Gender Differences in Breastfed Preterms

New Study Suggests Preterm Boys May Not Benefit From Breastfeeding as much as Preterm Girls

A study in June 2008 Pediatrics titled, “Differential Gender Response to Respiratory Infections and to the Protective Effect of Breast Milk in Preterm Infants” suggests that preterm boy infants with respiratory infections may not benefit from breast milk as much as preterm girls.  The study, which followed 119 high-risk, very low birth weight infants in Argentina for the first year of life, found that breast milk significantly protected girls (n=55) but not boys (n=64) against severe acute lung disease; respiratory infections were most severe in girls who were not breastfed.  However, there was no difference in the number of infections between the boys and girls, regardless of breastfeeding status.   

Although this study appears to have been performed according to high study management standards, several limitations exist.  First, there are some limitations with the statistics.  The sample size is very small; no a priori calculation of the number of subjects needed to show a significant difference was made to convince the reader that the sample size is large enough to detect significant differences.  Additionally, this study examines four groups, breastfed and non-breastfed girls and boys; baseline demographics were only provided for only two groups (boys and girls) so that it could not be determined if differences among the four groups existed at baseline.

Second, there are some nutritional limitations to this study.  The definition of breastfeeding in this study was vague, as it included exclusive breastfeeding (n=4) and any breastfeeding but this amount was not quantified (frequency, duration, etc).  Furthermore, no information was presented as to what other types of foods the breastfeeding group was receiving (preterm infants often receive supplemental sources of nutrition); further, there was no mention of infant formula feeding.  Conclusions about infant formula, therefore, cannot be made.  Likewise, due to the incomplete characterization of breastfeeding, making firm conclusions about the effects of breast milk per se is not possible.  The duration of breastfeeding is not presented in relation to the timing of respiratory disease onset; therefore, one cannot tell if the breastfed subjects were receiving breast milk at the time of respiratory infections and what the effect might be. 

Another limitation is that the infants had very low birth weights and thus represent a very specialized subgroup of infants with specific nutritional needs.  The generalizability of the study’s findings to term infants, as has been reported in the media recently,  is therefore questionable. 

The authors conclude that “these findings suggest that breast milk protection is not universally conferred.”  However, more studies are needed to determine if gender differences really do exist for respiratory infections and what those differences are.  The results of this study should not change the current advice that breastfeeding is the ideal source of infant nutrition.

*Klein, M. I., Bergel, E., Gibbons, L. et al.  Differential Gender Response to Respiratory Infections and to the Protective  Effect of Breast Milk in Preterm Infants.  Pediatrics 2008; 121; e1510-e1516.