The relationship between breastfeeding, asthma and atopy (idea)

The authors initiated their study with the hypothesis that breastfed children would display a lower incidence of asthma and atopy later in life.

The cohort

The 1037 children in the study were assessed for a number of conditions and their general health within a month of their date of birth at ages 3, 5, 7, 9, 11, 13, 15, 18, 21 and 26. They were accompanied by their parents during their assessments up to the age of 11, and were examined individually thereafter. Of course, not all children were present and tested for their respiratory condition at each year of investigation, but the representation was very good (from 71% for age 7 to 96% at age 26). Skin testing (for atopy) was conducted only at ages 13 and 21, again on a subset of the whole cohort (69% and 87% respectively).

Data Collection

At age 7, trained interviewers asked the parent whether the child suffered from asthma, hayfever, wheezing or other respiratory problems. During this same interview, the family history of asthma or respiratory problems was also recorded. At age 9 a more comprehensive questionnaire was introduced, wherein the frequency and occurrence of asthma or wheezing was recorded. At ages 18, 21 and 26 the participants completed questionnaires based on those from the American Thoracic Society and the International Union Against Tuberculosis and Lung Diseases. Those participants using bronchodilators were asked to withhold on the day of the study, and all individuals were tested using a spirometer. After an intial test in which the forced exhaled volume (FEV) and vital capacity were recorded, treatment with methacholine was performed and spirometry repeated. Presence of a respiratory problem or defect was recorded for the participant if their FEV was reduced by 20% or more following treatment with methacholine. In the case of children already showing signs of respiratory distress, the patients were treated instead with salbutamol and a problem or defect was recorded if their FEV improved by over 10%.

To test for the presence of atopy, skin prick tests were performed at ages 13 and 21. Participants were excluded from this trial if they had used antihistamines within the 2 days prior to the investigation. The allergens tested were dust mites, rye grass pollen, cat dander, dog dander, horse hair, kapok, cladosporium, Aspergillus fumigatus, Penicillium spp., Althernaria spp. and wool, with both a positive and negative control. After 15 minutes, an positive test was recorded if the pin prick produced a weal at least 2 mm larger than the negative control.

Statistical analyses were performed on these data to determine first whether there was any difference between the breastfed and non-breastfed children with respect to the occurrence of asthma and atopy. The initial tests used were simple chi-squared and likelihood ratio tests to compare the breastfed to non-breastfed groups. Later, multivariate methods were used in a step-forward manner in order to attempt to control for some confounding factors.

The results

At age 13, positive skin tests to cat, house dust mites, grass, alternaria or any other allergen were more likely to occur in the breastfed cohort. This same pattern was repeated in the tests conducted at age 21. In fact, the odds ratios (ie., the risk of showing a response in the breastfed group / risk in the non-breastfed group) ranged from 1.48 to 2.41 for these tests, meaning that breastfed children were as much as two and one-half times more likely to show an allergic response. The same pattern was repeated when examining the evidence of asthma or respiratory distress. At every age from 9 to 26, breastfed children were more likely to report having asthma than their counterparts. The odds ratios varied from 1.50 to 2.93 (three times!). The same pattern was uncovered when examining the respirometry results before and after treatment with methacholine or salbutamol: significant increases in the incidence of distress were observed in the breastfed cohort in every case, except for those children wheezing (not diagnosed as asthmatic) at age 15.

When the data were examined in a more intensive manner, it was found that any breastfeeding beyond the age of three weeks resulted in an increase in the risk of atopy at age 13. For example, while those children never breastfed or breastfed for fewer than four weeks had a 37% and 34% risk of displaying atopy to at least one of the tested allergens, those breastfed for 4-7, 8-11, 12-15, 16-25 and more than 26 weeks had 58%, 49%, 68%, 48% and 46% chances of showing such a response. This same pattern was found when testing for the incidence of atopy at 21 years or asthma at 9 years of age.

As should be expected, those children from families with a parental history of hayfever or asthma developed atopy or asthma at a greater rate than their counterparts, irrespective of whether they were breastfed. However, despite this family link the increased incidence in the breastfed group was still clearly visible and significant. Finally, in their multifactor analysis where they controlled for socioeconomic status, birth order, sheepskin use in infancy and the presence of asthma, atopy or smoking in/by the mother, all outcomes remained significant with the breastfed cohort again showing higher risk.

Discussion

The authors cite studies of Swedish, Californian, and Australian children who were breastfed had lower incidences of atopy at a young age, and in Kenya breastfeeding was related to a reduced severity of asthma, rather than incidence. A massive Brazilian study found lower rates of asthma in children breastfed for more than six months than those who were never breastfed. On the other hand, Tuscon, Italian and British children show that breastfeeding lead to increased risk of atopy and asthma.

It is important to note that while some the studies mentioned in the previous two paragraphs were undeniably massive in their undertaking, none can hold a torch to the Lancet study discussed here. All previous studies confined themselves to cases where the children were, or were not, genetically disposed to asthma and/or atopy. They did not attempt to control for smoking, the date of breast-feeding cessation, socioeconomic status and other factors. This study was superbly designed and produced results which are hard to deny.

To date the mechanism relating breastfeeding to asthma and atopy in children is poorly understood, which limits ability to make sense of these contradictory results. Without a causative mechanism, it is difficult to understand why one study produces one result while another produces something quite different. A study⁵ examining infants in Estonia and Sweden, countries with low and high prevalences of atopy, respectivley, found that one year old Estonian children had high densities of Lactobacilli and eubacteria in their guts, while Swedish children had high densities of clostridia instead. Those childern with lower lactobacilli densities and higher coliform densities were more likely to develop allergies. Thus, breastfeeding might affect the balance of the bacteria in a child's gut, giving way to future susceptibility to allergy.

Commentary and critique

Breastfeeding could be promoted for many reasons, including optimum nutrition and reduction of risk of infant infections. However, the role of breastfeeding in protection of children against atopy and asthma cannot be supported on the basis of the present balance of evidence.

I still think that breastfeeding should be encouraged and even pursued aggressively by the medical community, support groups and federal/state policies. It is cheaper, easier and better for the baby. However, it may not be the best nutritional course in every conceivable way. In the balance, by far better than formula. However, be aware that you might be increasing your child's risk of developing ashtma and/or allergies later in life.