Delaying the introduction of cereal-based foods into a child's diet could help avoid later gut problems, say University of Colorado scientists.
Babies had a lower chance of developing the digestive disorder coeliac disease if they were not fed grains until aged four to six months, they found.
This condition is caused by proteins in grains, which damage the gut and make it difficult to digest foods. The research appears in the Journal of the American Medical Association.
Exposure to gluten - a protein found in wheat - in the first three months of life increased the risk of coeliac disease five-fold, the study of more than 1,500 children found. Children not exposed until they were older than seven months were also more likely to develop coeliac disease than infants exposed when they were aged between four and six months.
The researchers said there were possible explanations for the apparent "safe window" for gluten exposure during infancy. For gluten to evoke an allergic reaction it has to cross the gut barrier so that it can be recognized by the body's immune cells.
At very young ages, such as the first three months of life, this barrier may not be as complete as at older ages, thus allowing gliadin to pass even with small amounts of intake. Conversely, when wheat products are introduced to an older child, such as those older than seven months, it tends to be in larger portion sizes, thus increasing the amount of gluten available to cross the gut.
Even if a small proportion of the available gluten crosses the gut, it may be sufficient to initiate an adverse response.
In the current study, infants first exposed to cereals at or after the seventh month were more likely to have been given one or more servings per day in the first month of exposure compared with children who were first exposed before four months.
This suggested that the frequency of exposure at initial introduction increased with age. But given that the children studied were all from families with a strong history of coeliac disease, the researchers said their findings might not apply to all children.
They also cautioned: "We cannot exclude the possibility that earlier exposure to gluten simply leads to earlier appearance of disease and that all exposed at-risk children will eventually develop coeliac disease."
They recommended that researchers look at many more children with coeliac disease to confirm whether their findings were correct. Dr Richard Farrell of the Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, the US, agreed. "Only then will it be possible to know the full effect of...exclusive verses partial breastfeeding, gluten amount versus gluten timing, and infant cereal versus follow-up formula on coeliac disease risk and prevention," he said.
Treatment for coeliac disease is a gluten-free diet, which essentially eliminates foods containing wheat, barley and rye from the diet.
Nicky Mendoza, a dietician for Coeliac UK, said: "Current guidelines suggest not weaning before six months of age, so, for most children, gluten should not be introduced before this. "Children that are weaned before this age should be given gluten-free foods until they reach six months."
She said children with a parent or other first degree relative with coeliac disease had a one in 10 chance of developing the intolerance themselves. "The recommendations are the same for the introduction of gluten to children with coeliac disease in the family, and previous studies have found that there is no benefit of holding off the introduction of gluten for longer than six months.
"Once gluten is introduced into the baby's diet, it is important that it is given on a regular basis so that symptoms are more easily recognised," she advised.