Levels of the sex-hormone mimic genistein that are found in soya-based infant formula can impair immune function, according to a new study in mice. Although food-safety groups maintain that it is still safe to feed babies with, concerns were raised in the media. After all, as HealthScout news pointed out, ∼750,000 babies in the United States alone drink soya-based formula.

The paper published by Paul Cooke and colleagues at the University of Illinois, in the 28 May edition of the Proceedings of the National Academy of Sciences, shows that the thymi of mice that are fed or injected with amounts of genistein that are similar to those that are found in soya formula are shrunken and contain 80% fewer T cells.

Paul Cooke, taking a moderate position, told BBC news, “although there is nothing definitive, there is enough concern in this area that you would tend to shy away from it if you were a parent”.

However, food-safety groups and food-industry bodies were quick to reassure parents. “If you look in the scientific literature for evidence of hormonal effects when human infants consume soya formula you don't find them,” claimed Mardi Mountford of the International Formula Council (HealthScout).

Heather Payne of the Infant and Dietetic Food Association (UK) argued that because soya milk has been used for decades, any detrimental effects on human immune systems should have come to light by now.