This study assessed the relationship between duration of breastfeeding and lung function in a well characterised whole population birth cohort at the Isle of Wight, UK. Children who were breastfed for at least 4 months were found to have a significantly better lung function profile at age 10 years compared with those who were not breastfed. Although there was a trend of higher FEV1 and higher PEF with increasing duration of breastfeeding, a statistically significant increase in lung volume was not seen in children who were breastfed for shorter durations (less than 4 months). Our data also showed no evidence of effect modification by maternal history of asthma and allergy.
The likelihood that our results are caused by selection bias is minimised by the fact that all births were eligible, and 1456 of 1536 eligible children (94.8%) agreed to participate. In addition, 94.1% (1370/1456) of the birth cohort participated at the 10 year follow-up. We compared our analytic sample with the total cohort available at age 10 years for evidence of selection bias but found no such evidence (). The prospective nature of the data collection (breastfeeding information was ascertained at ages 1 and 2 years) and the fact that the primary outcome variable was assessed instrumentally at the clinic at the 10 year visit, combine to reduce information bias (reporting bias). These factors increase both the internal and external validity of this study.
When testing for interaction, we used maternal history of asthma and allergy from standard questionnaires. We did not find any effect modification of breastfeeding by maternal history of asthma or allergy. This finding is in contrast with that of Guilbert and colleagues.22
Although there was no information on the test for interaction effect in their report, Guilbert et al
stratified their sample by maternal asthma and atopy and found a differential effect of the relation of breastfeeding to lung function depending on maternal asthmatic background. Of note, however, we classified maternal asthma and allergy using historical questionnaire information, because of the absence of clinical testing in the mothers. While this approach may not completely capture the effect of maternal asthma and atopy, the internal validity of the study is maintained. This approach limits the interpretation of our findings to historical ascertainment of asthma and allergy from questionnaire data, and not necessarily to parental atopy.
Our results showed a significant increase in FVC in children who were breastfed for longer than 4 months (, model 1). Including FVC in the explanatory model for FEV1
is comparable to the division of FEV1
by FVC (, model 1). The method of adjustment we used in model 2 has been applied in other studies and was shown to produce consistent results with minimal bias.29
The loss of statistical significance for breastfeeding duration on FEV1
after adjustment for FVC supports the notion that FVC may play a mediating role (breastfeeding → increased lung capacity → increased FEV1
). Thus the association of breastfeeding with FVC seems to explain the increase in FEV1
. Therefore, we believe that the effect of breastfeeding in our data is because of an increase in lung volume. Nevertheless, even after including FVC in the model, duration of breastfeeding was still associated with PEF, suggesting that breastfeeding may have some protective effect against airflow obstruction.
Unlike Guilbert and colleagues22
we did not find any effect of breastfeeding duration on the FEV1
/FVC ratio. This may be due to the fact that an effect of similar strength acted on both FEV1
and FVC in our population. In other words, the primary effect is on lung capacity and secondarily on FEV1
. If so, the use of a ratio would nullify the effect of breastfeeding. As the FEV1
/FVC ratio was not affected, our findings support an effect of prolonged breastfeeding on increased lung volumes, but no effect of breastfeeding duration on airway obstruction.
The results of previous studies suggest that factors in human breast milk, such as cytokines and chemokines, are responsible for the beneficial effects of breastfeeding on asthma and other allergic disorders.7 8
This view is supported by a recent report by Snijders et al30
who found that longer duration of breastfeeding was associated with a reduced risk for eczema when the mothers were non-allergic and non-asthmatic. They concluded that maternal allergic status was the major driver of this protective effect.30
Similar immune mechanisms may act by remodelling the airways and may explain the protective effect of breastfeeding duration on PEF.
While immunoactive factors are important, the effect of breastfeeding on the respiratory system may be a result of complex and interlinked mechanisms that may be accounted for by several variables. It has been demonstrated that during breastfeeding, for the 75 s prior to milk ejection, there is a negative pressure of up to 98 mm Hg, which is about three times higher than the pressure developed during bottle feeding.31
Sucking exercises during breastfeeding (an average of 8 min) is nearly twice as long as bottle feeding duration (4.4 min). In addition, bottle feeding has a higher rate of swallowing, more frequent interruption of breathing and decreased ventilatory efforts.32-34
Hence mechanical factors related to suckling may explain the differences in lung volume seen in breastfed infants compared with those who were not breastfed. For lung function, it may be that the physical exercise caused by suckling at the breast, about six times daily on average (for more than 4 months), may result in increased elasticity and efficiency of the lung parenchyma, resulting in increased lung capacity and increased airflow in breastfed children compared with bottle-fed children. This interpretation is supported by our finding that FVC (and only indirectly FEV1
) was higher in breastfed children.
Hence two explanatory mechanisms may compete. Firstly, prolonged suckling at the breast compared with the bottle may convey a mechanical stimulus that results in improved mechanics of ventilation (due to physical training). Secondly, the presence of protective immunoactive factors in breast milk may reduce allergic susceptibility of the lung tissue. This differentiation is relevant for future efforts to promote better lung growth. Pumping and subsequently bottle-feeding (indirect feeding) may eliminate the protective effects of breastfeeding, thus explaining the negative findings in some studies,16 21 35
especially in countries with minimal maternity leave periods. This mechanical explanation necessitates the collection of data on the method of breast milk administration employed in different studies. In addition, if suckling is essential and the newborn cannot be breastfed, then the design of the feeding bottle is important. For instance, an artificial increase of flow “resistance” from infant feeding bottles may be beneficial. On the other hand, if immune factors are essential, it would be important to investigate which constituents of breast milk provide the most protection.
In conclusion, this paper adds to the evidence supporting the promotion of prolonged breastfeeding for the improvement of lung volume in late childhood. In addition to repeating this analysis in other populations, future studies should assess the biological mechanisms that drive this phenomenon, in order to promote normal lung growth and development.