Breastfeeding 'cuts childhood asthma risk'

“Breastfeeding DOES cut the risk of asthma, says landmark study of 250,000 babies over 30 years,” the Mail Online reports. A major review has found a link between breastfeeding and reduced childhood asthma rates.  

The headlines follow a large review of the link between breastfeeding and the risk of asthma in children from the general population. The researchers included 117 observational studies published between 1983 and 2012, which looked at whether a child had previously had asthma, or had experienced recent asthma or wheezing (in the previous 12 months).

The researchers then pooled the results from these studies and found that breastfeeding was associated with a reduced risk of childhood asthma. The strongest link was seen among children up to two years of age.

According to the Mail, the charity Asthma UK has welcomed the results, with a spokesperson reported as saying: “The review provides good evidence that children who are breastfed have a lower risk of developing asthma”.

The study is subject to a number of limitations, with many of the included studies judged as having a low quality methodology and were, arguably, subject to bias. Even with these uncertainties, breastfeeding has been conclusively proven to benefit both mother and child.

Where did the story come from?

The study was carried out by researchers from University Campus Suffolk and the University of Bern, in Switzerland. It was funded by the Swiss National Science Foundation. The study was published in the peer-reviewed journal American Journal of Epidemiology.

The story was picked up by the Mail Online, which has interpreted the study appropriately, despite having a headline that overstates the review’s findings.

What kind of research was this?

This was a systematic review and meta-analysis of studies that looked at breastfeeding and the risk of asthma outcomes in children from the general population.

A systematic review is the best way to summarise all of the existing research on a topic, as it rigorously searches for and analyses the best available studies. These types of reviews use pre-defined criteria that potential studies must meet to be included, such as appropriate study design, population, exposure and outcomes assessed.

They are considered to be the strongest type of evidence. However, the strength of its conclusions are dependent on the quality and homogeneity (sameness) of the studies that it pools together.

Systematic reviews often, but not always, use meta-analysis to combine the results of included studies. A meta-analysis is a statistical technique that combines the results of individual studies addressing the same research question to arrive at an overall measure of the effect of a treatment or an exposure. It is important that the studies being pooled are similar enough (homogeneous) to make the results meaningful.

What did the research involve?

The researchers reviewed studies examining breastfeeding and the risk of asthma in children from the general population. They included cohort, cross-sectional and case-control studies published between 1983 and 2012.

There were no randomised controlled trials (RCTs) of breastfeeding compared to not breastfeeding included, because the researchers say it is not ethical to withdraw breastfeeding from children.

Studies performed in specific populations, such as including children with a family history of asthma, were excluded because this was not considered to be representative of the general population.

To be included, studies had to have asthma reported by a parent or confirmed with a physician or medical diagnosis.

Outcomes of interest were:

• ever having asthma
• recent asthma (in the previous 12 months)
• recent wheezing illness (in the previous 12 months)

These outcomes were further categorised into ages of asthma assessment into: up to two years of age; three to six years of age; or seven or more years.

The researchers also extracted information on duration of any breastfeeding and duration of exclusive breastfeeding, categorised into:

• ever vs. never
• three to four months or more vs. less than three to four months
• six or more months vs. less than six months

Using statistical methods, the researchers pooled the results and estimated the odds ratio of the association of breastfeeding with the risk of each outcome.

What were the basic results?

117 studies were included in the review, identified from 1,464 studies. All but four of these were included in the pooled meta-analysis. Most of the included studies were cohorts (49%), followed by cross-sectional (40%) and case-control studies (11%).

Breastfeeding was analysed as the duration of any breastfeeding in 62% of studies and as the duration of exclusive breastfeeding in 35% of studies.

The main findings in the meta-analysis of the review were:

• an odds ratio [OR] of 0.78 (95% confidence interval [CI] 0.74 to 0.84) for studies analysing ever having asthma. This means that breastfeeding was associated with a 22% reduced odds of a child ever having asthma. This analysis included 75 studies.
• an OR of 0.76 (95% CI 0.67 to 0.86) for studies analysing recent asthma in the previous 12 months. This means that breastfeeding was associated with a 24% reduced odds of a child having asthma. This analysis included 46 studies.
• an OR of 0.81 (95% CI 0.76 to 0.87) for studies analysing recent wheezing illness in the previous 12 months. This means breastfeeding was associated with a 19% reduced odds of a child having a recent wheezing illness. This analysis included 94 studies.
• there was a reduced risk in all age groups, but the strongest predictive association was found among the age group of up to two years

The researchers reported they found no evidence of differences by study design, study quality or between studies in Western and non-Western countries.

How did the researchers interpret the results?

The evidence, according to the researchers, suggests that breastfeeding protects against the development of childhood asthma. They found the strongest link in children aged up to two years and that the strength of the link decreases with age.

Conclusion

Breastfeeding is known to provide numerous health benefits for both infants and the mother. This review provides evidence that breastfeeding is associated with a reduced risk of developing childhood asthma and that it may have a protective effect.

However, there are some limitations to the study:

• The overall quality of the included studies was rated by the researchers using standardised methods as “low quality”. They say this was especially due to insufficient adjustment for potential confounders.
• The studies were heterogeneous (meaning they differed considerably); however, the researchers say that results were similar when they limited the selection of studies to only cohorts or high-quality studies.
• All of the included studies were observational and therefore prone to bias.
• Conference papers and abstracts were excluded from the review, as were non-English papers, and the researchers state this may have introduced publication bias.

To be fair to the researchers, and the studies included in the review, the gold standard of evidence, RCTs, cannot (or at least should not) be carried out, as it would be unethical to deny babies the benefits of breast milk.

Proven benefits of breastfeeding include a reduced risk of infection for the baby and reduced risk of ovarian and breast cancer for the mother.