Does breastfeeding really save lives, or are apparent benefits due to biases?

Analysis of mothers' recall data collected in 1976-1977 by a probability survey in Peninsular Malaysia shows an association between breastfeeding up to six months of age and improved survival of infants throughout the first year of life. Inappropriate sample selection and inadequate control of confounding can introduce large biases in these analyses. The magnitude and direction of these biases are presented. Even when these biases are dealt with, unsupplemented breastfeeding appears more beneficial than supplemented breastfeeding. The younger the infant and the longer the breastfeeding, the greater the estimated benefits in terms of deaths averted. The use of powdered infant formula did not appear to offset the detrimental effects of early weaning and supplementation. The positive relationships found in these analyses between breastfeeding and survival are not due to death precluding or terminating breastfeeding. Nor are they likely to be due to a shift away from breastfeeding because of recent illness, which was also controlled in the analyses. Nor are they likely to be due to other factors that both increase mortality risk and shorten breastfeeding; when such factors are taken into account, the beneficial effects of breastfeeding become stronger and imply that, if there had been no breastfeeding in this sample, twice as many babies would have died after the first week of life.