We read with great interest the letter by Wjst proposing an alternative hypothesis for margarine supplementation by vitamin D as the risk factor underlying its positive associations with asthma and allergies. In our recently published study (Thorax 2007;62:677–83), we found that margarine intake was a risk factor for asthma and rhinitis symptoms among children in Crete. We hypothesised that the high concentration of n-6 PUFAs in margarine could partially explain the observed associations through modulation of the synthesis of IgE and inflammatory mediators.1

The evidence regarding the effects of vitamin D on the development of asthma and allergies is controversial. Vitamin D has been shown to inhibit Th1 immune responses but its effects on Th2 responses are more complex and not fully elucidated. Moreover, genetic studies have provided early, although not clear, evidence of the potential role of gene polymorphisms of vitamin D receptors in asthma and allergies.2^–5 On the other hand, recent epidemiological data from birth cohort studies have shown beneficial associations between maternal vitamin D intake during pregnancy and early childhood asthma symptoms.6^–9

In Crete, the basic factor affecting vitamin D is probably exposure to sunlight. Margarine is fortified with vitamin D but is not commonly used among children, and olive oil remains the principal source of fat intake. A randomised clinical trial could provide strong evidence on the effect of fortified margarine intake in childhood allergies. We think, however, that there is an increasing need for more observational studies on this controversial field before embarking on a long term intervention study. We have recently started a mother–child cohort in Crete (Rhea study; http://rhea.med.uoc.gr/) aiming, among other issues, to investigate the role of early life dietary exposures on the development of asthma and allergies. We hope to answer some of the questions on the role of vitamin D in the development of allergic disorders.