Medical advisors are urging parents not to use soy-based baby formula if their infant can digest cow's milk following renewed calls to withdraw soy-based products from supermarket shelves.

Recent research suggests exposure to soy in the womb and in young children can affect female fertility, sparking calls to remove it from supermarkets.

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.

Midland clinical advisor Marianne Grant says soy-based formula is not recommended for general use in infant feeding.

'The only medical indication for soy-based formula is for managing galactosaemia. Soy-based formula contains no lactose or cows' milk protein.

'Formulae are available in New Zealand for infants with cow's milk protein allergy and lactose intolerance, and these formulae are recommended in preference to soy-based infant formula.”
Marianne says soy-based formula is an appropriate option for vegan infants if they are not breastfed.

Soy-based formula differs from cow's milk based formula because the protein in the formula is derived from refined soy protein isolate.

As soy protein lacks sufficient amounts of the essential amino acids and methionine, the formula must be fortified with methionine, which affects the taste.

Marianne says soy-based formulas contain higher levers of phytates, aluminium and fluoride than cow's milk based formula.

'The amount of aluminium and fluoride allowed in soy-based formula is controlled by the Food Standards Code (FSANZ 2002).

'The concentration of calcium in soy-based formula is greater than in cow's milk based formula to compensate for the presence of phytates in the soy protein isolate, which may inhibit calcium absorption.”

She says infants with existing hypothyroidism should not be fed a soy-based formula or foods containing soy as a major part of their diet, unless they are under close medical supervision and their thyroid function is closely monitored.

'There have been isolated cases of goitre in infants fed soy-based formula. They were resolved on withdrawal of the formula.”

Marianne says despite some health concerns, infants fed soy-based formula grow and develop normally and are reported to have normal metabolic functions.

'Concern has been expressed about the possible physiological effects of phytoestrogens in soy formula on the developing endocrine system of infants. There is no clear clinical evidence of adverse effects to infants.”