Waiting to introduce solids until the infant is 17 weeks old appears to be the first line of defense against allergies. Breast-feeding alongside "real food" is the other. British researchers had 1,140 moms keep food diaries on their babies from when they were born until they turned 2. Moms noted when and what they fed their kids and noted any suspected reactions to the foods.
If you're really trying to keep your kids from getting allergies, you might consider birthing and raising them somewhere besides the U.S.
Around 40 of the kids who had been suspected of allergies had confirmed it medically. These kids were compared with 82 kids of the same age who were part of the healthy control group. After controlling for variables like birth weight, duration of pregnancy and whether mom had allergies, lead researcher and nutritionist at the University of Southampton, Kate E. C. Grimshaw, said the team found that the sweet spot for starting solid food without upping the risk a child will develop allergies was the 17-week mark. Furthermore, continuing to breast-feed while starting kids on solids—even if parents wait well beyond the 17-week mark—appeared to lower the risk for allergies.
They also found that continuing to breast-feed while introducing cow's milk significantly lowered the child's risk of eventually developing milk allergies. But if you're really trying to keep your kids from getting allergies, you might consider birthing and raising them somewhere besides the U.S.