Soy-based formula tends to be used mainly by parents wanting to avoid problems with lactose intolerance, but some clinicians are concerned about its effect on fertility.

Calls for soy-based baby formula to be withdrawn from supermarket shelves have been renewed after fresh research suggesting exposure to soy in the womb or in childhood could affect female fertility.

Fears about the effects of soy were raised in New Zealand in the early 1990s after claims from Whanganui bird breeders that soy-feed led to infertile and deformed chicks.

The late Richard and Valerie James compared the problems with their birds to the experiences of their children, who were fed soy-based formula and had fertility problems as adults.

Soybeans contain phytoestrogens, which are like a weak form of the female hormone oestrogen. Earlier research suggested newborn female mice exposed to phytoestrogens were infertile as adults.

The latest research, published in The Society for the Study of Reproduction yesterday, goes a step further by suggesting soy damages the reproductive tract and immune system in mice.

The findings from the United States raise the possibility that human exposure to low levels of phytoestrogen during sensitive development stages of the female foetus through to puberty can alter the balance of the mucosal immune response in the uterus and fallopian tubes, affecting fertility in later years.

The authors suggested minimising use of soy-based baby formula as a step toward maintaining female reproductive health.

Soy-based formula tends to be used mainly by parents wanting to avoid problems with lactose intolerance.

Professor John Hutton, a Wellington-based Otago University reproductive medicine specialist, said New Zealand women should be more concerned about the effect of weight on fertility.

"Obesity is a much bigger issue because oestrogen is made in fat," he said.

"The amount of oestrogen that gets made in fat would be much more than they can get by ingesting soya."

The Health Ministry's current advice on soy-based formula, published in 2005, recommends that it "be used only where it is medically indicated, even though there is no conclusive evidence that phytoestrogens in soy-based infant formula harm human infants".

Plunket said that parents should raise any questions or concerns about soy-based formula with their family doctor or Plunket nurse.

Auckland-based toxicologist Mike Fitzpatrick was hired by the Jameses in the early 1990s when they concluded that the birds' beak and bone deformities, immune system disorders and pathological aggressive behaviours were caused by soy-feed.

Dr Fitzpatrick found the bird-feed contained high levels of phytoestrogens and his literature review uncovered evidence that soy consumption was linked to numerous disorders.

His findings spurred the Jameses to launch a campaign against soy-based foods.

Dr Fitzpatrick said yesterday that the latest research did not surprise him and he renewed his calls for soy-based formula to be banned in New Zealand, or at least limited to prescription only.

"It's not a surprise to me but it's tremendously saddening because I feel the warning signs had been there for a long time. Those chemicals in soy affect fertility, they also promote cancer ... and thyroid disease."