A Canadian-led study suggests peanut proteins can be passed to babies through breast milk, causing some to develop potentially fatal peanut allergies.

Women with a family history of allergies are being advised to avoid eating peanuts while nursing. Although children can't inherit specific food allergies, they can inherit a tendency to have an allergy.

Before an allergic reaction can occur, a person must first be exposed to allergy-prompting proteins called allergens. Finding peanut allergens in breast milk helps explain why more than 70 per cent of children with peanut allergies have reactions the first time they eat peanuts. Their first exposure likely came from breast milk, not the peanuts themselves.

Peanut allergies problematic

The report says peanut allergies affect about one per cent of preschoolers and are responsible for the majority of anaphylactic deaths caused by food allergies.

The study's lead author is Dr. Peter Vadas, an immunologist at St. Michael's Hospital in Toronto. "Because it's so difficult to manage peanut allergy, any action we can take to prevent its development in the first place is critical," said Vadas.

Scientists already knew that allergens in cows' milk, chicken eggs and wheat can be passed on in breast milk. This is the first study to show conclusively that peanut proteins are also released in breast milk.

The research team from the University of Toronto and the University of Arkansas looked for peanut allergens in the breast milk of 23 women. Traces of peanut protein were found in 11 samples of milk after the women ate about 50 grams of dry-roasted peanuts.

Proteins lingered in the milk up to six hours after the peanuts were eaten.

The study is published in the April 4 issue of the Journal of the American Medical Association.