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ELECTRONIC ARTICLE |
From the Epidemiology Branch, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONREFERENCES |
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Methods. Nationally representative samples of 1204 infants who died between 28 days and 1 year from causes other than congenital anomaly or malignant tumor (cases of postneonatal death) and 7740 children who were still alive at 1 year (controls) were included. We calculated overall and cause-specific odds ratios for ever/never breastfeeding among all children, conducted race and birth weight–specific analyses, and looked for duration–response effects.
Results. Overall, children who were ever breastfed had 0.79 (95% confidence interval [CI]: 0.67–0.93) times the risk of never breastfed children for dying in the postneonatal period. Longer breastfeeding was associated with lower risk. Odds ratios by cause of death varied from 0.59 (95% CI: 0.38–0.94) for injuries to 0.84 (95% CI: 0.67–1.05) for sudden infant death syndrome.
Conclusions. Breastfeeding is associated with a reduction in risk for postneonatal death. This large data set allowed robust estimates and control of confounding, but the effects of breast milk and breastfeeding cannot be separated completely from other characteristics of the mother and child. Assuming causality, however, promoting breastfeeding has the potential to save or delay ~720 postneonatal deaths in the United States each year.
Key Words: breastfeeding • infant mortality • cause of death • risk • logistic models
Abbreviations: SIDS, sudden infant death syndrome • NMIHS, National Maternal and Infant Health Survey • OR, odds ratio • CI, confidence interval
In developing countries, breastfeeding protects against diarrhea1 and respiratory diseases,2 important causes of infant death.3–9 In contemporary developed countries, however, where infectious diseases account for a smaller portion of infant mortality,10 what effect, if any, breastfeeding has on mortality is not clear. There is a large literature on the benefits to the child and the mother of breastfeeding,11 but almost all contemporary US data concern morbidity or are of a specific cause of death, such as sudden infant death syndrome (SIDS).12–14 The only US study of all-cause mortality and feeding method since the introduction of modern infant formulas in the late 1950s is an analysis of the 1988 and 1995 cycles of the National Survey of Family Growth15; Forste et al reported that breastfed children had substantially lower risk of dying between 1 month and 1 year, but they did not attempt control of confounding beyond race and birth weight. In Great Britain in the 1970s, Carpenter et al16 found that the infants of mothers who declared an intention to breastfeed had lower mortality from "preventable" causes, largely infectious diseases, trauma, and SIDS, out to 2 years of age. Knowing whether breastfed children have a survival advantage is important in its own right. In addition, developing policy or recommendations concerning potentially lethal hazards from breastfeeding, such as exposure to human immunodeficiency virus17 or chemical carcinogens in milk,18 requires some estimate of the mortality benefit as well.
Studying the salutary effects of breastfeeding presents some widely recognized problems in inference. In addition to the control of confounding by parity, maternal age, birth weight, and other factors that are plausibly associated both with the decision to breastfeed and the welfare of the infant, there is a special problem with reverse causality. Because infants who are sick from birth may be unable to breastfeed and children who become ill later may stop, breastfeeding infants may seem healthier because illness, especially mortal illness, prevents breastfeeding rather than because breastfeeding prevents illness. The recommended methods for dealing with this problem are to exclude deaths that occur in the neonatal period and to assign feeding category by how the child was fed at some time before death occurred.19 In addition, infants who die from congenital anomalies or malignant tumors (~15% of all postneonatal deaths in the United States in the late 1980s) may have been unable to initiate breastfeeding,20,21 and it is unlikely that their deaths are preventable by breastfeeding and they thus should be excluded. These tactics do not exclude reverse causality completely, but they should minimize its effects.
We use 1988 US National Maternal and Infant Health Survey (NMIHS) data to analyze the association between breastfeeding and postneonatal death using a case-control approach. We do not consider neonatal death (a liveborn child who dies before 28 days), because breastfeeding information was not gathered on the children who died so young and most such deaths are attributable to preterm birth or congenital anomalies; we also excluded deaths from congenital anomaly or malignancy occurring in the postneonatal period.
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METHODS |
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1 year old at survey; controls), is shown in Fig 1.
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Causes of postneonatal death (International Classification of Diseases, Ninth Revision) were obtained from death certificates. For some analyses, we divided the deaths into 4 categories: infections, injuries, SIDS, and others (Table 1).
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We were interested in determining whether prolonged breastfeeding had greater effects. Because these are case-control data, however, we cannot simply put breastfeeding duration in the logistic model, because the opportunity for the case infants to breastfeed extends only to their age at death, whereas the controls can breastfeed for up to 1 year. So, unless the case infants died very late in the infancy (clearly not true in this study), their opportunity for prolonged breastfeeding was significantly compromised. We addressed this problem by doing an analysis using the model described above but limiting the case group to those who had survived 3 months or more and using 3 months of breastfeeding versus <3 months or none in place of the ever/never breastfed variable. This equalizes the opportunity to breastfeed at 3 months in the cases and controls, at a cost of reduced sample size among the cases. We then can compare the OR of ever breastfeeding with the OR of breastfeeding for 3 months or more.
We used SAS 8.2 (SAS, Inc, Cary, NC) for preliminary tabulation and descriptive analysis and, because of the oversampling of black and low birth weight infants, SUDAAN 8.0.2 (Research Triangle Institute, Research Triangle Park, NC) to reweight the sample for the overall estimates and to calculate the ORs and 95% confidence intervals (CIs) in the final models. SUDAAN adjustment gives an estimate of the number of people in the US population with a given characteristic in that year; it uses different weights depending on the degree to which a given group was oversampled by design. For example, on the basis of the sampling frequencies, the sample of 7740 live births represents 3 186 497 live births in 1988, and the sample of 1204 postneonatal deaths represents 9145 deaths as a result of causes other than malignancy or congenital anomaly.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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Is it plausible that breastfeeding protects not only against infectious disease mortality, through familiar immune enhancing mechanisms, but also against SIDS, accidental death, and others? Although a satisfactory mechanism has not yet been proposed, the protection from SIDS has been seen in several studies and is under investigation. For accidental death, Carpenter also observed lower risk, and although the association may represent something as simple as physical proximity, it deserves additional study, having been seen both in his data and in ours.
To some extent, the policy implications of demonstrating benefits of breastfeeding depend on whether the benefits will be achieved by persuading a mother to breastfeed when she otherwise might not have. Strictly, though, causality is difficult to demonstrate for any specific part of the interaction between the breastfeeding mother and her child. It may be that breastfeeding represents a package of skills, abilities, and emotional attachments that mark families whose infants survive and that it is these factors that produce the benefits seen, rather than breastfeeding or breast milk per se. We cannot randomize breastfeeding, although it is possible to randomize breast milk: Lucas et al23 conducted an ingenious study in which premature infants who were fed their mother's milk from a bottle did better on follow-up testing than children who were fed formula.
Reverse causality, produced by the motivation or enthusiasm that marks a healthier child who can breastfeed or by specific characteristics of the child's illness, such as cleft palate and breathlessness during sucking, that prevent breastfeeding might produce an artificial benefit of breastfeeding. Eliminating deaths in the first month and deaths from congenital anomaly or malignant tumor, where infants who are unable to breastfeed are concentrated, and using the initial feeding method to categorize feeding should diminish but perhaps not eliminate this problem. However, excluding these deaths also excludes the chance to examine whether breastfeeding has any effects on these deaths, especially those who are not fatally ill at birth. In a prospective study, it might be possible to include neonatal deaths if careful attention were paid to the reason that a child was breastfed or not. We do not have such data; however, we can eliminate from the analysis any child, case or control, who was admitted to the neonatal intensive care unit. This yields a similar but less precisely estimated OR of 0.83 (95% CI: 0.67–1.03).
The NMIHS data are from cases and controls and depend on interviews done after the child had survived or not. There thus is opportunity for recall bias, if women report their feeding methods differently depending on whether the child survived. To produce the results that we see would require substantial underreporting of breastfeeding by mothers of children who died, which does not seem likely. For the analysis of duration, the case control data do not allow direct estimation of a duration effect, because the cases and controls have different opportunities to breastfeed for longer periods. When we limit the analysis to cases who survived at least 3 months and look at the effect of 3 months or more of breastfeeding, however, we see an increase in the protective effect, consistent with the idea that longer breastfeeding is more protective.
If more US mothers can be persuaded to breastfeed and indeed it is breastfeeding that accounts for the benefits, then the United States might improve its poor ranking among industrialized countries for postneonatal death. In 1986, 2 years before these data were collected, the United States ranked 16th (3.6/1000) in postneonatal death, well below Finland (first; 1.8/1000) and Sweden (second; 2.0/1000).24 The US breastfeeding prevalence in 1986 was 57% at birth and 22% at 6 months,25 whereas in Finland and Sweden, the prevalence at 6 months then was still ~60% and 50%, respectively.26 Although the United States still trails the Nordic countries both in breastfeeding and in postneonatal mortality, the US rate of postneonatal death has fallen steadily between the late 1980s and now, and breastfeeding has increased. In 2001, 70% of mothers left the hospital breastfeeding, and 33% were still breastfeeding at 6 months.25 If we assume that the risk structure has not changed as the overall rates have fallen, then the overall postneonatal mortality rate, a weighted average of the rate among those who were breastfed and those who were not, consists of 70% of children who are breastfed when they leave the hospital and who have a rate of 2.1 per 1000, and 30% of children who are not breastfed and have a rate of 2.7. If all children were breastfed, then it should prevent 1.8 postneonatal deaths per 10 000 live births. Because there are ~4 million births per year,27 720 postneonatal deaths might be prevented or delayed each year at little cost or risk. The benefit would be concentrated among young, less educated mothers who participate in Women, Infants, and Children and now have a relatively low rate of breastfeeding. The case for breastfeeding is already very strong, but this benefit on such a basic outcome might still increase encouragement of and support for breastfeeding in US children.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Reprint requests to (W.J.R.) Epidemiology Branch, NIEHS, PO Box 12233, Mail Drop A3-02, Research Triangle Park, NC 27709. E-mail: rogan{at}niehs.nih.gov
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REFERENCES |
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This article has been cited by other articles:
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 J. B. Wolf Is Breast Really Best? Risk and Total Motherhood in the National Breastfeeding Awareness Campaign Journal of Health Politics Policy and Law, August 1, 2007; 32(4): 595 - 636. [Abstract] [PDF]
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C M Wright and A J R Waterston Relationships between paediatricians and infant formula milk companies. Arch. Dis. Child., May 1, 2006; 91(5): 383 - 385. [Full Text] [PDF]
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