Preventing Development of Asthma

Factors associated with an increased incidence of asthma

Several factors associated with development of asthma have been identified. However, it remains uncertain whether manipulation of these factors can reduce the onset and duration of asthma.

1 Genetics

• a family history of atopy or asthma¹²
• a personal history of atopy or other allergic disease such as atopic dermatitis^{33, 34}
• ongoing asthma at 6 years of age is associated with the presence of atopy and a maternal history of asthma.¹²

2 Allergen sensitisation and exposure

Many studies have shown that sensitisation to environmental allergens is strongly associated with childhood asthma.^119-124 However the potential benefit of allergen avoidance in prevention of asthma is unproven.

• House dust mite avoidance, either alone or in combination with a hypoallergenic diet in breastfeeding mothers and infants, may reduce wheezing illness in the first year of life but does not affect asthma at 3 years of age. This suggests that these measures delay rather than prevent the development of asthma.^125-128
• There are conflicting findings regarding the relationship between pet exposure and development of asthma. Some studies have suggested that increased exposure to cats and dogs may be associated with reduced sensitisation and asthma,^{127, 129-131} whilst other studies have not found this to be the case.^{127, 132-135}

3 Smoking

Exposure to cigarette smoke and maternal smoking during pregnancy significantly increases the risk of wheezing illness in early childhood but probably does not increase the risk of atopic asthma later in life.^{12, 136, 137}

Factors associated with a reduced incidence of asthma

1 Breast feeding and diet

• Exclusive breastfeeding for 6 months led to reduced incidence of asthma up to 5 years of age and the incidence of allergic disease including asthma for up to 17 years of age.^{138, 139} However, a recent study has questioned these findings.¹⁴⁰
• Use of partially or extensively hydrolysed hypoallergenic formulas as compared with standard cow’s milk or soy formula was associated with a reduced incidence of atopy and allergic disease (including asthma) for up to 5 years.^139-144
• Diets high in omega-3 fatty acids found in fish oils are associated with a reduced incidence of asthma¹⁴⁵
• There is no evidence that dietary elimination during the third trimester of pregnancy is beneficial for the prevention of allergic disease. It may even result in an increased incidence of allergic disease ^{146, 147}

•  There is no evidence that elimination diet during lactation is of benefit for prevention of asthma.¹⁴⁸

2 Viral and bacterial infections

Study findings suggest that an increased incidence of infections in infancy may protect against the development of asthma.  The ‘hygiene hypothesis’ has been proposed to explain this.  The hypothesis suggests that reduced exposure to infections results in persistence of ‘pro-allergy’ immune responses that in turn lead to an increased susceptibility to allergic disease.^{149, 150}

3 Exposure to farm environments

Studies have shown that early exposure to farming environments and farm animals may prevent development of allergic disease including asthma. However, further studies are required before conclusions can be made.^151-155

4 Gut flora and probiotics

In a prospective, randomised, placebo controlled study, Lactobacillus GG given to pregnant women during gestation and to their infant offspring for the first 6 months of life protected against the development of atopic eczema in the first year of life.¹⁵⁶ The effect on development of asthma is yet to be shown.

Current suggestions

The following suggestions for preventing development of asthma are based on a summary of the evidence above.

It is important to note that even if these suggestions are carried out, there is still a significant risk that high risk infants will develop asthma.

1 Breastfeed exclusively for the first 6 months of life.

If a mother is unable or does not wish to breastfeed, use a partially hydrolysed formula in the first 6 months of life. These are available from most pharmacies and supermarkets.

2 Avoid smoking and exposure to smoke during pregnancy and infancy.

House dust mite avoidance measures in infants may be considered. However, it is unlikely this will influence wheezing or childhood asthma beyond the first year of life.

No recommendations can be made regarding exposure to pets or farm animals, dietary restrictions in pregnancy or lactation and probiotics for preventing the development of asthma at this time, as study findings are inconclusive.