A couple attend for preconception counselling to discuss approaches to reducing the risk of allergy in their future offspring. They already have one child who has a history of severe allergy to peanuts, who has so far experienced two episodes of life-threatening anaphylaxis. How should you advise them?
Geographical variations in the risk of developing allergies, and data showing marked increases in the prevalence of a number of allergic conditions clearly point to the importance of environmental factors in mediating risk of developing allergic problems.
Genetics are also important. If one first-degree relative (parent or sibling) is affected with an allergic disorder, the risk of the child developing an allergic problem is 50 per cent; if more than one first-degree relative is affected, the risk increases to 75 per cent.
Epidemiological studies have shown that most people who develop allergic problems first manifest symptoms in early life. Therefore, interventions before conception, during pregnancy and in early infancy are theoretically most likely to be effective.
However, there is currently no evidence to support measures taken before conception. General advice on ensuring that the mother is rubella immune, does not smoke, moderates alcohol consumption and is taking folic acid should always be given.
There is some evidence that reducing exposure to tobacco smoke, both in pregnancy and early life, can result in a reduction in the risk of infants developing a wheeze.
There is evidence from a randomised control trial that probiotics taken during the latter stages of pregnancy and continued during the first few months of life can substantially reduce the risk of infants developing atopic eczema.
The Food Standards Agency's Committee on Toxicology advises that women who are deemed high risk for allergic disorders should consider eliminating peanut exposure during pregnancy, lactation and in the first three years of the infant's life (www.eatwell.gov.uk). There is no evidence to underpin these recommendation regarding peanut exposure.
There is interesting evidence emerging that in countries where babies are routinely exposed to highly allergenic foods early in life, there is a much lower prevalence of peanut allergy.
A possible mechanism for this is repeated low dose exposure resulting in development of immune tolerance.
Reducing risk in infants
It is ethically not possible to randomise mothers to breast-or bottle-feeding. A number of good observational studies however, suggest that breast-feeding is likely to be beneficial in reducing the risk of developing allergic disorders. Given the wider benefits associated with breast-feeding, this should be encouraged in all women.
For infants who cannot be breast fed trials show that hydrolysed milk formula feeds can significantly reduce the reduce the risk of developing allergic conditions.
There have been no cost-effectiveness studies of probiotics or hydrolysed milk preparations. Neither are available on prescription.
Antenatal measures to prevent allergy are interventions in unborn children who are asymptomatic. Such interventions necessitate a high burden of proof before they can be routinely used outside a research context.
Most children with allergic problems are born into low-risk families. We have no idea how to identify which of these families will have children with allergic problems and or how to intervene.
Dr Sangeeta Dhami is a GP locum in Edinburgh and Professor Aziz Sheikh is Professor of Primary Care Research and Development at the University of Edinburgh
Food Allergy and Intolerance week is 22-26 January. For more information see http://www.allergyuk.org/