British researchers followed a group of 1,140 infants from birth to 2 years, while their mothers completed diaries detailing the babies' diets and noting suspected allergic reactions to food, which researchers later confirmed by testing. They found 41 babies with confirmed food allergies, and compared them with 82 age-matched healthy controls. All were born between January 2006 and October 2007.
After controlling for birth weight, the duration of pregnancy, maternal allergies and many other factors, they found that 17 weeks was the crucial age: babies who were introduced to solids before this age were significantly more likely to develop food allergies.
The study, published online in Pediatrics, found that continuing to breast-feed while introducing cow's milk also had a protective effect against allergies. The authors suggest that the immunologic factors in breast milk are what provide the advantage.
Image: Baby food, via Shutterstock