PEDIATRICS Vol. 99 No. 6 June 1997,
p. e5
Copyright ©1997 by the American Academy of Pediatrics
ELECTRONIC ARTICLE:
A Longitudinal Analysis of Infant Morbidity and the Extent of
Breastfeeding in the United States
, and
From the * Epidemic Intelligence Service, Epidemiology Program
Office and Division of Nutrition and Physical Activity, National Center
for Chronic Disease Prevention and Health Promotion, Centers for
Disease Control and Prevention, Atlanta, Georgia;
Division of
Nutrition and Physical Activity, National Center for Chronic Disease
Prevention and Health Promotion, Centers for Disease Control and
Prevention, Atlanta, Georgia; and the § Office of Scientific Analysis
and Support, Center for Food Safety and Applied Nutrition, Food and
Drug Administration, Washington, DC.
Background. Studies on the health benefits of breastfeeding in developed countries have shown conflicting results. These studies often fail to account for confounding, reverse causality, and dose-response effects. We addressed these issues in analyzing longitudinal data to determine if breastfeeding protects US infants from developing diarrhea and ear infections.
Methods. Mothers participating in a mail panel provided information on their infants at ages 2, 3, 4, 5, 6, and 7 months. Infants were classified as exclusively breastfed; high, middle, or low mixed breast- and formula-fed; or exclusively formula-fed. Diarrhea and ear infection diagnoses were based on mothers' reports. Infant age and gender; other liquid and solid intake; maternal education, occupation, and smoking; household size; family income; and day care use were adjusted for in the full models.
Results. The risk of developing either diarrhea or ear infection increased as the amount of breast milk an infant received decreased. In the full models, the risk for diarrhea remained significant only in infants who received no breast milk compared with those who received only breast milk (odds ratio = 1.8); the risk for ear infection remained significant in the low mixed feeding group (odds ratio = 1.6) and among infants receiving no breast milk compared with those who received only breast milk (odds ratio = 1.7).
Conclusions. Breastfeeding protects US infants against the development of diarrhea and ear infection. Breastfeeding does not have to be exclusive to confer this benefit. In fact, protection is afforded in a dose-response manner. The more breast milk an infant receives in the first 6 months of life, the less likely that he or she will develop diarrhea or ear infection. longitudinal analysis, diarrhea, ear infection, breastfeeding.
Does breastfeeding protect against infection? In developing countries, the answer clearly is yes.1 However, in industrialized countries the findings are mixed. Numerous nonprospective studies of industrialized populations in the literature examine the association between breastfeeding and infectious disease; many report a protective effect5 whereas others report little or no effect at all.13,14 Prospective studies show similarly conflicting findings.15 A 1986 review of the literature concluded that breastfeeding "has at most a minimal protective effect [against infection] in industrialized countries."26
The conflicting results have been attributed to flawed methodologies.20,23,26 Specific criticisms include failure to (1) collect data prospectively at frequent intervals; (2) control for known confounding variables such as day care, infant age, and indicators of socioeconomic status; (3) clearly define feeding and outcome variables; (4) apply appropriate statistical strategies to a population in which both feeding and exposure to infectious agents change over time; and (5) account for small samples with the potential for unspecified biases.
The objective of our study was to determine if breastfeeding protects US infants against the development of diarrhea and ear infection. If the answer was affirmative, we also wanted to know whether the breastfeeding has to be exclusive to confer these benefits.
Sampling Frame
The Infant Feeding Practices Study was a panel study of US mother-infant pairs followed from late pregnancy through the infant's first year. The Food and Drug Administration conducted the study between 1993 and 1994. The agency used as the sampling frame a consumer mail panel which yielded a set of households in which someone had agreed to respond to questionnaires. The panel included approximately 500 000 households, and the contractors attempted to maintain (compared with US Census data) representativeness on five characteristics: geographic region, annual income, population density, household size, and age. Members of the panel were recruited in a variety of ways: recommendation from other panel members, notices in places likely to attract the type of people needed, and lists compiled by companies who specialize in finding particular types of people. Prenatal intake questionnaires were sent to 3155 households identified as including a pregnant women. All infants enrolled in this study were born between March and October 1993.Exclusion Criteria
Subjects were considered ineligible for the study at the time of the prenatal questionnaire if the expected due date was more than 3 months away. After the prenatal questionnaire was administered, subjects were ineligible for the study if the infant weighed less than 5 pounds at birth, there were multiple infants, medical problems prevented the mother from feeding her infant for more than 1 week, the infant stayed in the intensive care unit for more than 3 days, the infant had medical problems that affected feeding, the mother or infant died at any time during the data collection period, or the infant was born too early for the neonatal questionnaire to be administered on time. Five hundred forty women were ineligible, yielding a sample base of 2615 mother-infant pairs.Data Collection
The Infant Feeding Practices Study was a series of 11 questionnaires administered by mail beginning when the mother was approximately 6 months pregnant and continuing until the infant was 1 year old. Infant feeding and health status information were collected at months 2, 3, 4, 5, 6, and 7 (ie, when the infant was 2, 3, 4, 5, 6, and 7 months old). Information on smoking status and day care use was collected at months 3 and 6. An additional demographic questionnaire collected information on education, employment, household size, and income.Nonresponse
A woman was considered a nonrespondent if she failed to complete the first (prenatal) or second (birth screener) questionnaire or if she failed to complete at least one of the first two questionnaires sent after the infant's birth (n = 812). We did not eliminate a mother from the study for failure to complete a subsequent questionnaire. The response rate was 69% (1803/2615).Population Characteristics
To better understand the characteristics of our sample, we compared them with a nationally representative population of mothers participating in the National Maternal and Infant Health Survey.31 In comparison, our cohort of mothers was more likely to belong to middle- and upper-income groups; to be older, white, and married; and to have taken a prenatal class. They were also less likely to drink alcohol or smoke.Classification of Predictor and Outcome Variables
Each month, the mother reported the number of times over the past 7 days that her infant received breast milk, formula, or both. Using this information, we created five feeding categories based on the percentage of breastfeedings an infant received: breast milk only (100% of feedings were breast milk), high mixed (89% to 99% of feedings were breast milk), middle mixed (58% to 88% of feedings were breast milk), low mixed (1% to 57% of feedings were breast milk), and formula only (0% of feedings were breast milk). The cutoffs for the mixed feeding categories (ie, high, middle, and low) were defined by dividing the mixed-feeding group into terciles. Table 1 lists the sample size for each breastfeeding group and the infant's age in months.|
Table 1. Sample Size by Age of Child and Feeding Category in the Preceding Month |
Confounding Variables
Each month, the mother reported the number of times in the past 7 days her infant received other liquids (cow's milk, other milk, other dairy products, and fruit juice) or solids (baby cereal, other cereals, fruit, vegetables, meat, peanut butter, eggs, and sweets). All analyses, including the crude models, were adjusted for whether or not the infant received in the previous month any other liquids and any solids.a The infant's intake of other liquids and solids was adjusted separately to allow us to examine the formula-breast milk differential independent of any other intake the infant received. We also adjusted all models for the infant's age as a categorical variable.
Table 2.
Confounding Variables Used in Multivariate Analysis
Analysis
We linked infection in any given month with feeding for the preceding month. Thus, infection in month 3 was linked to feeding in month 2; infection in month 4 was linked to feeding in month 3, and so on. This lagged association allowed us to rule out the possibility of reverse causality (ie, the type of milk given to an infant changes in response to illness rather than the illness results from the type of milk given) by ensuring that a reported illness occurred after a specified type of feeding. This information then was aggregated across all the groups so we could examine the effect of the previous month's feeding on diarrhea and ear infection.An analysis of our baseline data indicated that anywhere from 5.4% to 11.4% of our infants reportedly experienced diarrhea between 2 and 7 months of age; 6.8% to 13.2% experienced an ear infection (Table 3). In the crude model, the association between the percentage of breastfeedings and the development of diarrhea showed a dose-response effect (Fig 1). Specifically, when each feeding group was compared with infants who received breast milk only (referent), there was a small but steady increase in the risk of developing diarrhea as the amount of breast milk an infant received decreased. This was significant among low mixed (P = .02) and formula only (P < .001) infants. When we adjusted this model for confounders, the dose-response remained apparent, although its magnitude diminished and the effect remained significant only among infants receiving no breast milk (P < .001).
|
Table 3. Percentage of Infants 2 to 7 Months of Age Reported as Experiencing Diarrhea or Ear Infection, by Feeding Category in the Preceding Month |
Association between diarrhea and feeding in preceding 1 month.
Fig. 1. Asterisks indicate P < .05 relative to breast-milk-only group. Crude analyses takes into account infant's age and intake of other liquids and solids for the preceding month. Adjusted analyses take into account infant's age, gender, and intake of other liquids and solids the previous month; maternal education, occupation, and smoking pattern; household size and income; and day care use.[View Larger Version of this Image (20K GIF file)]
Association between ear infection and feeding in preceding 1 month.
[View Larger Version of this Image (21K GIF file)]
We conclude that breastfeeding protects US infants against the
development of diarrhea and ear infection. Exclusive breastfeeding is
not crucial to confer this benefit. In fact, protection is afforded in
a dose-response manner.
a Additional analyses controlling for the number of feedings of liquids or solids did not substantively alter our results.
Received for publication Sept 5, 1996; accepted Dec 23, 1996.
Reprint requests to (P.S.) Centers for Disease Control and Prevention, Mailstop K-25, Division of Nutrition and Physical Activity, 4770 Buford Highway, Atlanta, GA 30341-3724.
The authors thank Drs David Kleinbaum and Amrik Shah for sharing their expertise and advice concerning analyses of longitudinal data. We also thank Dr Kelley Scanlon for her careful review of this manuscript.
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Pediatrics (ISSN 0031 4005). Copyright ©1997 by the American Academy of Pediatrics
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