Soy isoflavones are thought to act similarly to the female hormone oestrogen, prompting researchers at the Arkansas Children's Nutrition Center (ACNC) in the US to embark on a six-year study to investigate the effects of soy-based infant formula compared with formulas made from human breast milk or cow's milk.
In rat studies, Thomas Badger, ACNC director, and part of the government's Agricultural Research Service, and his team found that the animals grew and developed normally except for very minor differences that have not been found in humans. There were no adverse effects, but many healthful ones, report the scientists.
They also found that diets containing soy proteins boost the activity of certain enzymes that help remove harmful, possible cancer-promoting, toxins from the body.
However, the ACNC studies did show that soy consumption changed certain enzymes in the liver, gut and other organs that break down many medications taken by people and animals which could affect how the drugs are used by the body.
In order to find out more the researchers are currently conducting a study to compare infants who are fed soy formula with those who are breast fed or who receive cow's milk formula.
Previous studies at the centre have found no apparent long-term positive or negative effects of feeding infants soy versus cow's milk formula, said the scientists.
But last year the UK's independent scientific Committee on Toxicity (COT) urged the government to review its advice on the use of soya-based infant formulas. This followed research on rats showing that exposure to large quantities of phytoestrogens damaged fertility, leading to concerns that similar effects could occur in humans, and particularly babies fed soya-based infant formulas.
According to ARS, soy infant formulas are now consumed by 20-25 per cent of formula-fed infants in America.