Relationship between infant feeding and infectious illness: a prospective study of infants during the first year of life

Prior studies investigating the relationship between infant feeding and infectious illnesses in developed countries have provided conflicting data about whether breast-feeding protects against common infectious illnesses early in life. These conflicts may in part be due to the failure to consider the following methodologic issues: (1) collecting data prospectively at frequent intervals for active surveillance of the detection of infections and of feeding practices, (2) specifying what is meant by infectious illnesses and breast-feeding, (3) controlling for confounding variables such as social class or presence of siblings in the household, and (4) applying appropriate analytical strategies to a population in which both feeding and exposure to illness change over time. A total of 500 infants born consecutively in a university-affiliated community hospital in Copenhagen, Denmark, were studied prospectively for the first 12 months of life by means of a detailed, monthly, mailed questionnaire that focused on feeding practices and illnesses (overall response rate, 73%). The percentage of infants who were completely or mostly breast-fed decreased from 88% at 1 month to 20% at 12 months of age. After adjustment for major covariates, no statistically significant relationship was found between the type of infant feeding and the incidence of four categories of infectious illnesses: gastroenteritis, upper respiratory illness, otitis media, and lower respiratory illness. The adjusted incidence density ratio for gastroenteritis was 1.067 (95% confidence interval = 0.982, 1.226) and for upper respiratory illnesses 0.984 (95% confidence interval = 0.883, 1.096). These data suggest there is no substantial protective effect of breast-feeding against the occurrence of infectious illnesses early in life in a largely middle-class urban population in a developed country