Soy infant formulas are mixtures of soy protein isolate (powdered concentrated soy protein), vegetable oils, sugars and vitamin and mineral supplements. Today about 20 per cent of babies consume these formulas exclusively, and many people believe them to be healthy alternatives to breast or cow's milk. However, according to the experts, a baby should not be given soy-based formula unless medically necessary or the child has vegan parents.

Soy formula is not recommended unless a baby has a specific medical need, such as an IgE-mediated allergy to milk protein or lactose intolerance, says Deborah L. O'Connor, director of clinical dietetics at The Hospital for Sick Children and an associate professor in the department of nutritional sciences at the University of Toronto. "For infants, human milk is the feeding of choice. If a woman cannot breast-feed, a baby should get a cow's-milk-based, iron-fortified formula. Soy formula is not even Plan B."

O'Connor says that there are some medical conditions where soy formula may be the only choice, such as lactase deficiency. Lactase is the enzyme responsible for digesting lactose, or sugar, in cow's milk and breast milk. Although a lactase deficiency is common in adults of certain racial backgrounds (for example, people of Asian and African descent), it's extremely rare in infants from any background. Babies who normally don't have a lactase deficiency may develop a temporary one after severe diarrhea or vomiting. In this case, a doctor might recommend a soy formula until the baby has recovered and is rehydrated, but most recommend that the baby go back to breast or cow's milk as soon as the illness is over.

A baby with an IgE-mediated allergy to cow's milk protein may also benefit from being put on a soy formula, says O'Connor. The alternative is to put him on a formula that is based on milk products, such as casein or whey, that has been hydrolyzed and broken down into smaller proteins. These casein- or whey-based formulas are more expensive and don't taste as good as soy formulas, but they are often a safe alternative for cow's-milk-protein-induced inflammatory conditions of the gut, which can also be caused by soy formulas.

Although these formulas have been blamed for everything from stunted growth to early sexual maturation, the studies to date are less than conclusive. Some animal studies show that phytoestrogens (compounds found in plants that act like human estrogen; soy isoflavones are a type of phytoestrogen) can reduce fertility and cause changes in uterine and behavioural development. The few reliable human studies in this area found little or no effect of soy on health. Blood levels of soy isoflavones are much higher in babies whose diet consists entirely of soy-based formula than in adults who consume soy foods, so babies may, in theory, be at greater risk of health consequences if a link is found in humans.