Risks remain theoretical

Soy protein is one of the cheapest sources of protein and has been used as a substitute for cow's milk since the turn of the century. Soy based infant formulas have been available in Britain for over 20 years and account for about 7% of infant formula sales (compared with 13% in New Zealand (C Wham, personal communication) and 10-20% in the USA).1 This represents gross sales in Britain of £10m annually. However, with rates of initial breastfeeding of only about 63%,2 many infants will be fed a soy based formula at some time in their first year of life.

Plant protein sources such as soy are quite complex and very different from milk proteins found in most infant formulas. Soy is a rich source of phytoestrogens, non-steroidal oestrogens of the isoflavone class.3 These compounds are structurally similar to oestrogens; they bind to oestrogen receptor sites and behave as partial oestrogen …