Pre-school children between the ages of 3 and 5 years were included in the study. The researchers deliberately chose two rural cities in Australia, one with a humid climate close to the coast and the other with a dry inland climate, to reflect the different types of prevalent allergens.
Parents of almost 1000 children completed a questionnaire, which included questions on asthma diagnosis, symptoms, and medicine in the preceding 12 months for their children, number of children in the family, whether the child was breastfed, and consumption of dietary polyunsaturated fats. Six hundred and fifty of these children also took an allergen (skin prick) test to assess their response to common allergens including dust mite, egg, cow's milk, and rye grass.
Around one in five children had asthma. A child with an allergic response to one or more components of the skin prick test was almost 2.5 times as likely to have recent asthma as children with a negative test. A parent with asthma doubled the risk and a serious respiratory infection before the age of 2 increased the risk by 93 per cent. But a diet high in polyunsaturated fat, equating to margarine usually spread on bread and foods regularly fried in polyunsaturated vegetable oils more than doubled the risk, accounting for up to 17 per cent of the cases studied.
Three or more older siblings and breastfeeding during the first days or weeks of life conferred protection. Not breastfeeding could account for 16 per cent of the cases, the authors suggest.
The authors suggest that high polyunsaturated fat consumption increases the levels of omega-6 fatty acid, which promotes the production of chemicals involved in inflammation. An increase in omega-6 means less omega-3 fatty acid, which inhibits inflammation.
The authors suggest that breastfeeding and polyunsaturated fat in the diet are modifiable factors, which, if changed, might make a substantial difference to the rising number of asthma cases.
Dr Michelle Haby, Centre for Community Child Health, Royal Children's Hospital, Melbourne, Australia.