Researchers warn men soy may harm fertility

Eating a lot of soy may be a risk to male fertility, a study shows.

Male rats fed a high-soy diet for three days had fewer pups in litters they fathered, Otago University researchers found.

But after a further nine days of high soy, their litter sizes returned to the same as those of rats on a low-soy diet.

Soy is high in a substance called phytoestrogen.

The researchers told the Medical Sciences Congress in Queenstown that the soy disrupted male rats' receptors for sex hormones.

The co-director of the university's Andrology Research Group, Dr Steve Assinder, said that even though the rats' sex hormone receptors were back to normal after six and 12 days of eating a high-soy diet, this might not apply to men.

He said people tended to eat soy one day and not the next, unlike the constantly high soy intake in the rat study.

"What's important is that men that are seeking treatment for fertility, if they are on the cusp, it could be pushing them below a low fertility threshold.'