Cow antibodies are a likely cause of infant colic - characterized by inconsolable crying - and mothers who consume dairy products can pass them on in breast milk, a new study says.

Colic affects about 20 percent of all babies, typically when they're between 1 and 4 months old. Dairy products long were suspected as a cause, but the study is the first to target antibodies.The findings are published in the April issue of Pediatrics, the journal of the American Academy of Pediatrics.

Doctors Patrick S. Clyne and Anthony Kulczycki Jr. compared 29 mothers with colicky babies to 30 mothers with non-colicky babies of the same age.

They said the average level of cow antibodies in the breast milk of mothers whose babies had colic was "significantly higher" than levels in mothers of non-colicky infants.

Eight mothers had undetectable or very low levels of cow antibodies and none of their babies had colic.

Levels of the antibodies also were high in milk-based formulas.

"Until now, physicians have been unable to explain why colic seems to occur with equal frequency in breast-fed infants and in formula-fed infants," said Kulczycki, the principal investigator and an associate professor of medicine at Washington University.

Kulzcycki said cow antibodies are a minor protein in cow's milk that had not been studied before.