difficult to interpret and non
comparables with those of other studies.
Sergio Conti Nibali, Family Pediatrician, Messina (Italy)
serconti@glauco.it
1. Oddy WH, Holt PG, Sly PD, Read AW, Landau LI, Stanley FJ, Kendall
GE, Burton PR. Association between breast feeding and asthma in 6 year old
children: findings of a prospective birth cohort study. BMJ 1999;319:815-
819
2. WHO. Division of diarhoeal and acute respiratory disease control.
Indicators for asse ssingbreastfeeding practices. Geneva: WHO, 1991.
Competing interests: No competing interests