The findings, published in the Journal of Experimental Medicine, is the first controlled investigation to demonstrate protection against food allergy from breast milk, while also pointing to a biological mechanism for inducing food tolerance.
This backs up current guidance for, which says women should not avoid common allergens like eggs and peanuts during pregnancy, and rejects previous advice urging mothers to avoid highly allergenic foods during pregnancy or while breastfeeding.
"Whether mothers should eat allergenic foods during pregnancy or avoid them has been controversial" commented study leader Michiko Oyoshi, PhD, of Boston Children's Division of Allergy and Immunology.
"Different studies have found different results, in part because it's hard in human studies to know when mothers and babies first encountered a specific food. But in a mouse model, we can control exposure to food."
Using a mouse model, the researchers from Boston Children's Hospital and Harvard Medical School tested whether maternal allergen intake affects offspring susceptibility to food allergy -- finding that when a nursing mother is exposed to a food protein, her milk contains complexes of the food protein combined with her antibodies, which are transferred to the offspring through breastfeeding.
These antibodies caused baby mice to produce allergen-specific regulatory T immune cells, which made them tolerate the allergenic foods.
Oyoshi and colleagues reported that breast milk from mothers who consumed allergenic foods protected against food allergy, prevented anaphylaxis and the production of immunoglobulin E and expansion of mast cells -- both of which are hallmarks of an allergic response.
Breast milk of mothers exposed to allergens was protective even when fed to unrelated offspring that were not exposed to food allergens in utero, said the team -- who added that further experiments showed protection for offspring via breastfeeding when mothers who had never consumed allergenic foods were given the food-specific antibodies from other mothers which had consumed the food.
In a further experiment, the team had mice born to allergen-exposed mothers nurse from mothers that had never consumed allergenic foods.
"We still saw protection from the in-utero exposure, but the protection was better when the mice were also exposed through breastfeeding," said Oyoshi. "If you combine both in utero and breastfeeding exposure, you have optimal induction of food tolerance."
The team also performed an early test as to whether the findings could be translated to humans, which suggested that there may be benefits and has resulted in plans for human studies in the area.
In a final experiment, the team gave human breast milk containing specific protein-antibody complexes to mice with humanised immune systems (tailored to respond to human antibodies). This also resulted in a protective -- suggesting that the mouse findings may translate to human infants, said Oyoshi and colleagues.
"This elegantly designed and controlled study shows that mothers should feel free to eat a healthy and diverse diet throughout pregnancy and while breastfeeding," said Dr James R. Baker, Jr, CEO and Chief Medical Officer at Food Allergy Research & Education (FARE), the world's largest private source of funding for food allergy research -- which supported the study. "Eating a range of nutritious foods during pregnancy and breastfeeding will not promote food allergies in developing babies, and may protect them from food allergy."
Oyoshi and colleagues are now collecting milk from actively breastfeeding mothers. They plan to compare milk from mothers whose infants are at high versus low risk of food allergy -- based on whether an older sibling is allergic, or whether the baby has early risk factors such as eczema.
"We are asking: Why is this protective mechanism not functioning in the allergic population? Is it just lack of antibodies?" said Oyoshi. "Maybe the mother cannot make the antibodies, or maybe something is blocking the protective pathway. We don't really know how tolerance is induced in normal circumstances, and what causes breakdown in normal tolerance."
In further tests, if an infant is at low risk for food allergy, the researchers will then compare breast milk samples before and after the mothers eat egg or peanut.
Pending the results of these studies, Oyoshi sees the possibility that infants at risk for food allergy could be supplied with purified antibodies to those foods.
In the meantime, she says, her team's findings indicate that "you should be eating every food to create antibodies to everything."
"Our study does not suggest that mothers' peanut eating will guarantee a healthy baby," she added.
"Given the complicated interactions between genetic and environmental factors, there is not going to be just one diet or one set of behaviors that will make children allergic or healthy."
Journal of Experimental Medicine